Kalpakjian 20

Transcript

Surface Treatments Coatings, and Cleaning ° The preceding chapters have described methods of producing desired shapes from a Wide variety of materials; although material and process selection is very important, often the surface properties of a component determine its performance or commercial success. This chapter describes various surfacefinishing operations that can be performed for technical and aesthetic reasons subsequent to manufacturing a part. The chapter presents the surface treatment, cleaning, and coating processes that are commonly performed and includes a discussion of mechanical surface treatments such as shot peening, laser peening, and roller burnishing, with the benefit of imparting compressive residual stresses onto metal surfaces. Coating operations are then examined, including cladding, thermal spray operations, physical and chemical vapor deposition, ion implantation, and electroplating. The benefits of diamond and diamond-like carbon coatings are also investigated. ° Finally, surface-texturing, painting, and cleaning operations are described. 1 ' ° ° 34.I 34.2 34.3 34.4 Hard Facing 34.5 34.6 34.7 34.8 34.9 34.l0 34.I! 34.l2 34.I3 34.I Introduction After a part is manufactured, some of its surfaces may have to be processed further in order to ensure that they receive certain properties and characteristics, It may be necessary to perform surface treatments in order to ° ° Improve resistance to wear erosion, and indentation (e.g., for machine-tool slidevvays, as shown in Figs. 23.2 and 35.1, Wear surfaces of machinery, and shafts, rolls, cams, and gears) Control friction (on sliding surfaces of tools, dies, bearings, and machine Ways) ° ° ° Introduction 973 Mechanical Surface Treatments 974 Mechanical Plating and Cladding 976 Case Hardeningand 34.I4 34.I5 34.I6 976 Thermal Spraying 977 Vapor Deposition 979 Ion lmplantation and Diffusion Coating 982 LaserTreatments 982 Electroplating, Electroless Plating, and Electroforming 983 Conversion Coatings 986 Hot Dipping 987 Porcelain Enameling; Ceramic and Organic Coatings 988 Diamond Coating and Diamond-like Carbon 989 Surface Texturing 990 Painting 990 Cleaning of Surfaces 99| EXAMPLES: 34.I 34.2 34.3 Repair ofaWorn Turbine-engine Shaft by Thermal Spraying 979 Applications of Laser Surface Engineering 983 Ceramic Coatings for High-temperature Applications 989 Reduce adhesion (of electrical contacts) Improve lubrication (surface modification to retain lubricants) Improve resistance to corrosion and oxidation (on sheet metals for automobile bodies, gas-turbine components, food packaging, and medical devices) 973 Chapter 34 Surface Treatments, Coatings, and Cleaning ° ° ° ° Improve fatigue resistance (of bearings and shafts with fillets) Rebuild surfaces (on worn tools, dies, molds, and machine components) Modify surface texture (appearance, dimensional accuracy, and frictional characteristics) Irnpart decorative features (color and texture). Numerous techniques are used to impart these characteristics to various types of metallic, nonmetallic, and ceramic materials on the basis of mechanisms that involve (a) plastic deformation of the workpiece surface, (b) chemical reactions, (c) thermal means, (d) deposition, (e) implantation, and (f) organic coatings and paints. We begin with surface-hardening techniques and then continue with descriptions of different types of coatings that are applied to surfaces by various means. Some of these techniques also are used in the manufacture of semiconductor devices, as described in Chapter 28. Techniques that are used to impart texture on workpiece surfaces and types of organic coatings used for various purposes are then described. The chapter ends with a discussion of methods used for cleaning manufactured surfaces before the components are assembled into the completed product and made ready for service. Environmental considerations regarding the fluids used and the waste material from various surface-treatment processes are also included, as they are an important factor to be considered. 34.2 Mechanical Surface Treatments Several techniques are used to mechanically improve the surface properties of manufactured components. The more common methods are the following: Shot Peening. In shot peening, the workpiece surface is impacted repeatedly with a large number of cast steel, glass, or ceramic shot (small balls), which make overlapping indentations on the surface. This action, using shot sizes that range from 0.125 to 5 mm in diameter, causes plastic surface deformation at depths up to 1.25 mm. Because the plastic deformation is not uniform throughout the part’s thickness, shot peening causes compressive residual stresses on the surface, thus improving the fatigue life of the component by delaying the initiation of fatigue cracks. Unless the process parameters are controlled properly, the plastic deformation of the surface can be so severe that it can damage the surface. The extent of deformation can be reduced by gravity peening, which involves larger shot sizes, but fewer impacts on the workpiece surface. Shot peening is used extensively on shafts, gears, springs, oil-well drilling equipment, and jet-engine parts, such as turbine and compressor blades. However, note that if these parts are subjected to high temperatures, the residual stress will begin to relax (thermal relaxation) and their beneficial effects will be diminished greatly. An example is gas-turbine blades performing at their operating temperatures. Laser Shot Peening. In this process, also called laser shock peening and first developed in the mid-19605 (but not commercialized until much later), the workpiece surface is subjected to planar laser shocks (pulses) from high-power lasers. This surface-treatment process produces compressive residual-stress layers that are typically 1 mm deep with less than 1% of cold working of the surface. Laser peening has been applied successfully and reliably to jet-engine fan blades and to materials such as titanium, nickel alloys, and steels for improved fatigue resistance and some corro- Section 34.2 Mechanical Surface Treatments sion resistance. Laser intensities necessary for the process are on the order of 100 to 300 ]/cmz and have a pulse duration of about 30 nanoseconds. Currently, the basic limitation of laser shot peening for industrial, cost-effective applications is the high cost of the high-power lasers (up to 1 kW) that must operate at energy levels of 100 J/pulse. Water-jet Peening. In this more recently developed process, a Water jet at pressures as high as 400 MPa impinges on the surface of the workpiece, inducing compressive residual stresses and surface and subsurface hardening at the same level as in shot peening. The water-jet peening process has been used successfully on steels and aluminum alloys. The control of process variables (jet pressure, jet velocity, the design of the nozzle, and its distance from the surface) is important in order to avoid excessive surface roughness and surface damage. Ultrasonic Peening. This process uses a hand tool based on a piezoelectric transducer. Operating at a frequency of 22 kHz, it can have a variety of heads for different applications. Roller Burnishing. In this process, also called surface rolling, the surface of the component is cold worked by a hard and highly polished roller or set of rollers. The process is used on various flat, cylindrical, or conical surfaces (Fig. 34.1). Roller burnishing improves surface finish by removing scratches, tool marks, and pits and induces beneficial compressive surface residual stresses. Consequently, corrosion resistance is improved, since corrosive products and residues cannot be entrapped. In a variation of this process called low-plasticity burnishing, the roller travels only once over the surface, inducing residual stresses and minimal plastic deformation. Internal cylindrical surfaces also are burnished by a similar process, called ballizing or ball burnishing. In this process, a smooth ball (slightly larger than the bore diameter) is pushed through the length of the hole. Roller burnishing is used to improve the mechanical properties of surfaces as Well as their surface finish. It can be used either by itself or in combination with other finishing processes, such as grinding, honing, and lapping. The equipment can be mounted on various CNC machine tools for improved productivity and consistency of performance. All types of metals (soft or hard) can be roller burnished. Roller burnishing is typically used on hydraulic-system components, seals, valves, spindles, and fillets on shafts. "2 Holler el Burnished Burnishing tools and roller burnishing of (a) the fillet of induce compressive surface residual stresses for improved fatigue life; (b) and (c) a flat surface. FIGURE 34.I Roller a a stepped shaft to conical surface; Chapter 34 Surface Treatments, Coatings, and Cleaning Explosive l-lardening. In explosive hardening, the surfaces are subjected to high transient pressures through the placement and detonation of a layer of an explosive sheet directly on the workpiece surface. The contact pressures that develop as a result can be as high as 35 GPa and can last about 2 to 3 /as. Major increases in surface hardness can be achieved with this method, with very little change (less than 5%) in the shape of the component. Railroad rail surfaces, for example, are explosively hardened. 34.3 Mechanical Plating and Cladding Mechanical Plating. In mechanical plating (also called mechanical coating, impact plating, or peen plating), fine metal particles are compacted over the workpiece surfaces by glass, ceramic, or porcelain beads that are propelled by rotary means (such as tumbling). This process, which is basically one of cold-welding particles onto a surface, typically is used for hardened-steel parts for automobiles, with plating thickness usually less than 25 um. Cladding. In this process, also called clad bonding, metals are bonded with a thin layer of corrosion-resistant metal through the application of pressure by rolls or other means. A typical application is the cladding of aluminum (Alclad), in which a corrosion-resistant layer of aluminum alloy (usually in sheet or tubular form) is clad over an aluminum-alloy body (core). The cladding layer is anodic to the core and usually has a thickness that is less than 10% of the total thickness. Examples of cladding are 2024 aluminum clad with 1230 aluminum, and 3003, 6061, and 7178 aluminum clad with 7072 aluminum. Gther applications are steels clad with stainless-steel or nickel alloys. The cladding material also may be applied with dies (as in cladding steel wire with copper) or explosives. Multiplelayer cladding is also utilized in special applications. Laser cladding consists of the fusion of a different material over the substrate. It has been applied successfully to metals and ceramics, especially for enhanced friction and good wear behavior of the components. 34.4 Case Hardening and Hard Facing Surfaces may be hardened by thermal means in order to improve their frictional and wear properties, as well as their resistance to indentation, erosion, abrasion, and corrosion. The most common methods are described next. Case Hardening. Traditional methods of case hardening (carhurizing, carhonitriding, cyaniding, nitriding, flame hardening, and induction hardening) are described in Section 4.10 and summarized in Table 4.1. In addition to common heat sources (gas and electricity), an electron beam or laser beam can be used as a heat source in surface hardening of both metals and ceramics. Case hardening, as well as some of the other surface-treatment processes described in this chapter, induces residual stresses on surfaces. The formation of martensite during case hardening causes compressive residual stresses on surfaces. Such stresses are desirable, because they improve the fatigue life of components by delaying the initiation of fatigue cracks. Hard Facing. In this process, a relatively thick layer, edge, or point of wearresistant hard metal is deposited on the workpiece surface by the fusion-welding ~ Section 34. 5 techniques described in Chapter 30. Numerous layers (known as weld overlay) can be deposited to repair worn parts. Hard facing enhances the wear resistance of the materials; hence, such materials are used in the manufacture of tools, dies, and various industrial components. Worn parts also can be hard faced for extended use. Spark Hardening. Hard coatings of tungsten, chromium, or molybdenum carbides can be deposited by an electric arc in a process called spark hardening, electric spar/2 hardening, or electrospar/Q deposition. The deposited layer is typically 250 /.rm thick. Hard-facing alloys can be used as electrodes, rods, wires, or powder in spark hardening. Typical applications for these alloys are as valve seats, oil-well drilling tools, and dies for hot metalworking. 34.5 Thermal Spraying Thermal spraying is a series of processes in which coatings of various metals, alloys, carbides, ceramics, and polymers are applied to metal surfaces by a spray gun with a stream heated by an oxyfuel flame, an electric arc, or a plasma arc. The earliest applications of thermal spraying (in the 1910s) involved metals (hence the term metallizing has also been used), and these processes are under continuous refinement. The surfaces to be sprayed are first cleaned of oil and dirt, and then roughened by, for example, grit blasting, to improve their bond strength (see Section 26.8). The coating material can be in the form of wire, rod, or powder, and when the droplets or particles impact the workpiece, they solidify and bond to the surface. Depending on the process, particle velocities typically range from a low of about 150 to 1000 m/s, but can be higher for special applications. Temperatures are in the range from 3000° to 8000°C. The coating is hard and wear resistant, with a layered structure of deposited material. However, the coating can have a porosity as high as 20% due to entrapped air and oxide particles because of the high temperatures involved. Bond strength depends on the particular process and techniques used and is mostly mechanical in nature (hence the importance of roughening the surface prior to spraying), but can be metallurgical in some cases. Bond strength generally ranges from 7 to 80 MPa, depending on the particular process used. Typical applications of thermal spraying include aircraft engine components (such as those used in rebuilding worn parts), structures, storage tanks, tank cars, rocket motor nozzles, and components that require resistance to wear and corrosion. In an automobile, thermal spraying typically can be applied to crankshafts, valves, fuel-injection nozzles piston rings, and engine blocks. The process is also used in the gas and petrochemical industries, for the repair of worn parts and to restore dimensional accuracy to parts that have not been machined or formed properly. The source of energy in thermal-spraying processes is of two types: combustion and electrical. Combustion Spraying ° ° Thermal wire spraying (Fig. 34.Za): The oxyfuel flame melts the wire and deposits it on the surface. The bond is of medium strength, and the process is relatively inexpensive. Thermal metal-powder spraying (Fig. 34_2b): This process is similar to flame wire spraying, but uses a metal powder instead of the wire. Thermal Spraying Chapter 34 Surface Treatments, Coatings, and Cleaning Wire or rod /- Gas Air cap Workpiece Combustion chamber _L Oxygen Fuel gas L Molten High-velocity gas P metal spray Deposited coating Powder orkpiece Flame Fuel gas Oxygen f Molten metal spray Deposited coating (D) Spray powder ClfCU|3T'l'lQ C°°|am Plasma suspended in Prepared base material (water cooled) Carrier 933 Plasma Deposited Spray Clrbbiiilgriiti DC Flame power to arc s ggg Nozzle Electrode aa,ss ggyg é _ Lsemimolte spray stream (C) Schematic illustrations of thermal-spray operations: (a) thermal wire spray, (b) thermal metal-powder spray, and lc) plasma spray. FIGURE 34.2 ° ' Detonation gun: Controlled and repeated explosions take place by means of an oxyfuel-gas mixture. The detonation gun has a performance similar to that of plasma. High-velocity oxyfuel-gas spraying (HVOF): This process produces a high performance similar to that of the detonation gun, but is less expensive. Electrical Spraying ° Twin-wire arc: An arc is formed between two consumable Wire electrodes. The resulting bond has good strength, and the process is the least expensive. Section 34.6 ° Vapor Deposition 979 Plasma: Either conventional, high-energy, or vacuum (Fig. 34.2c) plasma produces temperatures on the order of 8300°C and results in good bond strength with very low oxide content. Low-pressure plasma spray (LPPS) and vacuum plasma spray both produce coatings with high bond strength and with very low levels of porosity and surface oxides. Cold Spraying. In this more recent development, the particles to be sprayed are at a lower temperature and are not melted; thus, oxidation is minimal. The spray jet in cold spraying is narrow and highly focused; it has very high impact velocities, thereby improving the bond strength of the particles on the surface. EXAMPLE 34.1 Repair of a Worn Turbine-engine Shaft by Thermal Spraying The shaft of the helical gear for a GE T-38 gas-turbine engine had two worn regions on its nitrided surfaces. The case-hardened depth was 0.3 mm. Even though the helical gears were in good condition, the part was considered scrap because there was no approved method of repair. The worn regions first were machined undersize, grit blasted, and coated with tungsten carbide (12% 34.6 cobalt content; see Section 22.5) using the high-velocity oxyfuel-gas thermal-spraying (HVOF) technique. Then the part was finish machined to the dimensions of the original shaft. The total cost of repair was a fraction of the projected cost of replacing the part. Source: Courtesy of Plasma Technology, Inc. Vapor Deposition Vapor deposition is a process in which the substrate (workpiece surface) is subjected to chemical reactions by gases that contain chemical compounds of the material to be deposited. The coating thickness is usually a few microns, which is much less than the thicknesses that result from the techniques described in Sections 34.2 and 34.3. The deposited materials can consist of metals, alloys, carbides, nitrides, borides, ceramics, or oxides. Control of the coating composition, thickness, and porosity are important. The substrate may be metal, plastic, glass, or paper. Typical applications for vapor deposition are the coating of cutting tools, drills, reamers, milling cutters, punches, dies, and wear surfaces. There are two major vapor-deposition processes: physical vapor deposition and chemical vapor deposition. 34.6.I Physical Vapor Deposition The three basic types of physical vapor deposition (PVD) processes are (1) vacuum deposition, or arc evaporation; (2) sputtering; and (3) ion plating. These processes are carried out in a high vacuum and at temperatures in the range from 200° to 500°C. In PVD, the particles to be deposited are carried physically to the workpiece, rather than being carried by chemical reactions (as in chemical vapor deposition). Vacuum Deposition. In vacuum deposition (or evaporation), the metal is evaporated at a high temperature in a vacuum and is deposited on the substrate (which is usually at room temperature or slightly higher for improved bonding). Coatings of uniform thickness can be deposited, even on complex shapes. In arc deposition (PV/ARC), the coating material (cathode) is evaporated by several arc evaporators Surface Treatments, Coatings, and Cleaning Chapter 34 980 Evaporator Plasma Reactive gas Neutral gas Evaporator gg; Qifzizif' Substrate ;f:;f>, T Coating material g 0 iiirl 2% Q f’ -> I Evaporator Vacuum pump Evaporated material Power supply Schematic illustration of the physical-vapor(a) deposition process. Note that there are three arc evaporators and the parts to be coated are placed on a tray inside the chamber. FIGURE 34.3 Working gas feed Cathode Target lon flux g puttéfed H Hux H Plasma Subsme 1 ,§'l . v cuum pumps To FIGURE 34 4 .iw i,._i, , 343) using highly localized electric arcs. The arcs produce a highly reactive plasma, which consists of ionized vapor of the coating material. The vapor condenses on the substrate (anode) and coats it. Applications of this process are both functional (oxidation-resistant coatings for high-temperature applications, electronics, and optics) and decorative (hardware, appliances, and jewelry). Pulsed-laser deposition is a more recent, related process in which the source of energy is a pulsed (Fig. Ground Shleld. 5.>;g,;s;§2§22;2"§;;;; + ’°‘“°de Chamber laser. In sputtering, an electric field ionizes an inert gas (usually argon). The positive ions bombard the coating material (cathode) and cause sputtering Schematic illustration of the sputtering process. (ejection) of its atoms. The atoms then condense on the workpiece, which is heated to improve bonding (Fig. 34.4). In reactive sputtering, the inert gas is replaced by a reactive gas (such as oxygen), in which case the atoms are oxidized and the oxides are deposited. Carbides and nitrides also are deposited by reactive sputtering. Alternatively, very thin polymer coatings can be deposited on metal and polymeric substrates with a reactive gas, causing polymerization of the plasma. Radio-frequency (RF) sputtering is used for nonconductive materials, such as electrical insulators and semiconductor devices. " Sputtering. lon Plating. Ion plating is a generic term that describes various combined processes of sputtering and vacuum evaporation. An electric field causes a glow, generating a plasma (Fig. 34.5). The vaporized atoms in this process are ionized only partially. Ion-beam-enhanced (assisted) deposition is capable of producing thin films as coatings for semiconductor, tribological, and optical applications. Bulky parts can be coated in large chambers using high-current power supplies of 15 kW and voltages of 100,000 DC. Dual ion-beam deposition is a hybrid coating technique that combines PVD with simultaneous ion-beam bombardment. This technique results in good adhesion on metals, ceramics, and polymers. Ceramic bearings and dental instruments are examples of its applications. Section 34 6 _ Gas * insulator ? Variable leak Movable shutter ""',,<_.‘i__ _ _;5:1_-5¢g_-' itqighrvggage Ground shield Substrate f'f'ff_f_.2i?l'f",~,'§3`f`f{':f-f;fffff§,;'f-i;, "_*ff'.=`§`-=i Cathode dark space I Evaporator filament _ 11}~*'P|_@§m?‘~_f:1.1.= it Supply ~- + 151 f_».»_§`1ji-1{}f13_:;j»_';"-j§_';~f'_-ZffZj1 z'.': :;.;.;-;;~;;.\;- |_ Glass chamber High-current feedthroughs * » is Current mmf” T Vaffii Filament supply FIGURE 34.5 Schematic illustration of an ion-plating apparatus. Carrier gases Exhaust i -> -> -> Vi TlCl4 Electric furnace ' 7 it M ___; "=' _V rl“ ->‘ Exhaust scrubber Graphite shelves Tools to be coated Stainless-steel retort Schematic illustration of the chemical-vapor-deposition process. Note that parts and tools to be coated are placed on trays inside the chamber. FIGURE 34.6 34.6.2 Chemical Vapor Deposition Chemical vapor deposition (CVD) is a thermochemical process (Fig. 34.6). In a typical application, such as coating cutting tools with titanium nitride (TiN), the tools are placed on a graphite tray and heated at 950° to 105 0°C at atmospheric pressure in an inert atmosphere. Titanium tetrachloride (a vapor), hydrogen, and nitrogen are then introduced into the chamber. The chemical reactions form titanium nitride on the tool surfaces. For a coating of titanium carbide, methane is substituted for the other gases. Deposited coatings usually are thicker than those obtained with PVD. A typical cycle for CVD is long, consisting of (a) three hours of heating, (b) four hours of coating, and (c) six to eight hours of cooling to room temperature. The thickness of the coating depends on the flow rates of the gases used, the time, and the temperature. The types of coatings and the workpiece materials allowable are fairly unrestricted in CVD. Almost any material can be coated and any material can serve as a substrate, although bond strength may Vary. The CVD process is also used to produce diamond coatings without binders, unlike polycrystalline diamond films, which use Vapor Deposition Chapter 34 Surface Treatments, Coatings, and Cleaning 1 to 10% binder materials. A more recent development in CVD is medium-temperature CVD (MTCVD). This technique results in a higher resistance to crack propagation than CVD affords. 34.7 lon lmplantation and Diffusion Coating In ion implantation, ions (charged atoms) are introduced into the surface of the workpiece material. The ions are accelerated in a vacuum to such an extent that they penetrate the substrate to a depth of a few microns. Ion implantation (not to be confused with ion plating) modifies surface properties by increasing surface hardness and improving resistance to friction, wear, and corrosion. The process can be controlled accurately, and the surface can be masked to prevent ion implantation in un- wanted locations. Ion implantation is particularly effective on materials such as aluminum, titanium, stainless steels, tool and die steels, carbides, and chromium coatings. The process is typically used on cutting and forming tools, dies and molds, and metal prostheses, such as artificial hips and knees. When used in some specific applications, such as semiconductors (Section 283), ion implantation is called doping-meaning “alloying with small amounts of various elements.” Diffusion Coating. This is a process in which an alloying element is diffused into the surface of the substrate (usually steel), altering its surface properties. The alloying elements can be supplied in solid, liquid, or gaseous states. The process has acquired different names (depending on the diffused element), as shown in Table 4.1, which lists diffusion processes such as carburizing, nitriding, and boronizing. 34.8 Laser Treatments As described in various chapters of this book, lasers are having increasingly broader applications in manufacturing processes (laser machining, forming, joining, rapid prototyping, and metrology) and surface engineering (laser peening, alloying, surface treatments, and texturing). Powerful, efficient, reliable, and less expensive lasers are now available for a variety of cost~effective surface treatments, as outlined in Fig. 34.7. Laser surface treatments E;;;;‘:\;?atl°“ 9 ° ° Cleaning ° ° Thin-film deposition ° ~ Cladding Grain refinement ° lnfiltration ° ° (composite forming) FIGURE 34.7 Texturing Alloying ° Peening Vaporization Melting Heating Annealing Solid-state phase r Shock hardening Peening Marking . Scribing _ Etching An outline of laser surface-engineering processes. Source: After N.B. Dahotre Section 34.9 EXAMPLE 34.2 Applications Electroplating, Electroless Plating, and Electroforming of Laser Surface Engineering In this example, several applications of lasers in engi~ neering practice are given. The most commonly used lasers are Nd:YAG and CO2; excimer lasers are generally used for surface texturing (see also Table 27.2). _ Localized surface hardening--Cast irons: dieselengine eynnder nnefS> antofnonne Steering assemblies, and camshafts. Carbon steels: gears and eleennnleenanlenl Pens- Surface alloyirzg--Alloy steels: bearing components. Stainless steels: diesel-engine valves and seat inserts. Tool and die steels: dies for forming and die casting. Cladding-Alloy steels: automotive valves and Valve Seam Superalloysz turbine glad,” Ceramic coating-Aluminum-silicon alloys? automotivgengine bOre_ Surface texturing--Metals, plastics, ceramics, and Wood: all types of products. Electroplating, Electroless Plating, and Electroforming 34.9 Plating, like other coating processes, imparts the properties of resistance to Wear, resistance to corrosion, high electrical conductivity, and better appearance and reflectivity, as well as similar desirable properties. Electroplating. In electroplating, the workpiece (cathode) is plated with a different metal (anode), which is transferred through a Water-based electrolytic solution (Fig. 34.8). Although the plating process involves a number of reactions, the process consists basically of the following sequence: from the anode are discharged by means of the potential energy from the external source of electricity, or are delivered in the form of metal salts. I. The metal ions Cu” 303- SOX§ H+ Cu* H+ Q s § H+ ++ § Cu SO-- __ 304 4 + e Agitator tg \ so" 4 mn i 3 é g ”` Part to be plated mode) (°a _ ' Heating coils . ar Sacrificial (copper) anode (al (b) (a) Schematic illustration of the electroplating process. (b) Examples of electroplated parts. Source: Courtesy of BFG Electroplating. FIGURE 34.8 983 Chapter 34 Surface Treatments, Coatings, and Cleaning 2. The metal ions are dissolved into the solution. 3. The metal ions are deposited on the cathode. The volume of the plated metal can be calculated from the equation Volume = clt, (34.1) where I is the current in amperes, t is time, and c is a constant that depends on the plate metal, the electrolyte, and the efficiency of the system; typically, c is in the range from 0.03 to 0.1 mm3/amp-s. Note that, for the same volume of material deposited, the larger the workpiece surface plated, the thinner is the layer. The time required for elecroplating is usually long, because the deposition rate is typically on the order of 75 /.im/hour. Thin-plated layers are typically on the order of l /am; for thick layers, the plating can be as much as 500 /im. The plating solutions are either strong acids or cyanide solutions. As the metal is plated from the solution, it needs to be periodically replenished, and this is accomplished through two principal methods: salts of metals are occasionally added to the solution, or a sacrificial anode of the metal to be plated is used in the electroplating tank and dissolves at the same rate that the metal is deposited. There are three main forms of electroplating: rack plating, the parts to be plated are placed in a rack, which is then conveyed through a series of process tanks. 2. In barrel plating, small parts are placed inside a permeable barrel, which is placed inside the process tank(s). This form of electroplating is commonly performed with small parts, such as bolts, nuts, gears, and fittings. Electrolytic fluid can penetrate through the barrel and provide the metal for plating, and electrical contact is provided through the barrel and through Contact with I. In 3. other parts. In brush processing, the electrolytic fluid is pumped through a handheld brush with metal bristles. The workpiece can be very large in this circumstance, and the process is suitable for field repair or plating and can be used to apply coatings on large equipment without_ disassembly. Simple electroplating can be achieved in a single-process bath or tank, but more commonly, a sequence of operations is used in a plating line. For example, the following tanks and processes may be part of an electroplating operation: ° ° ° ° ° Chemical cleaning and degreasing tanks will be used to remove surface contaminants and enhance surface adhesion of the plated coating. The workpieces may be exposed to a strong acid bath (pickling solution) to reduce or eliminate the thickness of the oxide coating on the workpiece. A base coating may be applied. This may involve the same or a different metal than that of the ultimate surface. For example, if the desired metal coating will not adhere well to the substrate, an intermediate coating can be applied. Also, if thick films are desired, a plating tank can be used to quickly develop a film, and a subsequent tank with brightener additives in the electrolytic solution is used to develop the ultimate surface finish. A separate tank performs final electroplating. Rinse tanks will be used throughout the sequence. Rinse tanks are necessary for a number of reasons. Some plating is performed with cyanide salts delivering the required metal ions. If any residue acid (such as that from a pickling tank) is conveyed to the cyanide-solution tank, poisonous hydrogencyanide gas is exhausted. (This is a significant safety concern, and environmental Section 34.9 Electroplating, Electroless Plating and Electroformmg Coating Rounded corners Sharp , corners Sharp corner (H) ‘ Rounded corner (D) (a) Schematic illustration of nonuniform coatings (exaggerated) in electroplated parts. (b) Design guidelines for electroplating. Note that sharp external and internal corners should be avoided for uniform plating thickness. FIGURE 34.9 controls are essential in plating facilities.) Also, residue plating solution will contain some metal ions, and it is often desirable to recover those ions by capturing them in a rinse tank. The rate of film deposition depends on the local current density and is not necessarily uniform on a part. Workpieces with complex shapes may require an altered geometry because of varying plating thicknesses, as shown in Fig. 34.9. Common plating metals are chromium, nickel (for corrosion protection), cadmium, copper (corrosion resistance and electrical conductivity), and tin and zinc (corrosion protection, especially for sheet steel). Chromium plating is done by first plating the metal with copper, then with nickel, and finally with chromium. Hard chromium plating is done directly on the base metal and results in a surface hardness of up to 70 HRC (see Fig. 2.14) and a thickness of about 0.05 mm or more. This method is used to improve the resistance to wear and corrosion of tools, valve stems, hydraulic shafts, and diesel- and aircraft-engine cylinder liners. It is also used to rebuild worn parts. Examples of electroplating include copper-plating aluminum wire and phenolic boards for printed circuits, chrome-plating hardware, tin-plating copper electrical terminals (for ease of soldering), galvanizing sheet metal (see also Section 34.11), and plating components such as metalworking dies that require resistance to wear and galling (cold welding of small pieces from the workpiece surface). Metals such as gold, silver, and platinum are important electroplating materials in the electronics and jewelry industries for electrical contact and decorative purposes, respectively. Plastics (such as ABS, polypropylene, polysulfone, polycarbonate, polyester, and nylon) also can be electroplating substrates. Because they are not electrically conductive, plastics must be preplated by a process such as electroless nickel plating. Parts to be coated may be simple or complex, and size is not a limitation. Electroless Plating. This process is carried out by a chemical reaction and without the use of an external source of electricity. The most common application utilizes nickel as the plating material, although copper also is used. In electroless nickel plating, nickel chloride (a metallic salt) is reduced (with sodium hypophosphite as the reducing agent) to nickel metal, which is then deposited on the workpiece. The hardness of nickel plating ranges between 425 and 5 75 HV; the plating can subsequently be heat treated to 1000 HV. The coating has excellent wear and corrosion resistance. Cavities, recesses, and the inner surfaces of tubes can be plated successfully. Electroless plating also can be used with nonconductive materials, such as plastics 86 Chapter 34 Surface Treatments, Coatings, and Cleaning 2. 1. 3. 4. 5. (H) (D) FIGURE 34.10 (a) Typical sequence in electroforming. (1) A mandrel is selected with the correct nominal size. (Z) The desired geometry (in this case, that of a bellows) is machined into the mandrel. (3) The desired metal is electroplated onto the mandrel. (4) The plated material is trimmed if necessary. (5) The mandrel is dissolved through chemical machining. (b) A collection of electroformed parts. Source: Courtesy of Servometer®, Cedar Grove, Nj. and ceramics. The process is more expensive than electroplating. However, unlike that of electroplating, the coating thickness of electroless plating is always uniform. Electroforming. A variation of electroplating, electroforming actually is a metalfabricating process. Metal is electrodeposited on a mandrel (also called a mold or a matrix), which is then removed; thus, the coating itself becomes the product (Fig. 34.10). Both simple and complex shapes can be produced by electroforming, with wall thicknesses as small as 0.025 mm. Parts may weigh from a few grams to as much as 270 kg. Production rates can be increased through the use of multiple mandrels. Mandrels are made from a variety of materials: metallic (zinc or aluminum) or nonmetallic (which can be made electrically conductive with the proper coatings). Mandrels should be able to be removed physically without damaging the electroformed part. They also may be made of low-melting alloys, wax, or plastics, all of which can be melted away or dissolved with suitable chemicals. The electroforming process is particularly suitable for low production quantities or intricate parts (such as molds, dies, waveguides, nozzles, and bellows) made of nickel, copper, gold, and silver. The process is also suitable for aerospace, electronics, and electro-optics applications. 34.|0 Conversion Coatings Conversion coating, also called chemical-reaction priming, is the process of producing a coating that forms on metal surfaces as a result of chemical or electrochemical reactions. Various metals (particularly steel, aluminum, and zinc) can be conversion coated. Oxides that naturally form on their surfaces represent a form of conversion coating. Phospliates, chromates, and oxalates are used to produce these coatings, for purposes such as providing corrosion protection, prepainting, and decorative finishing. Section 34.11 Hot Dipping 987 An important application is the conversion coating of workpieces to serve as lubricant carriers in cold-forming operations, particularly zinc-phosphate and oxalate coatings (see Section 33.7.6). The two common methods of coating are immersion and spraying. The equipment required depends on the method of application, the type of product, and quality considerations. Anodizing. This is an oxidation process (anodic oxidation) in which the workpiece surfaces are converted to a hard and porous oxide layer that provides corrosion resistance and a decorative finish. The workpiece is the anode in an electrolytic cell immersed in an acid bath, which results in chemical adsorption of oxygen from the bath. Organic dyes of various colors (usually black, red, bronze, gold, or gray) can be used to produce stable, durable surface films. Typical applications for anodizing are aluminum furniture and utensils, architectural shapes, automobile trim, picture frames, keys, and sporting goods. Anodized surfaces also serve as a good base for painting, especially on aluminum, which otherwise is difficult to paint. Coloring. As the name implies, coloring involves processes that alter the color of metals, alloys, and ceramics. This change is caused by the conversion of surfaces (by chemical, electrochemical, or thermal processes) into chemical compounds such as oxides, chromates, and phosphates. An example is the blackening of iron and steels, a process that utilizes solutions of hot, caustic soda and results in chemical reactions that produce a lustrous, black oxide film on surfaces. 34.I I Hot Dipping In hot dipping, the workpiece (usually steel or iron) is dipped into a bath of molten metal, such as (a) zinc, for galvanized-steel sheet and plumbing supplies; (b) tin, for tinplate and tin cans for food containers; (c) aluminum (aluminizing); and (d) terne, an alloy of lead with 10 to 20% tin. Hot-dipped coatings on discrete parts provide long-term corrosion resistance to galvanized pipes, plumbing supplies, and many other products. A typical continuous hot-dipped galz/anizing line for sheet steel is shown in Fig. 34.1 1. The rolled sheet is first cleaned electrolytically and scrubbed by brushing. The sheet is then annealed in a continuous furnace with controlled atmosphere and temperature and dipped in molten zinc at about 450°C. The thickness of the zinc coating is controlled by a wiping action from a stream of air or steam, called an air knife (similar to air-drying in car washes). Proper draining for the removal of excess coating materials is important. The coating thickness is usually given in terms of coating weight per unit surface area of the sheet, typically 150 to 900 g/mz. The service life depends on the thickness of the zinc coating and the environment to which it is exposed. Various precoated sheet steels are used extensively in automobile bodies. Proper draining to remove excess coating materials is an important consideration. Accumulator ,T °I ~ ii = -: Welder I oll _Ll IIT-"ll-'-‘l,'lll _., :»:-:»:. 0 | Payoff reels --. Electrolytnc cleanlng , and brush scrubblng Coollng tower .... . Hllllllllllll K.. Chemlcal I treatment Molten sechon Galvanized sheet steel 1 Contlnuous anneallng furnace ZINC FIGURE 34.ll Flow line for the continuous hot-dipped galvanizing of sheet steel. The welder (upper left) is used to weld the ends of coils to maintain continuous material flow. Source: Courtesy of the American Iron and Steel Institute. 8 Chapter 34 Surface Treatments, Coatings, and Cleaning 34.12 Porcelain Enameling; Ceramic and Organic Coatings Metals can be coated with a variety of glassy (vitreous) coatings to provide corrosion and electrical resistance, and protection at elevated temperatures. These coatings usually are classified as porcelain enamels and generally include enamels and ceramics. The root of the word “porcelain” is porcellancz, in Italian meaning “marine shell.” Note that the word enamel also is used as a term for glossy paints, indicating a smooth, hard coating. Enamels. Porcelain enamels are glassy inorganic coatings that consist of various metal oxides and are available in various colors and transparencies. Enameling (which was a fully developed art by the Middle Ages) involves fusing the coating material to the substrate by heating both of them at 425° to 1000°C to liquefy the oxides. The coating may be applied by dipping, spraying, or electrodeposition, and thicknesses are usually from 0.05 to 0.6 mm. The viscosity of the material can be controlled using binders so that the coating adheres to vertical surfaces during application. Depending on their composition, enamels have varying resistances to alkali, acids, detergents, cleansers, and water. Typical applications for porcelain enameling are household appliances, plumbing fixtures, chemical-processing equipment, signs, cookware, and jewelry. Porcelain enamels also are used as protective coatings on jet-engine components. Metals coated are typically steels, cast iron, and aluminum. Glasses are used as a lining (for chemical resistance) where the thickness of_the glass is much greater than that of the enamel. Glazing is the application of glassy coatings onto ceramic wares to give them decorative finishes and to make them impervious to moisture. Ceramic Coatings. Materials such as powders of hard metals, aluminum oxide, and zirconium oxide are applied to a substrate at room temperature by means of binders. These coatings act as thermal barriers and have been applied (usually by thermal spraying techniques) to hot-extrusion dies, turbine blades, diesel-engine components, and nozzles for rocket motors to extend the life of these components. They also are used for electrical-resistance applications to withstand repeated arcing. Plasma arcs are used where temperatures may reach 15,000°C, which is much higher than those obtained by flames. Organic Coatings. Metal surfaces can be coated or precoated with a variety of organic coatings, films, and laminates to improve appearance and corrosion resistance. Coatings are applied to the coil stock on continuous lines (see Fig. 13.10), with thicknesses generally from 0.0025 to 0.2 mm. Such coatings have a wide range of properties: flexibility, durability, hardness, resistance to abrasion and chemicals, color, texture, and gloss. Coated sheet metal is subsequently formed into various products, such as TV cabinets, appliance housings, paneling, shelving, residentialbuilding siding, gutters, and metal furniture. More critical applications involve, for example, the protection of naval aircraft, which are subjected to high humidity, rain, seawater, pollutants (such as those from ship exhaust stacks), aviation fuel, deicing fluids, and battery acid, as well as being impacted by particles such as dust, gravel, stones, and deicing salts. For aluminum structures, organic coatings consist typically of an epoxy primer and a polyurethane topcoat with a lifetime of four to six years. Primer performance is an important factor in the durability of the coating. Section 34.13 EXAMPLE 34.3 Ceramic Coatings for 989 High-temperature Applications Certain product characteristics (such as Wear resistance and thermal and electrical insulation-particularly at elevated temperatures) can be imparted through ceramic coatings, rather than to the base metals or materials themselves. Selecting materials with such bulk properties can be expensive and may not meet the structural strength requirements of a particular application. TABLE 34.l Property Diamond Coating and Diamondlike Carbon For example, a wear-resistance component does not have to be made completely from a wear-resistant material, since the properties of only a thin layer of its surface are relevant to wear. Consequently, coatings have important applications. Table 34.1 shows various ceramic coatings and their typical applications at elevated temperatures. These coatings may be applied either singly or in layers-as is done in multiple-layer coated cutting tools (see Fig. 22.8). Type of ceramic Applications Wear resistance Chromium oxide, aluminum oxide, aluminum titania Pumps, turbine shafts, seals, and compressor rods for the petroleum industry; plastics extruder barrels; extrusion dies Thermal insulation Zirconium oxide (yttria stabilized), zironium oxide (calcia stabilized), magnesium zirconate Electrical insulation Magnesium aluminate, aluminum oxide Fan blades, compressor blades, and seals for gas turbines; valves, pistons, and combustion heads for automotive engines Induction coils, brazing fixtures, general electrical applications 34.I3 Diamond Coating and Diamond-like Carbon The properties of diamond that are relevant to manufacturing engineering were described in Section 8.7. Important advances have been made in the diamond coating of metals, glass, ceramics, and plastics using various techniques, such as chemical vapor deposition (CVD), plasma-assisted vapor deposition, and ion-beamenhanced deposition. Examples of diamond-coated products are scratchproof windows (such as those used in aircraft and military vehicles for protection in sandstorms); sunglasses; cutting tools (such as inserts, drills, and end mills); Wear faces of micrometers and calipers; surgical knives; razors; electronic and infrared heat seekers and sensors; light-emitting diodes; diamond-coated speakers for stereo systems; turbine blades; and fuel-injection nozzles. Techniques also have been developed to produce freestanding diamond films on the order of 1 mm thick and up to 125 mm in diameter. These films include smooth, optically clear diamond film, unlike the hazy gray diamond film formerly produced. This film is then laser cut to desired shapes and brazed onto cutting tools (for example). The development of these techniques, combined with the important properties of diamond (hardness, Wear resistance, high thermal conductivity, and transparency to ultraviolet light and microwave frequencies), has enabled the production of various aerospace and electronic parts and components. Studies also are continuing regarding the growth of diamond films on crystallinecopper substrate by the implantation of carbon ions. An important application is in making computer chips (see Chapter 28). Diamond can be doped to form p- and 0 Chapter 34 Surface Treatments, Coatings, and Cleaning n-type ends on semiconductors to make transistors, and its high thermal conductivity allows the closer packing of chips than would be possible with silicon or galliumarsenide chips, significantly increasing the speed of computers. Diamond is also an attractive material for future MEMS devices (see Chapter 29), because of its favorable friction and wear characteristics. Diamond-like Carbon. Diamond-like carbon (DLC) coatings, a few nanometers in thickness, are produced by a low-temperature, ion-beam-assisted deposition process. The structure of DLC is between that of diamond and graphite. Less expensive than diamond films, but with similar properties (such as low friction, high hardness, and chemical inertness, as well as having a smooth surface), DLC has applications in such areas as tools and dies, gears, engine components, bearings, MEMS devices, and microscale probes. As a coating on cutting tools, DLC has a hardness of about 5 O00 HV (compared with about double that for diamond). 34.l4 Surface Texturing throughout the preceding chapters, each manufacturing process (such as casting, forging, powder metallurgy, injection molding, machining, grinding, polishing, electrical-discharge machining, grit blasting, and wire brushing) produces a certain surface texture and appearance. Obviously, some of these processes can be used to modify the surface produced by a previous process-for example, grinding the surface of a cast part. However, manufactured surfaces can be modified further by secondary operations for technical, functional, optical, or aesthetic reasons. Called surface texturing, these additional processes generally consist of the following techniques: As stated ° ° ° ° Etching: Using chemicals or sputtering techniques. Electric arcs. Lasers: Using excimer lasers with pulsed beams; applications include molds for permanent-mold casting, rolls for temper mills, golf-club heads, and computer hard disks. Atomic oxygen: Reacting with surfaces to produce a fine, cone-like surface texture. The possible adverse effects of these processes on material properties and the performance of the textured parts are important considerations. 34.15 Painting Because of its decorative and functional properties (such as environmental protection, low cost, relative ease of application, and the range of available colors), paint has been widely used as a surface coating. The engineering applications of painting range from appliances and machine tools to automobile bodies and aircraft fuselages. Paints generally are classified as ° ° ° Enamels: Produce a smooth coat with a glossy or semiglossy appearance. Lacquers: Form a film by evaporation of a solvent. Water-based paints: Applied easily, but have a porous surface and absorb water, making them more difficult to clean than the first two types. Paints are available with good resistance to abrasion, temperature extremes, and fading; are easy to apply; and dry quickly. The selection of a particular paint Section 34.16 " Q? g Part . , Dip tank Zio oven ,I Conveyor ’ to Electrostatic paint Spray Paint Ouilel _ 'l ' . ¢;, ;““= Pump Paint supply Hi part Drainboard MT’ Overflow catch basin la) (D) 34.l2 Methods of paint application: (a) dip coating, (b) flow coating, and electrostatic spraying (used particularly for automotive bodies). FIGURE (c) depends on specific requirements. Among these are resistance to mechanical actions (abrasion, marring, impact, and flexing) and to chemical reactions (acids, solvents, detergents, alkalis, fuels, staining, and general environmental attack). Common methods of applying paint are dipping, brushing, rolling, and spraying (Fig. 34.12). In electrocoating or electrostatic spraying, paint particles are charged electrostatically and are attracted to surfaces to be painted, producing a uniformly adherent coating. Unlike paint losses in conventional spraying, which may be as much as 70% of the paint, the loss can be as little as 10% in electrostatic spraying. However, deep recesses and corners can be difficult to coat with this method. The use of robotic controls for guiding the spray nozzles is common (see Section 37.6.3). 34.l6 Cleaning of Surfaces Cleaning of Surfaces The importance of surfaces in manufacturing and the effects of deposited or adsorbed layers of various elements and contaminants on surface characteristics have been stressed throughout this text. A clean surface can have both beneficial and detrimental effects. Although a surface that is not clean may reduce the tendency for adhesion and galling, cleanliness generally is essential for a more effective application of coatings, painting, adhesive bonding, welding, brazing, and soldering, as well as for the reliable functioning of manufactured parts in machinery, assembly operations, and food and beverage containers. Cleaning involves the removal of solid, semisolid, or liquid contaminants from a surface and is an important part of manufacturing operations and the economics of production. The word clean or the degree of cleanliness of a surface is somewhat difficult to define. Two simple and common tests are as follows: Wiping the surface of, say, a dinner plate with a clean cloth and observing any residues on the cloth. 2. Observing whether water continuously coats the surface of a plate (the waterbrea/Q test). If water collects as individual droplets, the surface is not clean. (You can test this phenomenon by wetting dinner plates that have been cleaned to different degrees.) I. The type of cleaning process required depends on the type of metalworkingfluid residues and contaminants to be removed. For example, water-based fluids are easier and less expensive to remove than oil-based fluids. Contaminants (also called soils) may consist of rust, scale, chips (and other metallic and nonmetallic debris), h Vougge Chapter 34 Surface Treatments, Coatings, and Cleaning metalworking fluids, solid lubricants, pigments, polishing and lapping compounds, and general environmental elements. Basically, there are three types of cleaning methods: Mechanical Cleaning. This operation consists of physically disturbing the contaminants, often with wire or fiber brushing, abrasive blasting (jets), tumbling, or steam jets. Many of these processes are particularly effective in removing rust, scale, and other solid contaminants. Ultrasonic cleaning is also placed into this category. Electrolytic Cleaning. In this process, a charge is applied to the part to be cleaned in an aqueous (often alkaline) cleaning solution. The charge results in bubbles of hydrogen or oxygen (depending on polarity) being released at the part’s surface. The bubbles are abrasive and aid in the removal of contaminants. Chemical Cleaning. Chemical cleaning usually involves the removal of oil and grease from surfaces. The operation consists of one or more of the following processes: ° ° ° ° ° Solution: The soil dissolves in the cleaning solution. Saponification: A chemical reaction converts animal or vegetable oils into a soap that is soluble in water. Emulsification: The cleaning solution reacts with the soil or lubricant deposits and forms an emulsion; the soil and the emulsifier then become suspended in the emulsion. Dispersion: The concentration of soil on the surface is decreased by surfaceactive elements in the cleaning solution. Aggregation: Lubricants are removed from the surface by various agents in the cleanser and are collected as large dirt particles. Cleaning Fluids. Common cleaning fluids used in conjunction with electrochemical processes for more effective cleaning include the following: ° ° ° ° ° Alkaline solutions: A complex combination of water-soluble chemicals, alkaline solutions are the least expensive and most widely used cleaning fluids in manufacturing operations. Small parts may be cleaned in rotating drums or barrels. Most parts are cleaned on continuous conveyors by spraying them with the solution and rinsing them with water. Emulsions: Emulsions generally consist of kerosene and oil-in-water and various types of emulsifiers. Solvents: Typically petroleum solvents, chlorinated hydrocarbons, and mineral spirits, solvents generally are used for short runs. Fire and toxicity are major hazards. Hot vapors: Chlorinated solvents can be used to remove oil, grease, and wax. The solvent is boiled in a container and then condensed. This hot-vapor process is simple, and the cleaned parts are dry. Acids, salts, and mixtures of organic compounds: These are effective in cleaning parts covered with heavy paste or oily deposits and rust. Design Guidelines for Cleaning. Cleaning discrete parts with complex shapes can be difficult. Some design guidelines are as follows: ° ° ° Avoid deep, blind holes. Make several smaller components instead of one large component, which may be difficult to clean. Provide appropriate drain holes in the parts to be cleaned. Bibliography 993 The treatment and disposal of cleaning fluids, as well as of various fluids and waste materials from the processes described in this chapter, are among the most important considerations for environmentally safe manufacturing operations. (See also Section I.4.) SUMMARY Surface treatments are an important aspect of all manufacturing processes. They are used to impart specific chemical, physical, and mechanical properties, such as appearance, and corrosion, friction, wear, and fatigue resistance. Several techniques are available for modifying surfaces. The processes used include mechanical working and surface treatments, such as heat treatment, deposition, and plating. Surface coatings include enamels, nonmetallic materials, and paints. Clean surfaces can be important in the further processing (e.g., coating, painting, or welding) and use of the product. Cleaning can have a significant economic impact on manufacturing operations. Various mechanical and chemical cleaning methods may be utilized. KEY TERMS Anodizing Ballizing Blackening Case hardening Chemical cleaning Chemical vapor deposition Cladding Cleaning fluids Coloring Conversion coating Diamond coating Diamondlike carbon Diffusion coating Electroforming Electroless plating Electroplating Hard-chromium plating Hard facing Hot dipping Ion implantation Ion plating Enamel Explosive hardening Freestanding Laser peening Mechanical plating Metallizing Painting Physical vapor deposition diamond film Glazing Porcelain enamel Roller burnishing Shot peening Spraying Sputtering Surface texturing Thermal spraying Vacuum evaporation Vapor deposition Waterbreak test Water-jet peening BIBLIOGRAPHY ASM Handbook, Vol. 5, Surface Engineering, ASM International, 1994. Bhushan, B., and Gupta, B.K., Handbook of Tribology: Materials, Coatings, and Surface Treatments, McGrawHill, 1991. Bunshah, R.F. (ed.), Handbook of Hard Coatings: Deposition Technologies, Properties and Applications, Noyes, 2001. Burakowski, T., and Wierschon, T., Surface Engineering of Metals: Principles, Equipment, Technologies, CRC Press, 1998. Davis, ].R. (ed.), Surface Engineering for Corrosion and Wear Resistance, IOM Communications and ASM - International, 2001. (ed.), Handbook of Thermal Spray Technology, Thermal Spray Society and ASM International, 2004. --- (ed.), Surface Hardening of Steels, ASM International, 2002. Edwards, ]., Coating and Surface Treatment Systems for Metals: A Comprehensive Guide to Selection, ASM International, 1997. Handbook of Surface Treatments and Coatings, ASME Press, 2003. Holmberg, K., and Matthews, A., Coating Tribology: Properties, Techniques, and Applications, Elsevier, 1994. Inagaki, N., Plasma Surface Modification and Plasma Polymerization, Technomic, 1996. Kawai, S. (ed.), Anodizing and Coloring of Aluminum Alloys, Metal Finishing Information Services and ASM International, 2002. 994 Chapter 34 Surface Treatments, Coatings, and Cleaning Lambourne, R., and Strivens, T.A., Paint and Surface Coatings: Theory and Practice, 2nd ed., Woodhead, 1999. Liberto, N., User’s Guide to Powder Coating, 4th ed., Society of Manufacturing Engineers, 2003. Lindsay, ].H. (ed.), Coatings and Coating Processes for Metals, ASM International, 1998. Mattox, D.M., Handbook of Physical Vapor Deposition (PVD) Processing, Noyes, 1999. Park, J., Chemical Vapor Deposition, ASM International, 2001. Prelas, M.A., Popovichi, G., and Bigelow, L.K. (eds.), Handbook of Industrial Diamonds and Diamond Films, Marcel Dekker, 1998. Schlesinger, M., and Paunovic, M. (eds.) Modern Electroplating, 4th ed., Wiley, 2001. Stern, K.H. (ed.), Metallurgical and Ceramic Protective Coatings, Chapman SC Hall, 1997. Sudarshan, T.S. (ed.), Surface Modification Technologies, ASM International, 1998. Tamarin, Y, Protective Coatings for Turbine Blades, ASM International, 2002. Tracton, A.A. (ed.), Coatings Technology Handbook, CRC, 2006. Utech, B., A Guide to High Performance Coating, Society of Manufacturing Engineers, 2002. van Ooij, WJ., Bierwagen, G.P., Skerry, B.S., and Mills, D., Corrosion Control of Metals by Organic Coatings, CRC Press, 1999. REVIEW QUESTIONS 34.I. Explain why surface treatments may be necessary for various parts made by one or more processes. 34.2. What are the advantages of roller burnishing? 34.3. Explain the difference between case hardening and hard facing. 34.4. Describe the principles of physical and chemical vapor deposition. What applications do these processes have? 34.5. What is the principle of electroforming? What are the advantages of electroforming? 34.6. Explain the difference between electroplating and electroless plating. How is hot dipping performed? 34.8. What is an air knife? How does it function? 34.9. Describe the common painting systems presently 34.7. in use in industry. What conversion coating? Why is it so called? Describe the difference between thermal spraying and plasma spraying. 34.|2. What is cladding, and why is it performed? 34.l0. 34.| I. is a QUALITATIVE PROBLEMS Describe how roller-burnishing processes induce compressive residual stresses on the surfaces of parts. 34.|4. Explain why some parts may be coated with ceramics. Give some examples. 34.I5. Give examples of part designs that are suitable for hot-dip galvanizing. 34.I6. Comment on your observations regarding Fig. 34.9. 34.1 3. It is well known that coatings may be removed or depleted during the service life of components, particularly at 34.17. elevated temperatures. Describe the factors involved in the strength and durability of coatings. 34.I8. Make a list of the coating processes described in this chapter and classify them in relative terms as “thick” or “thin.” 34.I9. Why is galvanizing important for automotive-body sheet metals? 34.20. Explain the principles involved in various techniques for applying paints. QUANTITATIVE PROBLEMS 34.21. Taking a simple example, such as the parts shown in Fig. 34.1, estimate the force required for roller burnishing. (Hint: See Sections 2.6 and 14.4.) u34.22. Estimate the plating thickness in electroplating a 20-mm solid-metal ball using a current of 10 A and a plating time of 1.5 hours. Assume that c = 0.08 in Eq. (34.1). Synthesis, Design, and Projects 995 SYNTHESIS, DESIGN, AND PROIECTS 34.23. Which surface treatments are functional, and which are decorative? Are there any treatments that serve both func- tions? Explain. 34.24. An artificial implant has a porous surface area where it is expected that the bone will attach and grow into the implant. Without consulting the literature, make recommendations for producing a porous surface; then review the literature and describe the actual processes used. 34.25. If one is interested in obtaining a textured surface on a coated piece of metal, should one apply the coating first or apply the texture first? Explain. 34.26. It is known that a mirror-like surface finish can be obtained by plating workpieces that are ground; that is, the surface finish improves after coating. Explain how this occurs. 34.27. lt has been observed in practice that a thin layer of chrome plating, such as that on older model automobile bumpers, is better than a thick layer. Explain why, considering the effect of thickness on the tendency for cracking. 34.28. Outline the reasons that the topics described in this chapter are important in manufacturing processes and operations. 34.29. Shiny, metallic balloons have festive printed patterns that are produced by printing screens and then plated onto the balloons. How can metallic coatings be plated onto a rubber sheet? 34.30. Because they evaporate, solvents and similar cleaning solutions have adverse environmental effects. Describe your thoughts on what modifications could be made to render cleaning solutions more environmentally friendly. A roller-burnishing operation is performed on a shaft shoulder to increase fatigue life. It is noted that the resultant surface finish is poor, and a proposal is made to machine the surface layer to further improve fatigue life. Will this be advisable? Explain. 34.3 I. 34.32. The shot-peening process can be demonstrated with a ball-peen hammer (in which one of the heads is round). Using such a hammer, make numerous indentations on the surface ofa piece of aluminum sheet (a) 2 mm and (b) 10 mm thick, respectively, placed on a hard flat surface such as an anvil. Note that both pieces develop curvatures, but one becomes concave and the other convex. Describe your observations and explain why this happens. (Hint: See Fig. 2.14.) 34.33. Obtain several pieces of small metal parts (such as bolts, rods, and sheet metal) and perform the waterbreak test on them. Then clean the surfaces with various cleaning fluids and repeat the test. Describe your observations. Inspect various products, such as small and large appliances, silverware, metal vases and boxes, kitchen utensils, and hand tools, and comment on the type of coatings they may have and the reasons they are coated. 34.34. Engineering Metrology, Instrumentation and uality Assurance The preceding chapters have described the techniques used to modify the surfaces of components and products to obtain certain desirable properties, discussing the advantages and limitations of each technique along the way. Although dimensional accuracies obtained in individual manufacturing processes were described, We have not yet described how parts are measured and inspected before they are assembled into products. Dimensions and other surface features of a part are measured to ensure that it is manufactured consistently and Within the specified range of dimensional tolerances. The vast majority of manufactured parts are components or a subassembly of a product, and they must fit and be assembled properly so that the product performs its intended function during its service life. For example, (a) a piston should fit into a cylinder Within specified tolerances, (b) a turbine blade should fit properly into its slot on a turbine disk, and (c) the slideways of a machine tool must be produced with a certain accuracy so that the parts produced on that machine are accurate within their desired specifications. Measurement of the relevant dimensions and features of parts is an integral aspect of interchangeable parts manufacturing, the basic concept behind standardization and mass production. For example, if a ball bearing in a machine is Worn and has to be replaced, all one has to do is purchase a similar one with the same specification or part number. The same is now done with all products, ranging from bolts and nuts, to gears, to electric motors. The first of the next tvvo chapters describes the principles involved in, and the various instruments and modern machines used for, measuring dimensional features such as length, angle, flatness, and roundness. Testing and inspecting parts are important aspects of manufacturing operations; thus, the methods used for the nondestructive and destructive testing of parts are also described. One of the most important aspects of manufacturing is product quality. Chapter 36 discusses the technological and economic importance of building quality into a product rather than inspecting the product after it is made, as has been done traditionally. This concept is even more significant in view of competitive manufacturing in a global economy. Engineering Metrology and Instrumentation 35.l Introduction 998 Measurement Standards 999 35.3 Geometric Features of Parts; Analog and Digital Measurements |000 35.4 Traditional Measuring Methods and Instruments |00I 35.5 Modern Measuring Instruments and Machines |008 35.6 Automated Measurement and Inspection IOII 35.1 General Characteristics and Selection of Measuring Instruments I0l2 35.8 Geometric Dimensioning andTolerancing I0|2 35.2 EXAMPLES: 35.1 Length Measurements throughout History 999 35.2 Coordinate-measuring Machine for Car Bodies 998 A CH PT ER IOIO ° ° ° ° This chapter describes the importance of the measurement of manufactured parts, noting that measurement of parts and their certification to a certain standard is essential to ensurin S P art fit and thus Pro Per o Peration. A wide variety of measurement strategies, gages, equipment, and machines has been developed, as described in this chapter. The topics discussed include traditional measurement with simple rulers; gages and instruments, such as micrometers and calipers; and digital equipment and computer-controlled equipment, such as coordinate-measurement machines. The chapter describes features of measuring instruments and the importance of automated measurements, ending with an introduction to the principles of dimensioning and tolerancing. 35.1 Introduction This chapter presents the principal methods of measurement and the characteristics of the instruments used in manufacturing. Engineering metrology is defined as the measurement of dimensions such as length, thickness, diameter, taper, angle, flatness, and profile. Consider, for example, the slideways for machine tools (Fig. 35.1); these components must have specific dimensions, angles, and flatness in order for the machine to function properly and with the desired dimensional accuracy. Traditionally, measurements have been made after the part has been producedan approach known as postprocess inspection. Here, the term inspection means “checking the dimensions of what has been produced or is being produced and determining whether those dimensions comply with the specified dimensional tolerances and other specifications.” Today, however, measurements are being made while the part is being produced on the machine-an approach known as in-process, online, or real-time inspection. An important aspect of metrology in manufacturing processes is the dimensional tolerance (i.e., the permissible variation in the dimensions of a part). Tolerances are important because of their impact on the proper functioning of a product, part interchangeability, and manufacturing costs. Generally, the smaller the Section 35.2 tolerance, the higher are the production costs. The chapter ends with a discussion of dimensional limits and fits used in engineering practice. 35.2 Measurement Standards 2 Gap (exaggerated) Cam... Machine bed Measurement Standards 999 _~; i/&_,-Lv ‘- -__ Our earliest experience with measurement is usually with a FIGURE 35.l Cross section of a machine-tool simple ruler to measure lengths (linear dimensions). Rulers are slideway; see also Fig. 23.2. The width, depth, used as a standard against which dimensions are measured. angles, and other dimensions all must be produced Traditionally, in English-speaking countries, the units inch and and measured accurately for the machine tool to foot have been used, which originally were based on parts of function as expected. the human body. Consequently, it was common to find significant variations in the length of 1 foot. In most of the world, however, the meter has been used as a length standard. Originally, 1 meter was defined as one ten-millionth of the distance between the North Pole and the equator. The original meter length subsequently was standardized as the distance between two scratches on a platinum-iridium bar kept under controlled conditions in a building outside Paris. In 1960, the meter officially was defined as 1,65 0,763.73 wavelengths (in a vacuum) of the orange light given off by electrically excited krypton 86 (a rare gas). The precision of this measurement was set as 1 part in 109. The meter is now a unit of length in the Systeme International d’Unités (SI) and is the international standard. Numerous measuring instruments and devices are used in engineering metrology, each of which has its own application, resolution, precision, and other features. Two terms commonly used to describe the type and quality of an instrument are as follows: is the smallest difference in dimensions that the measuring instrument can detect or distinguish. A wooden yardstick, for example, has far less resolution than a micrometer. 2. Precision, sometimes incorrectly called accuracy, is the degree to which the instrument gives repeated measurements of the same standard. For example, an aluminum ruler will expand or contract depending on temperature variations in the environment in which it is used; thus, its precision can be affected even by being held by the hand. l. Resolution ln engineering metrology, the words instrument and gage often are used interchangeably. Temperature control is very important, particularly for making measurements with precision instruments. The standard measuring temperature is 20°C, and all gages are calibrated at this temperature. In the interest of accuracy, measurements should be taken in controlled environments maintaining the standard temperature, usually within d:0.3°C. EXAMPLE 35.| Length Measurements throughout History Many standards for length measurement have been developed during the past 6000 years. A common standard in Egypt around 4000 B.C. was the King’s elbow, which was equivalent to 0.4633 m. Gne elbow was equal to 1.5 feet (or 2 hand spans, 6 hand widths, or 24 finger thicknesses). In 1101 A.D., King Henry I declared a new standard called the yard (0.9144 rn), which was the distance from his nose to the tip of his thumb. During the Middle Ages, almost every kingdom and city established its own length standard-some with identical names. In 1528, the French physician |000 Chapter 35 Engineering Metrology and Instrumentation jean Fernel proposed the distance between Paris and Amiens (a city 120 km north of Paris) as a general length reference. During the 17th century, some scientists suggested that the length of a certain pendulum be used as a standard. In 1661, British architect Sir Christopher Wren suggested that a pendulum with a period of one-half second be used. The Dutch mathematician Christian Huygens proposed a pen~ dulum that had a length one~third of Wren’s and a period of 1 second. To put an end to the confusion of length measurement, a definitive length standard began to be de~ veloped in 1790 in France with the concept of a metre (from the Greek word metron, meaning “measure”). A gage block 1 meter long was made of pure platinum 35.3 with a rectangular cross section and was placed in the National Archives in Paris in 1799. Copies of this gage were made for other countries over the years. During the three years from 1870 to 1 872, inter~ national committees met and decided on an international meter standard. The new bar was made of 90% platinum and 10% iridium, with an X-shaped cross section and overall dimensions of 20 X 20 mrn. Three marks were engraved at each end of the bar. The standard meter is the distance between the central marks at each end, measured at 0°C. Today, extremely accurate measurement is based on the speed of light in a vacuum, which is calculated by multiplying the Wavelength of the standardized infrared beam of a laser by its frequency. Geometric Features of Parts; Analog and Digital Measurements In this section, we list the most common quantities and geometric features that typ ically are measured in engineering practice and in products made by the manufactur ing processes described throughout this book: ° ° ' ° ° ° ° ° ° ° Length-including all linear dimensions of parts. Diameter-outside and inside, including parts with different outside and in side diameters (steps) along their length. Roz/mdness-including out-of-roundness, concentricity, and eccentricity. Depth-such as that of drilled or bored holes and cavities in dies and molds Straightness-such as that of shafts, bars, and tubing. Platness-such as that of machined and ground surfaces. Parallelism-such as that of two shafts or slideways in machines. Perpendicalarity-such as that of a threaded bar inserted into a flat plate. Angles-including internal and external angles. Profile-such as curvatures in castings, in forgings, and on car bodies. A Wide variety of instruments and machines is available to accurately and rap idly measure the preceding quantities on stationary parts or on parts that are in con tinuous production. Because of major and continuing trends in automation and the computer control of manufacturing operations, modern measuring equipment is now an integral part of production machines. The implementation of digital instru- mentation and developments in computer-integrated manufacturing (described in Part IX of the book) have together led to the total integration of measurement technologies Within manufacturing systems. It is important to recognize the advantages of digital over analog instruments. As will be obvious from our description of traditional measuring equipment in Section 35.4, accurate measurement on an analog instrument, such as a vernier caliper or micrometer (Fig. 35.2a), relies on the skill of the operator to properly interpolate and read the graduated scales. In contrast, a digital caliper does not require any particular skills, because measurements are indicated directly (Fig. 35.2b). Section 35.4 (8) Traditional Measuring Methods and instruments (D) Digital gages Display examples * / car s.\;-1 I L e 3 Floppydisk drive Printer Bar-code reader (C) FIGURE 35.2 (a) A vernier (analog) micrometer. (b) A digital micrometer with a range of 0 to 1 in. (0 to 25 mm) and a resolution of 50 /tin. (1.25 /atm). Generally, it is much easier to read dimensions on this instrument than on analog micrometers. (c) Schematic illustration showing the integration of digital gages with miroprocessors for real-time data acquisition for statistical process control. Source: (a) Courtesy of L.C. Starrett Co. (b) Courtesy of Mitutoyo Corp. More importantly, digital equipment can be integrated easily into other equipment (Fig. 35.2c), including production machinery and systems for statistical process control (SPC), as described in detail in Chapter 36. 35.4 Traditional Measuring Methods and Instruments This section describes the characteristics of traditional measuring methods and instruments that have been used over many years and are still used extensively in many parts of the world. However, these instruments are rapidly being replaced with more efficient and advanced instruments and measuring machines, as described in Section 35.5. 35.4.l Line-graduated Instruments These instruments are used for measuring lengths or angles. Graduated means “marked to indicate a certain quantity.” A l00l |002 Chapter 35 Engineering Metrology and Instrumentation Linear Measurement (Direct Reading) Rules: The simplest and most commonly used instrument for making linear measurements is a steel rule (maclainist’s rule), bar, or tape with fractional or decimal graduations. Lengths are measured directly to an accuracy that is limited to the nearest division, usually 1 mm. ° Calipers: These instruments can be used to measure inside or outside lengths. Also called caliper gages and z/ernier calipers (named for P. Vernier, who lived FIGURE 35.3 A digital micrometer depth gage. in the 1600s), they have a graduated beam and a Source: Courtesy of Starrett Co. sliding jaw. Digital calipers are in increasingly wider use. ° Micrometers: These instruments are commonly used for measuring the thickness and inside or outside dimensions of parts. Digital micrometers are equipped with digital readouts (Fig. 35.2b) in metric or English units. Micrometers also are available for measuring internal diameters (inside micrometer) and depths (micrometer depth gage, Fig. 353). The anvils on micrometers can be equipped with conical or ball contacts to measure recesses, threaded-rod diameters, and wall thicknesses of tubes and curved sheets. ° Linear Measurement (Indirect Reading). These instruments typically are calipers and dividers without any graduated scales. They are used to transfer the measured size to a direct-reading instrument, such as a rule. Because of the experience required to use them and their dependence on graduated scales, the accuracy of indirectmeasurement tools is limited. Telescoping gages can be used for the indirect measurement of holes or cavities. Angle Measurement ° ° ° Bevel protractor: This is a direct-reading instrument similar to a common protractor, except that it has a movable element. The two blades of the protractor are placed in contact with the part being measured, and the angle is read directly on the vernier scale. Another common type of bevel protractor is the combination square, which is a steel rule equipped with devices for measuring 45° and 90° angles. Sine bar: Measuring with this method involves placing the part on an inclined bar (sine bar) or plate and adjusting the angle by placing gage blocks on a surface plate. After the part is placed on the sine bar, a dial indicator is used to scan the top surface of the part. Gage blocks (see Section 35.4.4) are added or removed as necessary until the top surface is parallel to the surface plate. The angle on the part is then calculated from trigonometric relationships. Surface plates: These plates are used to place both parts to be measured and the measuring instruments. They typically are made of cast iron or natural stones (such as granite) and are used extensively in engineering metrology. Granite surface plates have the desirable properties of being resistant to corrosion, being nonmagnetic, and having low thermal expansion, thereby minimizing thermal distortion. Comparative Length Measurement. Instruments used for measuring comparative lengths (also called deviation-type instruments) amplify and measure variations or deviations in the distance between two or more surfaces. These instruments, of which the most common example is a dial indicator (Fig. 35.4), compare dimensions Section 35.4 Dial indicator Depth gage Part M Part T; ` (H) gg" (C) (D) Three uses of dial indicators: to measure for multiple-dimension gaging of a part. FIGURE 35.4 (c) Traditional Measuring Methods and Instruments (a) roundness and (b) depth, and (hence the word comparative). They are all simple mechanical devices that convert linear displacements of a pointer to the amount of rotation of an indicator on a circular dial. The indicator is set to zero at a certain reference surface, and the instrument or the surface to be measured (either external or internal) is brought into contact with the pointer. The movement of the indicator is read directly on the circular dial (as either plus or minus some number) to accuracies as high as 1 /am. Dial indicators with electrical and fluidic amplification mechanisms and with a digital readout also are available. 35.4.2 Measuring Geometric Features Straightness. Straig/atness commonly can be checked with a straightedge or a dial indicator (Fig. 35.5 ). An autocollimator (which resembles a telescope with a light beam that bounces back from the object) is used to accurately measure small angular deviations on a flat surface. Laser beams are now commonly used to align individual machine elements in the assembly of machine components. Flatness. Flatness can be measured by mechanical means with a surface plate and a dial indicator. This method can be used to measure perpendicularity, which also can be measured by precision-steel squares. Another method for measuring flatness is interferometry, which uses an optical fiat. This device is a glass disk or fused-quartz disk with parallel flat surfaces that is placed on the surface of the workpiece (Fig. 35.6a). When a monochromatic light beam (a light beam with one wavelength) is aimed at the surface at an angle, the optical flat splits the light beam into two beams, appearing as light and dark bands to the naked eye (Fig. 35.6b). The number of fringes that appear is related to the distance between the surface of the part and the bottom surface of the optical flat (Fig. 35.6c). Consequently, a truly flat workpiece surface (i.e., one in which the angle between the two surfaces is zero) will not split the light beam, and fringes will not appear. When surfaces are not flat, the fringes are curved (Fig. 35.6d). The interferometry method is also used for observing surface textures and scratches (Fig. 35.6e). Diffraction gratings consist of two optical flat glasses of different lengths with closely spaced parallel lines scribed on their surfaces. The grating on the shorter glass is inclined slightly. As a result, interference fringes develop when it is viewed over the longer glass. The position of these fringes depends on the relative position of the two sets of glasses. With modern equipment and with the use of electronic counters and photoelectric sensors, a resolution of 2.5 /,tm can be obtained with gratings having 40 lines/mm. Roundness. This feature usually is described as a deviation from true roundness (which, mathematically, is manifested in a circle). The term out-of-roundness (ovality) is actually more descriptive of the shape of the part (Fig. 35.7a) than the word |003 |004 Chapter 35 Engineering Metrology and Instrumentation Dial indicator 7 _,_r,, Knife if Pan edge Part A, . (D) (H) Measuring straightness manually with indicator. Source: After F.T. Farago. FIGURE 35.5 Optical flat , g (a) a knife-edge rule and (b) a dial Fang.. 1_5 (H) (D) EE (C) (d) (G) (a) Interferometry method for measuring flatness with an optical flat. (b) Frmges on a flat, inclined surface. An optical flat resting on a perfectly flat workpiece surface will not split the light beam, and no fringes will be present. (c) Fringes on a surface with two inclinations Note: The greater the incline, the closer together are the fringes. (d) Curved fringe patterns indicate curvatures on the workpiece surface. (e) Fringe pattern indicating a scratch on the surface. FIGURE 35.6 roundness. True roundness is essential to the proper functioning of rotating shafts bearing races, pistons, cylinders, and steel balls in bearings. Methods of measuring roundness generally fall into two categories: round part is placed on a V-block or between centers (Figs. 35.7b and c, respectively) and is rotated while the point of a dial indicator is in Contact with the part surface. After a full rotation of the workpiece, the difference between the maximum and minimum readings on the dial is noted. This difference is called the total indicator reading (TIR) or the full indicator movement This method can also be used to measure the straightness (squareness) of end faces of shafts that are machined, such as the facing operation shown in Fig. 23 le I. The 2. In circular tracing, the part is placed on a platform, and its roundness is meas ured by rotating the platform (Fig. 35.7d). Alternatively, the probe can be rotated around a stationary part to take the measurement. Section 35.4 Dial indicator Circle ; _ probe 4| 5 Pall Centering clamps ~ S (8) |005 Traditional Measuring Methods and Instruments i`iii (D) iii; Rotating precision table i i iiiiii (C) (Ci) (a) Schematic illustration of out-of-roundness (exaggerated). Measuring roundness with (b) a V-block and dial indicator, (c) a round part supported on centers and rotated, and (d) circular tracing. Source: After F.T. Farago. FIGURE 35.7 Profile. Profile may be measured by means such as (a) comparing the surface with a template or profile gage (as in the measurement of radii and fillets) for conformity and (b) using a number of dial indicators or similar instruments. The best method, however, is using the advanced measuring machines described in Section 35.5. Screw Threads and Gear Teeth. Threads can be measured by means of thread gages of various designs that compare the thread produced against a standard thread. Some of the gages used are threaded plug gages, screw-pitch gages, micrometers with coneshaped points, and snap gages (see Section 35.4.4) with anvils in the shape of threads. Gear teeth are measured with (a) instruments that are similar to dial indicators, (b) calipers (Fig. 35.8a), and (c) micrometers using pins or balls of various diameters (Fig. 35 .8b). Advanced methods include the use of optical projectors and coordinate-measuring machines. 7 f. . I' ,z Camper 2 " L_ .. Lgywil ,, gg? _ , if f tmm. . 5. s. 1 'ib I~~i¥s`i‘-51 Jr. ;f‘ivf< gm you-i_av - __ Z _V V 4. I 3 X rs EQ? ‘ YI? °‘;'iela,,+ P.: ff. ¢f*:.i.‘i_,'~‘¢=;’ A Q _ c.f,.,,.. ,_..:=i- , .~~§e= ‘M » ~»<».:’.},“»tf§‘ f¥i.‘f»=¢€'.,i “ia” (a) FIGURE 35.8 Measuring gear-tooth thickness and profile with (a) a gear-tooth caliper and (b) pins or balls and a micrometer. Source: Courtesy of American Gear Manufacturers Association. 35.4.3 Optical Contour Projectors These instruments, also called optical comparators, were first developed in the 1940s to check the geometry of cutting tools for machining screw threads, but are now used for checking all profiles (Fig. 35.9). The part is mounted on a table or between centers, and the image is projected onto a screen at magnifications of 100>< or higher. Linear and angular measurements are made directly on the screen, which is marked with reference lines and circles. For angular measurements, the screen can be rotated. 35.4.4 Gages This section describes several common gages that have simple solid shapes and cannot be classified as instruments although they are very valuable in metrology. Gage Blocks. Gage blocks are individual square, rectangular, or round blocks of various sizes. For general use, they are made from heat-treated and stress-relieved alloy |006 Chapter 35 Engineering Metrology and Instrumentation steels. The better gage blocks are made of ceramics (often zirconia) and chromium carbide-unlike steels, these materials do not rust, but they are brittle and must be handled carefully. Angle bloc/as are made the same way and are used for angular gaging. Gage blocks have a flatness within 1.25 /im. Control panel Environmental temperature control is important when gages are used for high-precision measurements. Projected screen image Clips for __ -- Fixed Gages. These gages are replicas of the shapes of the parts to be measured. Although /Qxed gages are easy to use and inexpensive, they indicate only whether a part is too small or too large compared with an established standard. template Pan Table Plug gages are commonly used for holes (Figs. 35.1Oa and b). The GO gage is smaller than the NOT GO (or NO GO) gage and slides into any hole that has a dimension smaller than the diameter of the gage. The NOT GO gage must not go into the hole. Two gages are required for such FIGURE 35.9 A bench-model horizontal-beam contour projecmeasurements, although both may be on tor with a 16-in.-diameter (400-mm) screen with 150-W tungsten halogen illumination. Source: Courtesy of L.S. Starrett Company, the same device-either at opposite ends or Precision Optical Division. in two steps at one end (step-type gage). Plug gages also are available, for measuring internal tapers (in which deviations between the gage and the part are indicated by the looseness of the gage), splines, and threads (in which the GO gage must screw into the threaded hole). ° Ring gages (Fig. 35.10c) are used to measure shafts and similar round parts. Ring thread gages are used to measure external threads. The GO and NOT ° L, ....... aww, = =”"" GO NOT oo (3) _ I ......ii ,,., oo | “_” __ ___ *Q* oo (D) _'__ |. || | ppgyggfwiiit; GO NOT °° "'“ | GO NOT _ ___ G0 ,M Nor GO (C) (Ci) 35.I0 (a) Plug gage for holes, with GO and NOT GO on opposite ends of the gage. (b) Plug gage with GO and NOT GO on one end. (c) Plain ring gages for gaging round rods. Note the difference in knurled surfaces to identify the two gages. (d) Snap gage with adjustable anvils. FIGURE Section 35.4 ° Traditional Measuring Methods and Instruments GO features on these gages are identified by the type of knurling on the outside diameters of the rings. Snap gages (Fig. 35 .10d) commonly are used to measure external dimensions. They are made with adjustable gaging surfaces for use with parts that have different dimensions. One of the gaging surfaces can be set at a different gap from the other, thus making the device a one-unit GO-and-NOT-GO gage. The basic operation of an air gage (also called a pneumatic gage) is shown in Fig. 35.lla. The gage head (air plug) has two or more holes, typically 1.25 mm in diameter, through which pressurized air (supplied by a constant~pressure line) escapes. The smaller the gap between the gage and the hole, the more difficult it is for the air to escape, and hence, the higher is the back pressure. The back pressure, which is sensed Air Gages. i) i' i ’ Air supply A 1Z0-- }1 Controls Pressure gage Part Hole Gage head Air filter (H) Display Pneumatic lines Workpiece (crankshaft) Air fork (b) (C) (a) Schematic illustration of the principle of an air gage. (b) Illustration of an air-gage system used to measure the main bearing dimension on a crankshaft. (c) A conical head for air gaging; note the three small airholes on the conical surface. Source: (b) Courtesy of Mahr Federal, Inc. (c) Courtesy of Stotz Gaging Co. FIGURE 35.I I |007 |008 Chapter 35 Engineering Metrology and Instrumentation and indicated by a pressure gage, is calibrated to measure the dimensional variations of holes. The air gage can be rotated during use to indicate and measure any out-ofroundess of the hole. The outside diameters of parts (such as pins and shafts) also can be measured when the air plug is in the shape of a ring slipped over the part. In cases where a ring is not suitable, a fork-shaped gage head (with the airholes at the tips) can be used (Fig. 35.11b). Various shapes of air heads, such as the conical head shown in Fig. 35.11c, can be prepared for use in specialized applications on parts with different geometric features. Air gages are easy to use, and the resolution can be as fine as 0.125 um. If the surface roughness of the part is too high, the readings may be unreliable. The compressed-air supply must be clean and dry for proper operation. The part being measured does not have to be free of dust, metal particles, or similar contaminants, because the air will blow them away. The noncontacting nature and the low pressure of an air gage has the benefit of not distorting or damaging the measured part, as could be the case with mechanical gages-thus giving erroneous readings. 35.5 Modern Measuring Instruments and Machines wide variety of measuring instruments and gages has been developed. They range from simple, hand-operated devices to computer-controlled machines with very large workspaces. A Electronic Gages. Unlike mechanical systems, electronic gages sense the movement of the contacting pointer through changes in the electrical resistance of a strain gage, inductance, or capacitance. The electrical signals are then converted and displayed as linear dimensions with a digital readout. A handheld electronic gage for measuring bore diameters is shown in Fig. 35.12. When its handle is squeezed slightly, the tool can be inserted into the bore, and the bore diameter is read directly. A microprocessorassisted electronic gage for measuring vertical length is shown in Fig. 35.13. A commonly used electronic gage is the linear-variable differential transformer (LVDT), for measuring small displacements. Electronic caliper gages with diamondcoated edges are available. The chemical vapor deposition (CVD) coating on these gages has a wear resistance superior to that of steel or tungsten-carbide edges; it also resists corrosion. Although they are more expensive than other types of gages, electronic gages have advantages in ease of operation, rapid response, a digital readout, less possibility of human error, versatility, flexibility, and the capability to be integrated into automated systems through microprocessors and computers. 35.l2 An electronic gage for measuring bore diameters. The measuring head is equipped with three carbide-tipped steel pins for wear resistance. The LED display reads 29.158 mm. Source: Courtesy of TESA SA. FIGURE Laser Micrometers. In this instrument, a laser beam scans the workpiece (Fig. 35.14), typically at a rate of 350 times per second. Laser micrometers are capable of resolutions as high as 0.125 um. They are suitable not only for stationary parts, but also for in-line measurement of stationary, rotating, or vibrating parts, as well as parts in continuous, highspeed production. In addition, because there is no physical contact, they can measure parts that are at elevated temperatures or are too flexible to be measured by other means. The laser beams can be of various types (such as scanning or Section 35.5 Modern Measuring Instruments and Machines |009 rastoring for stationary parts), yielding point-cloud descriptions of part surfaces. Laser micrometers are of the shadow type or are charge-coupled device (CCD) based for in-line measurement while a part is in production. Laser micrometers are available with various capacities and features. They can be handheld for manual operation, or they can be mounted on and integrated with computer-controlled machines and statistical-process control units. Laser lnterferometry. This technique is used to check and calibrate machine tools for various geometric features during assembly. The method has better accuracies than those of gages or indicators. Laser interferometers are also used to automatically compensate for positioning errors in coordinatemeasuring machines and computer-numerical control machines. Photoelectric Digital Length Measurement. This type of measurement is done by an instrument that can measure the overall dimensions, thickness, and depth of a variety of parts. Resolution settings can range from 5 to 0.01 pm. 35.5.l Coordinate-measuring Machines FIGURE 35.l3 An electronic verticallength measuring instrument with a resolution of 1 /.tm(40 pin). Source: Courtesy of TESA SA. As schematically shown in Fig. 35 .15 a, a coordinate-measuring machine (CMM) consists basically of a platform on which the workpiece being measured is placed and is then moved linearly or rotated. A probe (Fig. 35 .15 b; see also Fig. 25.6) is attached to a head (capable of various movements) and records all measurements. In addition to the tactile probe shown, other types of probes are scanning, laser (Fig. 35.15c), and vision probes, all of which are nontactile. A CMM for inspection of a typical part is shown in Fig. 35.15d. Coordinate-measuring machines are very versatile and capable of recording measurements of complex profiles with high resolution (0.25 um) and high speed. They are built rigidly and ruggedly to resist environmental effects in manufacturing plants, such as temperature variations and vibration. They can be placed close to machine tools for efficient inspection and rapid feedback; that way, processing parameters are corrected before the next part is made. Although large CMMS can be expensive, most machines with a touch probe and computer-controlled three-dimensional movement are suitable for use in small shops and generally cost under $20,000. __.,1fff¢.¢~;;>2s, A x Laser beam iff.. ..aaaa if Laser beam it ....,. aeee Direct measurement of Fiunout of ..... yaasa shaft in it rotation diameter D (H) ; (D) (C) 35.l4 (a) and (b) Two types of measurements made with a laser scan micrometer. Two types of laser micrometers. Note that the instrument in the front scans the part (placed in the opening) in one dimension; the larger instrument scans the part in two dimensions. Source: Courtesy of BETA LaserMike. FIGURE (c) I0 0 I Chapter 35 Engineering Metrology and Instrumentation -1Z_aXiS --1 Spimne z-axis fine-feed knob x-axis fine-feed knob Probe adapter Clamp knobs for x, y, and zaxes |V|€aSU|'l“Qtable O t' Machine stand pera Iona pane y-axis fine-feed knob (al f (C) (D) (Ci) (a) Schematic illustration of a coordinate-measuring machine. (b) A touch signal probe. (c) Examples of laser probes. (d) A coordinate-measuring machine with a complex part being measured. Source: (b) through (d) Courtesy of Mitutoyo America Corp. FIGURE 35.l5 EXAMPLE 35.2 Coordinate-measuring Machine for Car Bodies large horizontal CNC coordinate-measuring machine used to measure all dimensions ofa car body is shown in Fig. 35.16. This machine has a measuring range of 6 X 1.6 X 2.4 m high and a resolution of 0.1 /rm. The system has temperature compensation within a range from 16° to 26°C to maintain A measurement accuracy. For efficient measurements, the machine has two heads with touch-trigger probes that are controlled simultaneously and have full threedimensional movements. The measuring speed is 5 mm/s. The probes are software controlled, and the machine is equipped with safety devices to prevent the Section 35.6 Automated Measurement and Inspection I0 Um FIGURE 35.I6 A large coordinate-measuring machine with two heads measuring various dimensions on a car body. probes from inadvertently hitting any part of the car body during their movements. The equipment shown around the base of the machine includes supporting hardware and software that controls all movements and records all measurements Source: Courtesy of Mitutoyo America Corporation 35.6 Automated Measurement and Inspection Automated measurement and inspection is based on various online sensor systems that monitor the dimensions of parts while they are being made and, if necessary, use these measurements as input to make corrections (Section 37.7). Manufacturing cells and flexible manufacturing systems (Chapter 39) have led to the adoption of advanced measuring techniques and systems. To appreciate the importance of online monitoring of dimensions, consider the following question: If a machine has been producing a certain part with acceptable dimensions, what factors contribute to the subsequent deviation in the dimensions of the same part produced by the same machine? There are several technical, as Well as human, factors involved: ° ° ° ° Static and dynamic deflections of the machine because of vibrations and fluctuating forces are caused by machine characteristics and variations in the properties and dimensions of the incoming material. Distortion of the machine because of thermal effects are caused by such factors as changes in the temperature of the environment, changes of metalworking fluids, and changes of machine bearings and various components. Wear of tools, dies, and molds can affect the dimensional accuracy of the parts produced. Human errors and miscalculations cause problems. As a result of these factors, the dimensions of parts tinuous monitoring during production necessary. Will vary, thus making con- I I0 2 I Chapter 35 Engineering Metrology and Instrumentation 35.1 General Characteristics and Selection of Measuring Instruments The characteristics and quality of measuring instruments are generally described by various specific terms, defined as follows (in alphabetical order): ° ° ° ° ° ° ° ° ° ° ° ° ° Accuracy: The degree of agreement of the measured dimension with its true magnitude. Amplification: The ratio of instrument output to the input dimension; also called rnagnihcation. Calibration: The adjustment or setting of an instrument to give readings that are accurate within a reference standard. Drift: An instrument’s capability to maintain its calibration over time; also called stability. Linearity: The accuracy of the readings of an instrument over its full working range. Magnification: The ratio of instrument output to the input dimension; also called amplification. Precision: Degree to which an instrument gives repeated measurement of the same standard. Repeat accuracy: The same as accuracy, but repeated many times. Resolution: Smallest dimension that can be read on an instrument. Rule of 10 (gage rna/eer’s rule): An instrument or gage should be 10 times more accurate than the dimensional tolerances of the part being measured. A factor of 4 is known as the rnil standard rule. Sensitivity: Smallest difference in dimension that an instrument can distinguish or detect. Speed of response: How rapidly an instrument indicates a measurement, particularly when a number of parts are measured in rapid succession. Stability: An instrument’s capability to maintain its calibration over time; also called drift. The selection of an appropriate measuring instrument for a particular application also depends on (a) the size and type of parts to be measured, (b) the environment (temperature, humidity, dust, and so on), (c) the operator skills required, and (d) the cost of equipment. 35.8 Geometric Dimensioning and Tolerancing Individually manufactured parts and components eventually are assembled into products. We take it for granted that when a thousand lawn mowers are manufactured and assembled, each part of the mower will mate properly with its intended components. For example, the wheels of the lawn mower will slip easily into their axles, or the pistons will fit properly into the cylinders, being neither too tight nor too loose. Likewise, when we have to replace a broken or worn bolt on an old machine, we purchase an identical bolt. We are confident from similar experiences in the past that the new bolt will fit properly in the machine. The reason we feel confident is that the bolt is manufactured according to certain standards and the dimensions of all similar bolts will vary by only a small, specified amount that do not affect their function. In other words, all bolts are manufactured within a certain range of dimensional tolerance; thus, all similar bolts are interchangeable. We also expect that the new bolt will function satisfactorily for a certain length of time, unless it is abused Section 35.8 Geometric Dimensionmg and Tolerancmg or misused. Bolts are periodically subjected to various tests during their production to make sure that their quality is within certain specifications. Dimensional Tolerance. Dimensional tolerance is defined as the permissible or acceptable variation in the dimensions (height, width, depth, diameter, and angles) of a part. The root of the word “tolerance” is the Latin tolerare, meaning “to endure” or “to put up with.” Tolerances are unavoidable, because it is virtually impossible and unnecessary to manufacture two parts that have precisely the same dimensions. Furthermore, because close dimensional tolerances can increase the product cost significantly, a narrow tolerance range is economically undesirable. However, for some parts, close tolerances are necessary for their proper functioning and are worth the added expense associated with narrow tolerance ranges. Examples are precision measuring instruments and gages, hydraulic pistons, and bearings for aircraft engines. Measuring dimensional tolerances and features of parts rapidly and reliably can parts on a Boeing 747-400 aircraft requires the measurement of about 25 features, representing a total of 150 million measurements. Surveys have shown that the dimensional tolerances on state-of-the-art manufactured parts are shrinking by a factor of 3 every 10 years and that this trend will continue. It is estimated that accuracies of (a) conventional turning and milling machines will rise from the present 7.5 to 1 /im, (b) diamond-wheel wafer-slicing machines for semiconductor fabrication to 0.25 /sum, (c) precision diamond turning machines to 0.01 um, and (d) ultraprecision ion-beam machines to less than 0.001 /J.m. (See also Fig. 25.16.) be a challenging task. For example, each of the 6 million <1> E as \. 2 IE' _ E 5 .Q U>- U # (D CD _ §§> _eg ¢’§ cu E '_ U FIGURE "' "*lZ>"I5 "°'»3Ol\->D-"‘ F0023 Dona/1~'O (°»-f-r:5f-rum!-' "Y HE at BED P-10 """"'O O°'°E"'3° QQ.. agaa~» ae~Di.= E 5 ge 'E "” 2 § -'U l L_ <\> 5 _Q V9 <5 an San ‘D O _ ,Qiii 'Scv -O g |-|o|e DL _I E? “XE 3 3 S E 5 -D _ _ is § o "V § " D gf ge I l gg E '_ _E E _ :Q 3 2 qi CU 55 35.I1 Basic size, deviation, and tolerance on Zero line, or line of Z9l‘O d€VlallOI'l V 2 a .Q 'o <1> E .Q (D <6 an shaft, according to the ISO system. l0l 3 Unilateral 40.00 + 0.05 Bilateral 40.05 + 0.00 7 ...,. Various methods of assigning tolerances on (b) unilateral tolerance, and (c) limit dimensions. FIGURE 35.18 1- (C) (D) (H) a L 40.05 Limit shaft: (a) bilateral tolerance, have a slightly different diameter. Machines with the same setup may produce rods of slightly different diameters, depending on a number of factors, such as speed of operat\§\“g/ xo °‘@ 10 tion, temperature, lubrication, and variations ot>@"' 6 MQ/ in the properties of the incoming material. If o@5“ 'o<\¢ tQ\“9/ g,aY‘ \ e,,`\t\>5\ 6_(§\e '\° we now specify a range of diameters for both the rod and the hole of the wheel, we can pre_ \t\Q “\o\¢ <5\j\Y\“ dict the type of fit correctly. \(\@\\ Z “\\ 5 _G Ga9\\<\<;9\0\\ 'tot Certain terminology has been established to clearly define these geometric quantities. One such system is the International W _ ‘“e(\\ oagwg‘“e\'5\\\i é=== r:>."°f'l “,_,5u :VNUI Allowance: The specified difference in dimensions between mating parts; also called functional dimension or sum dimension. Basic size: Dimension from which limits of size are derived with the use of tolerances and allowances. Bilateral tolerance: Deviation (plus or minus) from the basic size. Section 35.8 é/ 2_0 L o.5- E E \/ o X; <<\@ Q5 0`$C\’®Q, Q* <<\"> U) § // 0°\6&§S ‘\ T a 'Q §<©5 /a<@°\`* /<`° \\a‘\ 0.01 _ <<§ é\ & ~<\ \` \g99 0% Q9 0,69 v <\°` / Q,Q0 Q°<\0 \\§`<\ <§®b` 9\&é<{Z§? o°\ o 05 / Qe ®`I`\ 9/¢,<<\/ <\&e, aéée lo /1' /;;k°6 - ef? <~ §,%"" ¢%&‘° QQ/\ /Q CI) 8 g \e>/ / 1.0 - A Geometric Dimensiomng and Tolerancing ®" <<»<>° 9\I‘ \@/ .ar <> ®°@I\S“~\aI)` 0.0250.05 0.1 0.2 0.4 0.8 1.6 3.2 6.3125 25 N1 N2 N3 N4 N5 N6 N7 N8 N9 N10 N11 50 /dm N12ISO NO. Surface roughness (Ra) Dimensional tolerance range and surface roughness obtained in various manufacturing processes. These tolerances apply to a 25-mm workpiece dimension. FIGURE 35.20 Source: After ].A. Schey. Clearance: The space between mating parts. Clearance fit: Fit that allows for rotation or sliding between mating parts. Datum: A theoretically exact axis, point, line, or plane. Feature: A physically identifiable portion of a part, such as hole, slot, pin, or chamfer. Fit: The range of looseness or tightness that can result from the application of a specific combination of allowance and tolerance in the design of mating-part features. Geometric tolerances: Tolerances that involve shape features of the part. Hole-basis system: Tolerances based on a zero line on the hole; also called standard hole practice or hasic hole system. Interference: Negative clearance. Interference fit: A fit having limits of size so prescribed that an interference always results when mating parts are assembled. Intemational tolerance (IT) grade: A group of tolerances that vary with the basic size of the part, but provide the same relative level of accuracy within a grade. Limit dimensions: The maximum and minimum dimensions of a part; also called limits. Maximum material condition (MMC): The condition whereby a feature of a certain size contains the maximum amount of material within the stated limits of that size. Nominal size: An approximate dimension that is used for the purpose of general identification. Positional tolerancing: A system of specifying the true position, size, and form of the features of a part, including allowable variations. Shaft-basis system: Tolerances based on a zero line on the shaft; also called standard shaft practice or basic shaft system. Standard size: Nominal size in integers and common subdivisions of length. Transition fit: A fit with small clearance or interference that allows for accurate location of mating parts. I0|5 |016 Chapter 35 Engineering Metrology and Instrumentation ° ° Unilateral tolerancing: Deviation from the nominal dimension in one direction only. Zero line: Reference line along the basic size from which a range of tolerances and deviations are specified. Because the dimensions of holes are more difficult to control than those of shafts, the hole-basis system is commonly used for specifying tolerances in shaft and hole assemblies. The symbols used to indicate geometric characteristics are shown in Figs. 35.21a and b. °f feature TYP° Type °f tolerance Characteristic Flatness Individual (no datum FOrm reference) individual Frome °' related Orientation Related (datum reference Location "9°lU"'edl Symbol D Straightness - Circularity (roundness) O Cylindrlcity ,U Profile of a line f°\ Profile of a surface Cb Perpendicularity _l_ Angularity 4 Parallelism // Position G9 Concentricity 9 Circular runout / Flunout Total runout A/ la) ® Basic or exact dimension Projected tolerance zone o Diametrical (cylindrical) tolerance zone or feature ® Maximum material condition Ei Regardless of feature size 'Ill Datum feawfe SY"‘b°l ® Feature control frame Datum target symbol Least material condition (D) FIGURE 35.2l Geometric characteristic symbols to be indicated on engineering drawings of parts to be manufactured. Source: Courtesy of The American Society of Mechanical Engineers Bibliography l0l7 Limits and Fits. Limits and #ts are essential in specifying dimensions for holes and shafts. There are two standards on limits and fits, as described by the American National Standards Institute (see ANSI/ASME B4.1, B4.2, and B4.3). One standard is based on the traditional inch unit. The other is based on the metric unit and has been developed in greater detail. In these standards, capital letters always refer to the hole and lowercase letters to the shaft. SUMMARY ° ° ° ° ° In modern manufacturing technology, many parts are processed to a high degree of precision and thus require measuring instrumentation with several features and characteristics. Many devices are available for inspection-from simple gage blocks to electronic gages with high resolution. The selection of a particular measuring instrument depends on factors such as the type of measurement for which it will be used, the environment in which it will be used, and the accuracy of measurement required. Major advances have been made in automated measurement, linking measuring devices to microprocessors and computers for accurate in-process control of manufacturing operations. Reliable linking, monitoring, display, distribution, and manipulation of data are important factors, as are the significant costs involved in implementing them. Dimensional tolerances and their selection are important factors in manufacturing. Tolerances not only affect the accuracy and operation of all types of machinery and equipment, but also can influence product cost significantly. The smaller (tighter) the range of tolerances specified, the higher is the cost of production. Tolerances should be as broad as possible, but should also maintain the functional requirements of the product. KEY TERMS Air gage Analog instruments Autocollimator Bevel protractor Comparative lengthmeasuring instruments Coordinate-measuring machine Dial indicator Diffraction gratings Digital instruments Dimensional tolerance Electronic gages Fits Fixed gage Gage block Interferometry Laser micrometer Limits Line-graduated instruments Measurement standards Micrometer Optical contour projector Optical flat Plug gage Resolution Ring gage Sensitivity Snap gage Tolerance Total indicator reading Vernier caliper Pneumatic gage Precision BIBLIOGRAPHY Bentley, ].P., Principles of Measurement Systems, 4th ed., Prentice Hall, 2005. Bosch, ].A. (ed.), Coordinate Measuring Machines and Systems, Marcel Dekker, 1995. Cogorno, G., Geometric Dimensioning and Tolerancing for Mechanical Design, McGraw-Hill, 2006. Creveling, C.M., Tolerance Control: A Handbook for Developing Optimal Specifications, Addison-Wesley, 1996. Curtis, M.A., Handbook of Dimensional Measurement, 4th ed., Industrial Press, 2007. I0 8 I Chapter 35 Engineering Metrology and Instrumentation Drake, P.]., Dimensioning and Tolerancing Handbook, McGraw-Hill, 1999. Gooldy, G., Geometric Dimensioning and Tolerancing, rev. ed., Prentice Hall, 1995. Kimura, F., Computer-Aided Tolerancing, Springer, 1997. Krulikowski, A., Fundamentals of Geometric Dimensioning and Tolerancing, Delmar, 1997. Liggett, ].V., Dimensional Variation Management Handbook: A Guide for Quality, Design, and Manufacturing Engineers, Prentice Hall, 1993. Madsen, D.A., Geometric Dimensioning and Tolerancing, Goodheart-Wilcox, 2003. Meadows, ].D., Measurement of Geometric Tolerances in Manufacturing, Marcel Dekker, 1998. -, Morris, A.S., Measurement and Calibration for Quality Assurance, Wiley, 1998. Measurement and Instrumentation Principles, Butterworth-Heinemann, 2001. Puncochar, D.E., Interpretation of Geometric Dimensioning and Tolerancing, 2nd ed., Industrial Press, 1997. Whitehouse, D.]., Handbook of Surface Metrology, IOP Publishing, 1994. Wilson, B.A., Design Dimensioning and Tolerancing, Goodheart-Wilcox, 1996. Dimensioning and Tolerancing Handbook, Genium, -, 1998. Winchell, W, Inspection and Measurement in Manufacturing, Society of Manufacturing Engineers, 1996 _ REVIEW QUESTIONS Explain what is meant by standards for measurement. 35.2. What is the basic difference between direct-reading and indirect-reading linear measurements? Name the instruments used in each category. 35.3. What is meant by comparative length measurement? 35.4. Explain how flatness is measured. What is an optical flat? Describe the principle of an optical comparator. 35.5 35.6. Why have coordinate measuring machines become important instruments? 35.|. 35.7. What is the difference between gage? ring tolerance? 35.l I. How is straightness measured? ,_.._.__._..__._,__.____.._._ 35.|8. Why do manufacturing processes produce parts with wide range of tolerances? Explain, giving several examples. 35.l9. 35.I4. Explain the need for automated inspection. 35.I5. Dimensional tolerances for nonmetallic parts usually are wider than for metallic parts. Explain why. Would this also be true for ceramics parts? 35.l6. Comment on your observations regarding Fig. 35.20. Why does dimensional tolerance increase with increasing surface roughness? 35.17. Review Fig. 35.19, and comment on the range of tolerances and part dimensions produced by various manufacturing processes. 35.20. a a What are dimensional tolerances? Why is their control important? 35.9. Explain the difference between tolerance and allowance. 35.I0. What is the difference between bilateral and unilateral Why are the words “accuracy” and “precision” often incorrectly interchanged? 35.I3. plug gage and 35.8. °UAUTAT'VE PROBLEMS 35.|2. a ___,__.__._.__.____._______._____ In the game of darts, is it better to be accurate or to be precise? Explain. What are the advantages and limitations of GO and NOT GO gages? Comment on your observations regarding Fig. 35.18. 35.2I. Why is it important to Control temperature during the measurement of dimensions? Explain, with examples. 35.22. Describe the characteristics of electronic gages. 35.23. What method would you use to measure the thickness of a foam-rubber part? Explain. QUANTITATIVE PROBLEMS |]35.24. Assume that a steel rule expands by 0.07% due to an increase in environmental temperature. What will be the indicated diameter of a shaft with a diameter of 30.00 mm at room temperature? |]35.25. If the same steel rule as in Problem 35.24 is used to measure aluminum extrusions, what will be the indicated diameter at room temperature? What if the part were made of a thermoplastic? |l35.26. A shaft must meet a design requirement of being at least 28.0 mm in diameter, but it can be 0.380 mm oversized. Express the shaft’s tolerance as it would appear on an engineering drawing. Synthesis, Design, and Projects IOI9 SYNTHESIS, DESIGN, AND PROIECTS 35.27. Describe your thoughts on the merits and limitations of digital measuring equipment over analog instruments. Give specific examples. 35.28. Take an ordinary vernier micrometer (see Fig. 35.2a) and a simple round rod. Ask five of your classmates to measure the diameter of the rod with this micrometer. Comment on your observations. 35.29. Cbtain a digital micrometer and a steel ball of, say, 6-mm diameter. Measure the diameter of the ball when it (a) has been placed in a freezer, (b) has been put into boiling Water, and (c) when it has been held in your hand for different lengths of time. Note the variations, if any, of measured dimensions, and comment on them. 35.30. Repeat Problem 35.29, but with the following parts: (a) the plastic lid of a small jar, (b) a thermoset part such as the knob or handle from the lid of a saucepan, (c) a small juice glass, and (d) an ordinary rubber eraser. 35.3|. What is the significance of the tests described in Problems 35.29 and 35.3O? 35.32. Explain the relative advantages and limitations of tactile probe versus a laser probe. a 35.33. Make simple sketches of some forming- and cuttingmachine tools (as described in Parts III and IV of the book) and integrate them with the various types of measuring equipment described in this chapter. Comment on the possible difficulties involved in doing so. 35.34. Ins P ect various P arts and comP onents in consumer products, and comment on how tiht dimensional tolerances have to be in order for these products to function properly. 35.35. As you know, very thin sheet-metal parts can distort differently when held from various locations and edges of the part, just as a thin paper plate or aluminum foil does. How, then, could you use a coordinate-measuring machine for “accurate” measurements? Explain. 35.36. Explain how you would jusify the considerable cost of a coordinate-measuring machine such as that shovvn in Fig. 35.16.