Manufacturing 35

A typical roughness profile includes the peaks and valleys that are considered separately from waviness. Flaws also add to texture but should be measured independent of it. Changes in the surface layer, as a result of processing, include plastic deformation, residual stresses, cracks, and other metallurgical changes (hardness, overaging, phase changes, recrystallization,intergranular attack,and hydrogen embrittlement).See Figure 30-2.The surface layer will always contain local surface deformation due to any machining passes. The material removal processes generate a wide variety of surfaces textures, generally referred to as surface finish.The cutting processes leave a wide variety of surface patterns on the materials.Lay is the term used to designate the direction of the predominant surface pattern produced by the machining process. In addition, certain other terms and symbols have been developed and standardized for specifying the surface quality.The most important terms are surface roughness, waviness, and lay (Figure 35-3). Roughness refers to the finely spaced surface irregularities. It results from machining operations in the case of machined surfaces. Waviness is surface irregularity of greater spacing than in roughness. It may be the result of warping, vibration, or the work being deflected during machining

A variety of instruments are available for measuring surface roughness and surface profiles. The majority of these devices use a diamond stylus that is moved at a constant rate across the surface, perpendicular to the lay pattern.The rise and fall of the stylus is detected electronically [often by a Linear Variable Differential Transformer Device (LVTD)], is amplified and recorded on a strip-chart, or is processed electronically to produce average or root-mean-square readings for a meter (Figure 35-4). The unit containing the stylus and the driving motor may be handheld or supported by skids that ride on the workpiece or some other supporting surface. Roughness is measured by the height of the irregularities with respect to an average line. These measurements are usually expressed in micrometers or microinches

ELECTROPOLISHING Electropolishing is the reverse of electroplating (discussed later in this chapter) since material is removed from the surface rather than being deposited.A DC electrolytic circuit is constructed with the workpiece as the anode.As current is applied, material is stripped from the surface, with material removal occurring preferentially from any raised location. Unfortunately, it is not economical to remove more than about 0.001 in. of material from any surface. However, if the initial surface is sufficiently smooth (less than 8 in. rms), and the grain size is small, the result will be a smooth polish with irregularities of less than 2 µin.—a mirrorlike finish. Electropolishing was originally used to prepare metallurgical specimens for examination under the microscope. It was later adopted as a means of polishing stainless steel sheets and other stainless products. It is particularly useful for polishing irregular shapes that would be difficult to buff. ■ 35.3 CHEMICAL CLEANING Chemical cleaning operations are effective means of removing oil, dirt, scale, or other foreign material that may adhere to the surface of a product, as a preparation for subsequent painting or plating. Because of environmental, health, and safety concerns, however, many processes that were once the industrial standard have now been eliminated or substantially modified.While the major concern with the mechanical methods has usually been airborne particles, the chemical methods often require the disposal of spent or contaminated solutions, and they occasionally use hazardous, toxic, or environmentally unfriendly materials. Chlorofluorocarbons (CFCs) and carbon tetrachloride, for example, have been identified as ozone-depleting chemicals and have been phased out of commercial use. Process changes to comply with added regulations can significantly shift process economics. Manufacturers must now ask themselves if a part really has to be cleaned, what soils have to be removed, how clean the surfaces have to be, and how much they are willing to pay to accomplish that goal. Selection of the cleaning method will depend on cost of the equipment, power, cleaning materials, maintenance and labor, plus the cost of recycling and disposal of materials. Specific processes will depend on the quantity of parts to be processed (part per hour), part configuration, part material, desired surface finish, temperature of the process, and flexibility. Manufacturers want machines they can integrate with manufacturing cells so changes in products can be quickly handled. ALKALINE CLEANING Alkaline cleaning is basically the "soap and water" approach to parts cleaning and is a commonly used method for removing a wide variety of soils (including oils, grease, wax, fine particles of metal, and dirt) from the surfaces of metals.The cleaners are usually complex solutions of alkaline salts, additives to enhance cleaning or surface modification, and surfactants or soaps that are selected to reduce surface tension and displace, emulsify, and disperse the insoluble soils.The actual cleaning occurs as a result of one or more of the following mechanisms: (1) saponification, the chemical reaction of fats and other organic compounds with the alkaline salts; (2) displacement, where soil particles are lifted from the surface; (3) dispersion or emulsification of insoluble liquids; and (4) dissolution of metal oxides. Alkaline cleaners can be applied by immersion or spraying, and they are usually heated to accelerate the cleaning action. The cleaning is then followed by a water rinse to remove all residue of the cleaning solution, as well as flush away some small amounts of remaining soil. A drying operation may also be required since the aqueous cleaners do not evaporate quickly, and some form of corrosion inhibitor (or rust preventer) may be required, depending on subsequent use. Environmental issues relating to alkaline cleaning include (1) reducing or eliminating phosphate effluent, (2) reducing toxicity and increasing biodegradability, and (3) recycling the cleaners to extend their life and reduce the volume of discard.

INFLUENCE OF SURFACE FINISH ON FATIGUE Fatigue failure occurs as the result of repeated loading at some point typically below the yield strength of the material. Fatigue failures have been shown to almost always nucleate on or near the surface of a component. Fine surface cracks begin at discontinuities (such as microcracks, grooves, ridges, cavities, machining marks, imbedded particles, etc.) at the surface, and the cracks propagate with repeated cyclic loads. Tensile residual stresses in the altered surface layer have an additive effect on the applied stresses in the component. This means that tensile residual stresses in the material add to external stresses to the component, reducing its fatigue strength. Alternatively, as shown in Figure 35-23, compressive residual stresses subtract from tensile external stresses, and since tensile stresses are those ultimately responsible for fatigue failure, the fatigue strength of the material is increased. Figure 35-23 shows how residual stresses couple with applied stresses to affect product performance. Suppose that a round beam has a load applied to it so that it is bent while rotating. At the top of the rotation, the surface is in tension, and at the bottom, it is in compression.The result is a condition of cyclic fatigue and the likelihood of a service life limited by fatigue failure. If the part is roller burnished or shot peened, the compressive residual stress pattern of the middle figure is added to the applied stresses, producing the net pattern shown at the bottom.The net effect is a lowering of the peak tensile stress experienced by the surface and a related extension in fatigue life.The specific results will depend on the details of the process. For shot peening, the key variables include shot size, shot velocity, exposure time, distance between the nozzle and the surface, and the angle of impact. Figure 35-24 presents the results of a study in which specimens were prepared by milling and turning and then either polished, shot peened, or roller burnished. If an applied stress between 41,000 and 42,000 psi is experienced in a fatigue application, the difference in fatigue life between a milled specimen and one that has been milled and roller burnished is 610,000 cycles (90,000 cycles as opposed to 700,000 cycles). In essence, roller burnishing serves to induce a sevenfold extension to the fatigue life of the product. Similar results have been observed in the resistance to stress-corrosion cracking. As the data above show, both the designer and the manufacturer need to be aware of the effects that manufacturing processes can have on the performance of a product. Maintaining the proper sequence of operations may be as important to the surface properties as the selection of the processes and control of the operating parameters.

In a process variation known as color anodizing, a sulfuric acid bath is used to produce a layer of microscopically porous oxide that is transparent on pure aluminum and somewhat opaque on alloys.When this material is immersed in a dye solution, capillary action pulls the dye into the pores. The dye is then trapped in place by a sealing operation, usually performed simply by immersing the anodized metal in a bath of hot water.The aluminum oxide coating is converted to a monohydrate, with accompanying increase in volume.The pores close and become resistant to further staining or the leaching out of the dye. While most people are familiar with the variety of colors in aluminum athletic goods, such as softball hats, the actual applications range from giftware, through automotive trim, to architectural use. Aluminum can be made to look like gold, copper, or brass, or it can take on a variety of colors with a combined metallic luster that cannot be duplicated by other methods. If PTFE (Teflon) is introduced into the pores, coatings can be produced that couple high hardness and low friction. The porous oxide layer can also be used to enhance the adhesion of an additional layer of material, such as paint, or carry lubricant during a subsequent forming operation. Since the coating is integral to the part, subsequent operations can often be performed without destroying its integrity or reducing its protective qualities. Anodizing can also be performed on other metals, such as magnesium, and the process is similar to the passivation of stainless steel. ELECTROLESS PLATING When using electroplating, it is almost impossible to obtain a uniform plating thickness on even moderately complex shapes, the platings cannot be applied to nonconductors, and a large amount of energy is required. For these reasons, a substantial effort has been directed toward the development of plating techniques that do not require an external source of electricity. These methods are known as electroless, or autocatalytic, plating. Considerable success has been achieved with nickel, but copper and cobalt, as well as some of the precious metals, can also be deposited. In the electroless process, complex plating solutions (containing metal salts, reducing agents, complexing agents, pH adjusters, and stabilizers) are brought into contact with a substrate surface that acts as a catalyst or has been pretreated with catalytic madega-c35_933-968-hr 1/9/07 4:59 PM Page 956 SECTION 35.4 Coatings 957 Base metal Coating 0.02 cm 0.008 in. Base metal Coating FIGURE 35-17 (Left) Photomicrograph of nickel carbide plating produced by electroless deposition. Notice the uniform thickness coating on the irregularly shaped product. (Right) High-magnification cross section through the coating. (Courtesy of Electro-Coatings Inc.) terial.The metallic ion in the plating solution is reduced to metal and deposits on the surface. Since the deposition is purely a chemical process, the coatings are uniform in thickness, independent of part geometry. Unfortunately, the rate of deposition is considerably slower than with electroplating. Probably the most popular of the electroless coatings is electroless nickel, and various methods exist for its deposition using both acid and alkaline solutions.The coatings offer good corrosion resistance, as well as hardnesses between Rockwell C 49 and 55. In addition, the hardness can be increased further to as high as Rockwell C 80 by subsequent heat treatment. ELECTROLESS COMPOSITE PLATING A very useful adaptation of the electroless process has been developed wherein minute particles are co-deposited along with the electroless metal to produce composite-material coatings. Finely divided solid particles, with diameters between 1 and 10 in., are added to the plating bath and deposit up to 50 vol% with the matrix.While it may appear that a large variety of materials could be co-deposited, commercial applications have largely been limited to diamond, silicon carbide, aluminum oxide, and Teflon (PTFE). Figure 35-17 shows a deposit of silicon carbide particles in a nickel-alloy matrix, where the particles constitute about 25% by volume. The coating offers the same corrosion resistance as nickel, but the high hardness of the silicon carbide particles (about 4500 on the Vickers scale, where tungsten carbide is 1300 and hardened steel is about 900) contributes outstanding resistance to wear and abrasion. Since the deposition is electroless, the thickness of the coating is not affected by the shape of the part. Applications include the coating of plastic-molding dies, for use where the polymer resin contains significant amounts of abrasive filler. MECHANICAL PLATING Mechanical plating, also known as peen plating or impact plating, is an adaptation of barrel finishing in which coatings are produced by cold-welding soft, malleable metal powder onto the substrate. Numerous small products are first cleaned and may be

Processes such as roller burnishing (described in Chapter 18) produce a smooth surface with compressive residual stresses.Shot peening (and tumbling) can increase the hardness in the surface and introduce a residual compressive stress, as shown in Figure 35-21a and 35-21b. Welding processes produce tensile residual stresses as the deposited material shrinks upon cooling. Similar shrinkage occurs in castings, but the resulting stresses may be complex due to the variation of shrinkage or the lack of restraint.Tensile stresses on the surface can often be offset by a subsequent exposure to shot peening or tumbling. In summary, the surface and subsurface regions of a material can be significantly altered due to (1) plastic strain or plastic deformation, (2) high temperatures, (3) differential expansions or contractions due to temperature changes or variations, and (4) chemical reactions. To illustrate the complex nature of surface effects, consider Figure 35-22, which shows the depth of "surface damage" due to machining as a function of the rake angle of the tool.To increase the cutting speed (and thereby increase the rate of production), an engineer might change from a high-speed tool steel cutter with a large rake angle (such as 30°) to a carbide tool with a zero rake. While the resulting surface finish may be similar, the depth of "surface damage" is doubled. Failures may occur in service, whereas previous parts had performed quite admirably.

Since the solvent is water, no fire hazard exists (as with the use of many solvents), and air and water pollution is reduced significantly. In addition, the process can be readily adapted to conveyor line production. DRYING Most paints and enamels used in manufacturing require from 2 to 24 hours to dry at normal room temperature.This time can be reduced to between 10 minutes and 1 hour if the temperature can be raised to between 275° and 450°F. As a result, elevated-temperature drying is often preferred. Parts can be batch processed in ovens or continuously passed through heated tunnels or under panels of infrared heat lamps. Elevated-temperature drying is rarely a problem with metal parts, but other materials can be damaged by exposure to the moderate temperatures. For example, when wood is heated, the gases, moisture, and residual sap are expanded and driven to the surface beneath the hardening paint. Small bubbles tend to form that roughen the surface, or break, producing small holes in the paint. POWDER COATING Powder coating is yet another variation of electrostatic spraying, but here the particles are solid rather than liquid. Several coats, such as primer and finish, can be applied and then followed by a single baking, in contrast to the baking after each coat that is required in the conventional spray processes. In addition, the overspray powder can often be collected and reused. While volatilized solvents are no longer a concern, operators must now address the possibility of powder explosion, as well as the health hazards of airborne particles. Modern powder technology can produce a high-quality finish with superior surface properties and usually at a lower cost than liquid painting. Powder painting is more efficient in the use of materials (the overspray can be captured and reused) and lower energy requirements.The economic advantages must be weighed against the limitations of powder coating. Dry systems have a longer color change time than wet systems. The process is not good for large objects (massive tanks) or heat-sensitive objects. It is not easy to produce film thickness less than 1 mil (0.03 mm). Table 35-4 provides details on powders that are used in powder coatings. Thermoplastics can also be used, but thermosetting powders are most common.The elements

The most basic way to detect a burr is to run your finger or fingernail over the edges of the part. Probes and visual inspection techniques (microscopes) are used to find burrs as well. A number of different processes have been used for burr removal, including some discussed previously in this chapter and others presented as special types of machining. These include grinding, chamfering, barrel tumbling, vibratory finishing, centrifugal and spindle finishing, abrasive jet machining, water jet cutting, wire brushing, belt sanding, chemical machining, electropolishing, buffing, electrochemical machining, filing, ultrasonic machining, and abrasive flow machining (see Chapter 26). Other burr removal methods may be quite specialized, such as thermal-energy deburring. Here the parts are loaded into a chamber, which is then filled with a combustible gas mixture.When the gas is ignited, the short-duration wavefront heats the small burrs to as much as 6000°F, while the remainder of the workpiece rarely exceeds 300°F. The burrs are vaporized in less than 20 ms, including those in inaccessible or difficult-to

are lowered into successive plating, washing, and fixing tanks. Ordinarily, only one type of workpiece is plated at a time, because the details of solutions, immersion times, and current densities are usually changed with changes in workpiece size and shape. In the electroforming process, the coating becomes the final product. Metal is electroplated onto a mandrel (or mold) to a desired thickness and is then stripped free to produce small quantities of molds or other intricate-shaped sheet-metal type products. ANODIZING Anodizing is an electrochemical process, that is somewhat the reverse of electroplating, which produces a conversion-type coating on aluminum that can improve corrosion and wear resistance and impart a variety of decorative effects. If the workpiece is made the anode of an electrolytic cell, instead of a plating layer being deposited on the surface, a reaction progresses inward, increasing the thickness of the hard hexagonal aluminum oxide crystals on the surface. The hardness depends on thickness, density, and porosity of the coating, which are controlled by the cycle time and applied currents along with the chemistry, concentration, and temperature of the electrolyte. The surface texture very nearly duplicates the prefinishing texture, so a buffing prefinish produces a smooth, lustrous coating while sand blasting produces a grainy or satiny coating. The flow diagram in Figure 35-16 shows the anodizing process. Coating thicknesses range from 0.1 mils to 0.25 mils. Note that the product dimensions will increase, however, because the aluminum oxide coating occupies about twice the volume of the metal from which it formed. The nature of the developed coating is controlled by the electrolyte. If the oxide coating is not soluble in the anodizing solution, it will grow until the resistance of the oxide prevents current from flowing. The resultant coating, which is thin, nonporous, and nonconducting, is used in a variety of electrical applications. If the oxide coating is slightly soluble in the anodizing solution, dissolution competes with oxide growth and a porous coating will be produced, where the pores provide for continued current flow to the metal surface.As the coating thickens, the growth rate decreases until it achieves steady state, where the growth rate is equal to the rate of dissolution. This condition is determined by the specific conditions of the process, including voltage, current density, electrolyte concentration, and electrolyte temperature. Sulfuric, chromic, oxalic, and phosphoric acids all produce electrolytes that dissolve oxide, with a sulfuric acid solution being the most common.

ering Angle-cut cylinder Angle-cut triangle Diamond Star Arrowhead Sphere Cone Pyramid Angle-cut prism Angle-cut star Shapes of media used for finishing Ball Ballcone Cone Diagonal Ovalball Pin Steel media shapes used for burnishing FIGURE 35-9 Synthetic abrasive media are available in a wide variety of sizes and shapes. Through proper selection, the media can be tailored to the product being cleaned. Increasing the speed of rotation adds centrifugal forces that cause the material to rise higher in the barrel. The enhanced action can often accelerate the process, provided that the speed is not so great as to destroy the cascading action and that the additional action does not damage the workpiece. By a suitable selection of abrasives, filler, barrel size, ratio of workpieces to abrasive, fill level, and speed, a wide range of parts can be tumbled successfully. Delicate parts may have to be attached to racks within the barrel to reduce their movement while permitting the media to flow around them. Natural and synthetic abrasives are available in a wide range of sizes and shapes, including those depicted in Figure 35-9, that enable the finishing of complex parts with irregular openings.The various media are often mixed in a given load, so that some will reach into all sections and corners to be cleaned. Tumbling is usually done dry, but it can also be performed with an aqueous solution in the barrel. Chemical compounds can be added to the media to assist in cleaning, or descaling, or to provide features such as rust inhibition. Support equipment usually assists with loading and unloading the barrels as well as, with the separation of the workpieces from the abrasive media. The latter operation often uses mesh screens with selected size openings. Barrel tumbling can be a very inexpensive way to finish large quantities of small parts and produce rounded edges and corners. Unfortunately, the abrasive action occurs on all surfaces and cannot be limited to selected areas.The cycle time is often long, and the process can be quite noisy. In the barrel burnishing process, no cutting action is desired. Instead, the parts are tumbled against themselves or with media such as steel balls, shot, rounded-end pins, or ballcones. If the original material is free of visible scratches and pits, the combination of peening and rubbing will reduce minute irregularities and produce a smooth, uniform surface. Barrel burnishing is normally done wet, using a solution of water and lubricating or cleaning agents, such as soap or cream of tartar. Because the rubbing action between the work and the media is very important, the barrel should not be loaded more than half full, and the volume ratio of media to work should be about 2:1 so the workpieces rub against the media, not each other. The speed of rotation should be set to maintain the cascading action and not fling the workpieces free of the tumbling mass. Centrifugal barrel tumbling places the tumbling barrel at the end of a rotating arm. This adds centrifugal force to the weight of the parts in the barrel and can accelerate the process by as much as 25 to 50 time

given a thin galvanic coating of either copper or tin. They are then placed in a tumbling barrel, along with a water slurry of the metal powder to be plated, glass or ceramic tumbling media, and chemical promoters or accelerators. The media particles peen the metal powder onto the surface, producing uniform-thickness deposits (possibly a bit thinner on edges and thicker in recesses—the opposite of electroplating!). Any metal that can be made into fine powder can be deposited, but the best results are obtained for soft materials, such as cadmium, tin, and zinc. Since the material is deposited mechanically, the coatings can be layered or involve mixtures with bulk chemistries that would be chemically impossible due to solubility limits. The fact that the coatings are deposited at room temperature, and in an environment that does not induce hydrogen embrittlement, makes mechanical plating an attractive means of coating hardened steels. PORCELAIN ENAMELING Metals can also be coated with a variety of glassy, inorganic materials that impart resistance to corrosion and abrasion, decorative color, electrical insulation, or the ability to function in high-temperature environments. Multiple coats may be used, with the first or ground coat being selected to provide adhesion to the substrate and the cover coat to provide the surface characteristics.The material is usually applied in the form of a multicomponent suspension or slurry (by dipping or spraying), which is then dried and fired. An alternative dry process uses electrostatic spraying of powder and subsequent firing. During the firing operation, which may require temperatures in the range of 800° to 8000°F, the coating materials melt, flow, and resolidify. Porcelain enamel is often found on the inner, perforated tubs of many washing machines and may be used to impart the decorative exterior on cookpots and frying pans. ■ 35.5 VAPORIZED METAL COATINGS Vapor deposition processes can be classified into two main categories: physical vapor deposition (PVD) and chemical vapor deposition (CVD).While sometimes used as though it were a specific process, the term PVD applies to a group of processes in which the material to be deposited is carried physically to the surface of the workpiece. Vacuum metallizing and sputtering are key PVD processes, as are complex variations, such as ion plating.All are carried out in some form of vacuum, and most are line-of-sight processes in which the target surfaces must be positioned relative to the source. In contrast, the CVD processes deposit material through chemical reactions and generally require significantly higher temperatures.Tool steels treated by CVD may have to be heat treated again,while most PVD processes can be conducted below normal tempering temperatures. See Chapter 21 for additional discussions on PVD and CVD processes. ■ 35.6 CLAD MATERIALS Clad materials are actually a form of composite in which the components are joined as solids, using techniques such as roll bonding, explosive welding, and extrusion.The most common form is a laminate, where the surface layer provides properties such as corrosion resistance, wear resistance, electrical conductivity, thermal conductivity, or improved appearance, while the substrate layer provides strength or reduces overall cost. Alclad aluminum is a typical example. Here surface layers of weaker but more corrosionresistant single-phase aluminum alloys are applied to a base of high-strength but less corrosion-resistant, age-hardenable material. Aluminum-clad steel meets the same objective but with a heavier substrate, and stainless steel can be used to clad steels, reducing the need for nickel- and chromium-alloy additions throughout. Wires and rods can also be made as claddings. Here the surface layer often imparts conductivity, while the core provides strength or rigidity. Copper-clad steel rods that can be driven into the ground to provide electrical grounding for lightning rod systems are one example

molten zinc. Nickel plating provides good corrosion resistance but is rather expensive and does not retain its lustrous appearance. Consequently, when lustrous appearance is desired, a chromium plate is usually specified. Chromium is seldom used alone, however. An initial layer of copper produces a leveling effect and makes it possible to reduce the thickness of the nickel layer that typically follows to less than 0.0006 in. The final layer of chromium then provides the attractive appearance. Gold, silver, and platinum platings are used in both the jewelry and electronics industries, where the thin layers impart the desired properties while conserving the precious metals. Hard chromium plate, with Rockwell hardnesses between 66 and 70, can be used to build up worn parts to larger dimensions and to coat tools and other products that need reduced surface friction and good resistance to both wear and corrosion. Hard chrome coatings are always applied directly to the base material and are usually much thicker than the decorative treatments, typically ranging from .003 to .010 in. thick. Even thicker layers are used in applications such as diesel cylinder liners. Since hard chrome plate does not have a leveling effect, defects or roughness in the base surface will be amplified. If smooth surfaces are desired, subsequent grinding and polishing may be necessary. Figure 35-14 depicts the typical electroplating process. A DC voltage is applied between the parts to be plated (which is made the cathode) and an anode material that is either the metal to be plated or an inert electrode. Both of these components are immersed in a conductive electrolyte, which may also contain dissolved salts of the metal to be plated as well as additions to increase or control conductivity. In response to the applied voltage, metal ions migrate to the cathode, lose their charge, and deposit on the surface. While the process is simple in its basic concept, the production of a high-quality plating requires selection and control of a number of variables, including the electrolyte and the concentrations of the various dissolved components, the temperature of the bath, and the electrical voltage and current.The interrelation of these features adds to the complexity and makes process control an extremely challenging problem. The surfaces to be plated must also be prepared properly if satisfactory results are to he obtained. Pinholes, scratches, and other surface defects must be removed if a smooth, lustrous finish is desired. Combinations of degreasing, cleaning, and pickling are used to ensure a chemically clean surface, one to which the plating material can adhere. As shown in Figure 35-15, the plated metal tends to be preferentially attracted to corners and protrusions. This makes it particularly difficult to apply a uniform plating to irregular shapes, especially ones containing recesses, corners, and edges. Design features can be incorporated to promote plating uniformity, and improved results can often be obtained through the use of multiple spaced anodes or anodes whose shape resembles that of the workpiece. Electroplating is frequently performed as a continuous process, where the individual parts to be plated are hung from conveyors.As they pass through the process, they

nontraditional methods. Such methods can leave stress-free surfaces, remelted layers, and excessive surface roughness.Finishing treatments can be used to negate or remove the impact of both traditional and nontraditional processes as well as providing good surface finish. For example, residual tensile stresses can be removed by shot peening or roller burnishing. Chemical milling can remove the recast layer left by EDM. The objectives of the surface-modification processes can be quite varied. Some are designed to clean surfaces and remove the kinds of defects that occur during processing or handling (such as scratches, pores, burrs, fins, and blemishes). Others further improve or modify the products' appearance, providing features such as smoothness, texture, or color. Numerous techniques are available to improve resistance to wear or corrosion, or to reduce friction or adhesion to other materials. Scarce or costly materials can be conserved by making the interior of a product from a cheaper, more common material and then coating or plating the product surface. As with all other processes, surface treatment requires time, labor, equipment, and material handling, and all of these have an associated cost. Efficiencies can be realized through process optimization and the integration of surface treatment into the entire manufacturing system. Design modifications can often facilitate automated or bulk finishing, eliminating the need for labor-intensive or single-part operations. Process selection should further consider the size of the part, the shape of the part, the quantity to be processed, the temperatures required for processing, the temperatures encountered during subsequent use, and any dimensional changes that might occur due to the surface treatment. Through knowledge of the available processes and their relative advantages and limitations, finishing costs can often be reduced or eliminated while maintaining or improving the quality of the product. In addition to the above, the field of surface finishing has recently undergone another significant change. Many chemicals that were once "standard" to the field, such as cyanide, cadmium, chromium, and chlorinated solvents, have now come under strict government regulation. Wastewater treatment and waste disposal have also become significant concerns. As a result, processes may have to be modified or replacement processes may have to be used. Because of their similarity to other processes, many surface finishing techniques have been presented elsewhere in the book. The case hardening techniques, both selective heating (flame, induction, and laser hardening) and altered surface chemistry (diffusion methods such as carburizing, nitriding, and carbonitriding), are presented in Chapter 5 as variations of heat treating. Shot peening and roller burnishing are presented in Chapter 17 as cold-working processes. Roll bonding and explosive bonding are discussed in Chapter 15 as means of producing laminar composites. Hard facing and metal spraying are included in Chapter 33 as adaptations of welding techniques. Chemical vapor deposition and physical vapor deposition are discussed in Chapters 19 and 21. Sputtering and ion implantation are also discussed in Chapter 19 as processes needed in electronics manufacturing. In this chapter we focus on techniques for cleaning and surface preparation as well as the remaining methods of surface finishing or surface modification. ■ 35.2 MECHANICAL CLEANING AND FINISHING BLAST CLEANING It is not uncommon for the various manufacturing processes to produce certain types of surface contamination. Sand from the molds and cores used in casting often adheres to product surfaces. Scale (metal oxide) can be produced whenever metal is processed at elevated temperatures. Oxides such as rust can form if material is stored between operations. These and other contaminants must be removed before decorative or protective surfaces can be produced. While vibratory shaking can be useful, some form of blast cleaning is usually required to remove the foreign material. Blast cleaning uses a media (abrasive) propelled into the surface using air, water, or even a wheel (wheel blasting uses a high-rpm blocked wheel to deliver the media). The bulk of the work is done by kinetic energy of the impacting media; KE 1 /2 MV 2 where m mass of the median and v the velocity. Abrasives, steel grit, metal shot, fine glass shot, plastic

of a powder coating system are shown in Figure 35-13. The following aspects of the process must be considered: • Types of guns—corona charged or tribo charged • Number of guns—depends on many factors, such as parts per hour, size of parts, line speed, and powder types • Color change time/frequency • Safety • Curing oven—coated parts put in ovens to melt, flow, and cure the powder HOT-DIP COATINGS Large quantities of metal products are given corrosion-resistant coatings by direct immersion into a bath of molten metal. The most common coating materials are zinc, tin, aluminum, and tene (an alloy of lead and tin). Hot-dip galvanizing is the most widely used method of imparting corrosion resistance to steel.(The zinc acts as a sacrificial anode,protecting the underlying iron.) After the products,or sheets,have been cleaned to remove oil,grease,scale,and rust,they are fluxed by dipping into a solution of zinc ammonium chloride and dried. Next, the article is completely immersed in a bath of molten zinc.The zinc and iron react metallurgically to produce a coating that consists of a series of zinc-iron compounds and a surface layer of nearly pure zinc. The coating thickness is usually specified in terms of weight per unit area. Values between 0.5 and 3.0 oz/ft2 are typical, with the specific value depending on the time of immersion and speed of withdrawal. Thinner layers can be produced by incorporating some form of air jet or mechanical wiping as the product is withdrawn. Since the corrosion resistance is provided through the sacrificial action of the zinc, the thin layers do not provide long-lasting protection. Extremely heavy coatings, on the other hand, may tend to crack and peel. The appearance of the coating can be varied through both the process conditions and alloy additions of tin, antimony, lead, and aluminum. When the dega-c35_933-968-hr 1/9/07 4:59 PM Page 952 SECTION 35.4 Coatings 953 coatings are properly applied, bending or forming can often follow galvanizing without damage to the integrity of the coating. Zinc-galvanized sheet can be heat treated with a zinc-iron alloy coating. The 10% iron content adds strength and makes for good corrosion and pitting/chipping resistance. In auto applications, galvannealing beats out pure zinc on several counts: spot weldability, pretreatability, and ease of painting. Electrogalvanized zinc-nickel coatings that contain 10 to 15% Ni can be used in thinner layers (5-6 microns) and are easier to form and spot weld. The primary limitations to hot-dip galvanizing are the size of the product (which is limited to the size of the tank holding the molten zinc) and the"damage"that might occur when a metal is exposed to the temperatures of the molten material (approximately). Tin coatings can also be applied by immersing in a bath of molten tin with a covering of flux material. Because of the high cost of tin and the relatively thick coatings applied by hot dipping, most tin coatings are now applied by electroplating. Terne coating utilizes an alloy of 15 to 20% tin and the remainder lead.This material is cheaper than tin and can provide satisfactory corrosion resistance for many applications. CHEMICAL CONVERSION COATINGS In chemical conversion coating, the surface of the metal is chemically treated to produce a nonmetallic, nonconductive surface that can impart a range of desirable properties.The most popular types of conversion coatings are chromate and phosphate.Aluminum, magnesium, zinc, and copper (as well as cadmium and silver) can all be treated by a chromate conversion process that usually involves immersion in a chemical bath. The surface of the metal is convened into a layer of complex chromium compounds that can impart colors ranging from bright clear through blue, yellow, brown, olive drab, and black. Most of the films are soft and gelatinous when they are formed but harden upon drying. They can be used to (1) impart exceptionally good corrosion resistance; (2) act as an intermediate bonding layer for paint, lacquer, or other organic finishes; or (3) provide specific colors by adding dyes to the coating when it is in its soft condition. Phosphate coatings are formed by immersing metals (usually steel or zinc) in baths where metal phosphates (iron, zinc, and manganese phosphates are all common) have been dissolved in solutions of phosphoric acid. The resultant coatings can be used to precondition surfaces to receive and retain paint or enhance the subsequent bonding with rubber or plastic. In addition, phosphate coatings are usually rough and can provide an excellent surface for holding oils and lubricants. This feature can be used in manufacturing, where the coating holds the lubricants that assist in forming, or in the finished product, as with black-color bolts and fasteners, whose corrosion resistance is provided by a phosphate layer impregnated with wax or oil. BLACKENING OR COLORING METALS Many steel parts are treated to produce a black, iron oxide coating—a lustrous surface that is resistant to rusting when handled. Since this type of oxide forms at elevated temperatures, the parts are usually heated in some form of special environment, such as spent carburizing compound or special blackening salts. Chemical solutions can also be used to blacken, blue, and even "brown" steels. Brown, black, and blue colors can also be imparted to tin, zinc, cadmium, and aluminum through chemical bath immersions or wipes. The surfaces of copper and brass can be made to be black, blue, green, or brown, with a full range of tints in between. ELECTROPLATING Large quantities of metal and plastic parts are electroplated to produce a metal coating that imparts corrosion or wear resistance, improves appearance (through color or luster), or increases the overall dimensions. Virtually all commercial metals can be plated, including aluminum, copper, brass, steel, and zinc-based die castings. Plastics can be electroplated, provided that they are first coated with an electrically conductive material. The most common platings are zinc, chromium, nickel, copper, tin, gold, platinum, and silver.The electrogalvanized zinc platings are thinner than the hot-dip coatings and can be produced without subjecting the base metal to the elevated temperatures of

pproach the magnitude of the tip of the stylus, great caution should be used in interpreting the output from these devices.As a case in point, Figure 35-5 shows a scanning electron micrograph of a face-milled surface on which has been superimposed (photographically) a scanning electron micrograph of the tip of a diamond stylus (tip radius of 0.0005 in.). Both micrographs have the same final magnification. Surface flaws of the same general size as the roughness created by the machining process are difficult to resolve with the stylus-type device, where both these features are about the same size as the stylus tip. This example points out the difference between resolution and detection. Stylus tracing devices can often detect the presence of a surface crack, step, or ridge on the part but cannot resolve the geometry of the defect when the defect is of the same order of magnitude as the stylus tip or smaller. Another problem with these devices is that they produce a reading (a line on the chart) where the stylus tip is not touching the surface, as is demonstrated in Figure 35-6a, which shows the S from the word TRUST on a U.S. dime.The scanning electron microscope (SEM) micrograph was made after the topographical map of Figure 35- 6b had been made. Both figures are at about the same magnification.The tracks produced by the stylus tip are easily seen in the micrograph. Notice the difference between the features shown in the micrograph and the trace, indicating that the stylus tip was not in contact with the surface many times during its passage over the surface (left no track in the surface), yet the trace itself is continuous

reach locations. Since the process does not use abrasive media, there is no change to any of the product dimensions.The product surfaces are rarely affected by the generated heat, and the cycle (including loading and unloading) can be repeated as many as 100 times an hour. Unfortunately, there is a thin recast layer and heat-affected zone that forms where the burrs were removed. This region is usually less than 0.001 in. thick but may be objectionable in hardened steels and highly stressed parts. Of all of the burr removal methods, tumbling and vibratory finishing are usually the most economical, typically costing in the neighborhood of a few cents per part. Since most of the common methods also remove metal from exposed surfaces and produce a radius on all edges, it is important that the parts be designed for deburring.Table 35-5 provides a listing of the various deburring processes, as well as the edge radius, stock loss, and surface finish that would result from removal of a "typical burr" of 0.003 in. thickness. DESIGN TO FACILITATE OR ELIMINATE BURR REMOVAL By knowing how and where burrs are likely to form, the engineer may be able to design parts to make the burrs easy to remove or even eliminate them.As shown in Figure 35-19, extra recesses or grooves can eliminate the need for deburring, since the burr produced by a cutoff tool or slot milling cutter will now lie below the surface. In this approach, one must determine whether it is cheaper to perform another machining operation (undercutting or grooving) or to remove the resulting burr. Chamfers on sharp corners can also eliminate the need to deburr.The chamfering tool removes the large burrs formed by facing, turning, or boring and produces a relief for mating parts.The small burr formed during chamfering may be allowable or can easily be removed. Often, it may be preferable to give the manufacturer the freedom to use either a chamfer (produced by machining) or an edge radius (formed during the deburring operation) on all exposed corners or edges. ■ 35.10 SURFACE INTEGRITY Surface integrity has become the subject of intense interest because the traditional, nontraditional,and posttreatment methods used to manufacture hardware can change the material's properties.Although the consequence of these changes becomes a design problem, the preservation of properties is a manufacturing consideration. Designs that require a high degree of surface integrity are the ones that display the following qualities: • Are highly stressed • Employ low safety factors • Operate in severe environments • Must have prime reliability • Have a high surface areas-to-volume ratio • Are made with alloys that are sensitive to processing

PAINT APPLICATION METHODS In manufacturing, almost all painting is done by one of four methods:dipping, hand spraying, automatic spraying, or electrostatic spray finishing. In most cases, at least two coats are required. The first (or prime) coat serves to (1) ensure adhesion, (2) provide a leveling effect by filling in minor porosity and other surface blemishes, and (3) improve corrosion resistance and thus prevent later coatings from being dislodged in service.These properties are less easily attainable in the more highly pigmented paints that are used in the final coats to promote color and appearance. When using multiple coats, however, it is important that the carrying vehicles for the final coats do not unduly soften the underlayers. Dipping is a simple and economical means of paint application when all surfaces of the part are to be coated. The products can be manually immersed into a paint bath or passed through the bath while on or attached to a conveyor. Dipping is attractive for applying prime coats and for painting small parts where spray painting would result in a significant waste due to overspray. Conversely, the process is unattractive where only some of the surfaces require painting or where a very thin, uniform coating would be adequate, as on automobile bodies. Other difficulties are associated with the tendency of paint to run, producing both a wavy surface and a final drop of paint attached to the lowest drip point. Good-quality dipping requires that the paint be stirred at all times and be of uniform viscosity. Spray painting is probably the most widely used paint application process because of its versatility and the economy in the use of paint. In the conventional technique, the paint is atomized and transported by the flow of compressed air. In a variation known as airless spraying, mechanical pressure forces the paint through an orifice at pressures between 500 and 4500 psi. This provides sufficient velocity to produce atomization and also propel the particles to the workpiece. Because no air pressure is used for atomization, there is less spray loss (paint efficiency may be as high as 99%) and less generation of gaseous fumes. Hand spraying is probably the most versatile means of application but can be quite costly in terms of labor and production time. When air or mechanical means provide the atomization, workers must exercise considerable skill to obtain the proper coverage without allowing the paint to "run" or "drape." Only a very thin film can be deposited at one time, usually less than 0.001 in. As a result, several coats may be required with intervening time for drying. One means of applying thicker layers in a single application is known as hot spraying. Special solvents are used that reduce the viscosity of the material when heated. Upon atomization, the faster-evaporating solvents are removed, and the drop in temperature produces a more viscous, run-resistant material that can be deposited in thicker layers. When producing large quantities of similar or identical parts, some form of automatic system is usually employed. The simplest automatic equipment consists of some form of parts conveyor that transports the parts past a series of stationary spray heads. dega-c35_933-968-hr 1/17/07 9:09 AM Page 949 While the concept is simple, the results may be unsatisfactory. A large amount of paint is wasted, and it is difficult to get uniform coverage. Industrial robots can be used to move the spray heads in a manner that mimics the movements of a human painter, maintaining uniform separation distance and minimizing waste. This is an excellent application for the robot, since a monotonous and repetitious process can be performed with consistent results. In addition, use of a robot removes the human from an unpleasant, and possibly unhealthy, environment. Nowdays, cars are painted almost exclusively with robots. Both manual and automatic spray painting can benefit from the use of electrostatic deposition.A DC electrostatic potential is applied between the atomizer and the workpiece. The atomized paint particles assume the same charge as the atomizer and are therefore repelled.The oppositely charged workpiece then attracts the particles, with the actual path of the particle being a combination of the kinetic trajectory and the electrostatic attraction.The higher the DC voltage, the greater the electrostatic attraction. Overspraying can be reduced by as much as 60 to 80%, as can the generation of airborne particles and other emissions. Unfortunately, part edges and holes receive a heavier coating than flat surfaces due to the concentration of electrostatic lines of force on any sharp edge. Recessed areas will receive a reduced amount of paint, and a manual touch-up may be required using conventional spray techniques. Despite these limitations, electrostatic spraying is an extremely attractive means of painting complex-shaped products where the geometry would tend to create large amounts of overspray. In an electrostatic variation of airless spraying, the paint is fed onto the surface of a rapidly rotating cone or disk that is also one electrode of the electrostatic circuit. Centrifugal force causes the thin film of paint to flow toward the edge, where charged particles are spun off without the need for air assist.The particles are then attracted to the workpiece, which serves as the other electrode of the electrostatic circuit. Because of the effectiveness of the centrifugal force, paints can be used with high-solids content, reducing the amount of volatile emissions and enabling a thicker layer to be deposited in a single application. Electrocoating or electrodeposition applies paint in a manner similar to the electroplating of metals.As shown schematically in Figure 35-12, the paint particles are suspended in an aqueous solution and are given an electrostatic charge by applying a DC voltage between the tank (cathode) and the workpiece (anode).As the electrically conductive workpiece enters and passes through the tank, the paint particles are attracted to it and deposit on the surface, creating a uniform, thin coating that is more than 90% resin and pigment. When the coating reaches a desired thickness, determined by the bath conditions, no more paint is deposited. The workpiece is then removed from the tank, rinsed in a water spray, and baked at a time and temperature that depends on the particular type of paint. Baking of 10 to 20 minutes at 375°F is somewhat typical. Electrocoating combines the economy of ordinary dip painting with the ability to produce thinner, more uniform coatings.The process is particularly attractive for applying the prime coat to complex structures, such as automobile bodies, where good corrosion resistance is a requirement. Hard-to-reach areas and recesses can be effectively coated.

SOLVENT CLEANING In solvent cleaning, oils, grease, fats, and other surface contaminants are removed by dissolving them in organic solvents derived from coal or petroleum, usually at room temperature. The common solvents include petroleum distillates (such as kerosene, naphtha, and mineral spirits), chlorinated hydrocarbons (such as methylene chloride and trichloroethylene), and liquids such as acetone, benzene, toluene, and the various alcohols. Small parts are generally cleaned by immersion, with or without assisting agitation, or by spraying. Products that are too large to immerse can be cleaned by spraying or wiping. The process is quite simple, and capital equipment costs are rather low. Drying is usually accomplished by simple evaporation. Solvent cleaning is an attractive means of cleaning large parts, heat-sensitive products, materials that might react with alkaline solutions (such as aluminum, lead, and zinc), and products with organic contaminants (such as soldering flux or marking crayon). Virtually all common industrial metals can be cleaned, and the size and shape of the workpiece are rarely a limitation. Insoluble contaminants, such as metal oxides, sand, scale, and the inorganic fluxes used in welding, brazing, and soldering, cannot be removed by solvents. In addition, resoiling can occur as the solvent becomes contaminated. As a result, solvent cleaning is often used for preliminary cleaning. Many of the common solvents have been restricted because of health, safety, and environmental concerns. Fire and excessive exposure are common hazards.Adequate ventilation is critical. Workers should use respiratory devices to prevent inhalation of vapors and wear protective clothing to minimize direct contact with skin.In addition,solvent wastes are often considered to be hazardous materials and may be subject to high disposal cost. VAPOR DEGREASING In vapor degreasing, the vapors of a chlorinated or fluorinated solvent are used to remove oil, grease, and wax from metal products.A nonflammable solvent, such as trichloroethylene, is heated to its boiling point, and the parts to be cleaned are suspended in its vapors. The vapor condenses on the work and washes the soluble contaminants back into the liquid solvent. Although the bath becomes dirty, the contaminants rarely volatilize at the boiling temperature of the solvent. Therefore, vapor degreasing tends to be more effective than cold solvent cleaning, since the surfaces always come into contact with clean solvent. Since the surfaces become heated by the condensing solvent, they dry almost instantly when they are withdrawn from the vapor. Vapor degreasing is a rapid, flexible process that has almost no visible effect on the surface being cleaned. It can be applied to all common industrial metals, but the solvents may attack rubber, plastics, and organic dyes that might be present in product assemblies. A major limitation is the inability to remove insoluble soils, forcing the process to be coupled with another technique, such as mechanical or alkaline cleaning. Since hot solvent is present in the system, the process is often accelerated by coupling the vapor cleaning with an immersion or spray using the hot liquid. Unfortunately, environmental issues have forced the almost complete demise of the process. While the vapor degreasing solvents are chemically stable, have low toxicity, are nonflammable, evaporate quickly, and can be recovered for reuse, the CFC materials have been identified as ozone-depleting compounds and have essentially been banned from use. Solvents that can be used in the same process, or in a replacement process that offers the necessary cleaning qualities, include chlorinated solvents (methylene chloride, perchloroethylene, and trichloroethylene), most manufacturers have converted to some form of water-based process using alkaline, neutral, or acid cleaners or to a process using chlorine-free, hydrocarbon-based solvents. Sealed chamber machines use non-VOC, nonchlorinated solvents that are continuously recycled. ULTRASONIC CLEANING When high-quality cleaning is required for small parts, ultrasonic cleaning may be preferred. Here the parts are suspended or placed in wire-mesh baskets that are then immersed in a liquid cleaning bath, often a water-based detergent. The bath contains an

ultrasonic transducer that operates at a frequency that causes cavitation in the liquid. The bubbles that form and implode provide the majority of the cleaning action, and if gross dirt, grease, and oil are removed prior to the immersion, excellent results can usually be obtained in 60 to 200 seconds. Most systems operate at between 10 and 40 kHZ. Because of the ability to use water-based solutions, ultrasonic cleaning has replaced many of the environmentally unfriendly solvent processes. ACID PICKLING In the acid-pickling process, metal parts are first cleaned to remove oils and other contaminants and then dipped into dilute acid solutions to remove oxides and dirt that are left on the surface by the previous processing operations.The most common solution is a 10% sulfuric acid bath at an elevated temperature between 150° and 185° F. Muriatic acid is also used, either cold or hot. As the temperature increases, the solutions can become more dilute. After the parts are removed from the pickling bath, they should be rinsed to flush the acid residue from the surface and then dipped in an alkaline bath to prevent rusting.When it will not interfere with further processing, an immersion in a cold milk of lime solution is often used. Caution should be used to avoid overpickling, since the acid attack can result in a roughened surface. ■ 35.4 COATINGS Each of the surface finishing methods previously presented has been a material removal process, designed to clean, smooth, and otherwise reduce the size of the part. Many other techniques have been developed to add material to the surface of a part. If the material is deposited as a liquid or organic gas (or from a liquid or gas medium), the process is called coating. If the added material is a solid during deposition, the process is known as cladding. PAINTING, WET OR LIQUID Paints and enamels are by far the most widely used finish on manufactured products, and a great variety are available to meet the wide range of product requirements. Most of today's commercial paints are synthetic organic compounds that contain pigments and dry by polymerization or by a combination of polymerization and adsorption of oxygen. Water is the most common carrying vehicle for the pigments. Heat can be used to accelerate the drying, but many of the synthetic paints and enamels will dry in less than an hour without the use of additional heat.The older oil-based materials have a long drying time and require excessive environmental protection measures. For these reasons they are seldom used in manufacturing applications. Paints are used for a variety of reasons, usually to provide protection and decoration but also to fill or conceal surface irregularities, change the surface friction, or modify the light or heat absorption or radiation characteristics.Table 35-3 provides a list of some of the more commonly used organic finishes, along with their significant characteristics. Nitrocellulose lacquers consist of thermoplastic polymers dissolved in organic solvent. Although fast drying (by the evaporation of the solvent) and capable of producing very beautiful finishes, they are not sufficiently durable for most commercial applications. The alkyds are a general-purpose paint but are not adequate for hard-service applications. Acrylic enamels are widely used for automotive finishes and may require catalytic or oven curing. Asphaltic paints, solutions of asphalt in a solvent, are used extensively in the electrical industry, where resistance to corrosion is required and appearance is not of prime importance. When considering a painted finish, the temptation is to focus on the outermost coat, to the exclusion of the underlayers. In reality, painting is a complex system that includes the substrate material, cleaning and other pretreatments (such as anodizing, phosphating, and various conversion coatings), priming, and possible intermediate layers. The method of application is another integral feature to be considered.

BELT SANDING In the belt sanding operation, the workpieces are held against a moving abrasive belt until the desired degree of finish is obtained. Because of the movement of the belt, the resulting surface contains a series of parallel scratches with a texture set by the grit of the belt. When smooth surfaces are desired, a series of belts may be employed, with progressively finer grits. The ideal geometry for belt sanding is a flat surface, for the belt can be passed over a flat table where the workpiece can be held firmly against it. Belt sanding is frequently a hand operation and is therefore quite labor intensive. Furthermore, it is difficult to apply when the geometry includes recesses or interior corners. As a result, belt sanding is usually employed when the number of parts is small and the geometry is relatively simple. See Chapter 28. WIRE BRUSHING High-speed rotary wire brushing is sometimes used to clean surfaces and can also impart some small degree of material removal or smoothing. The resulting surface consists of a series of uniform curved scratches. For many applications, this may be an acceptable final finish. If not, the scratches can easily be removed by barrel finishing or buffing. Wire brushing is often performed by hand application of a small workpiece to the brush or the brush to a larger workpiece. Automatic machines can also be used where the parts are moved past a series of rotating brushes. In another modification the brushes are replaced with plastic or fiber wheels that are loaded with abrasive. BUFFING Buffing is a polishing operation in which the workpiece is brought into contact with a revolving cloth wheel that has been charged with a fine abrasive, such as polishing rouge. The "wheels", which are made of disks of linen, cotton, broadcloth, or canvas, achieve the desired degree of firmness through the amount of stitching used to fasten the layers of cloth together. When the operation calls for very soft polishing or polishing into interior corners, the stitching may be totally omitted, the centrifugal force of the wheel rotation being sufficient to keep the layers in the proper position. Various types of polishing compounds are also available, with many consisting of ferric oxide particles in some form of hinder or carrier. The buffing operation is very similar to the lapping process that was discussed in Chapter 27. In buffing, however, the abrasive removes only minute amounts of metal from the workpiece. Fine scratch marks can be eliminated and oxide tarnish can be removed.A smooth, reflective surface is produced. When soft metals are buffed, a small amount of metal flow may occur, which further helps to reduce high spots and produce a high polish. In manual buffing, the workpiece is held against the rotating wheel and manipulated to provide contact with all critical surfaces. Once again, the labor costs can be quite extensive.If the workpieces are not too complex,semiautomatic machines can be used,where the workpieces are held in fixtures and move past a series of individual buffing wheels. By designing the part with buffing in mind, good results can be obtained quite economically

In most cases, the arithmetical average (AA) is used. In terms of the measurements, the AA would be as follows: Cutoffrefers to the sampling length used for the calculation of the roughness height. When it is not specified,a value of 0.030 in.(0.8 mm) is assumed.In the previous equation, yi is a vertical distance from the centerline and n is the total number of vertical measurements taken within a specified cutoff distance.This average roughness value is also called Ra, occasionally used is the root-mean-square (rms) value, which is defined as The resolution of stylus profile devices is determined by the radius or the diameter of the tip of the stylus. When the magnitude of the geometric features begins to

In spindle finishing the workpieces are attached to rotating shafts, and the assembly is immersed in media moving in a direction opposite to part rotation. This process is commonly applied to cylindrical parts and avoids the impingement of workpieces on one another.The abrasive action is accelerated, but time is required for fixturing and removal of the parts. VIBRATORY FINISHING Vibratory finishing is a versatile process widely used for deburring, radiusing, descaling, burnishing, cleaning, brightening, and fine finishing. In contrast to the barrel processing, vibratory finishing is performed in open containers. As illustrated in Figure 35-10, tubs or bowls are loaded with workpieces and media and are vibrated at frequencies between 900 and 3600 cycles per minute. The specific frequency and amplitude are determined by the size, shape, weight, and material of the pan, as well as the media and compound. Because the entire load is under constant agitation, cycle times are less than with barrel operations.The process is less noisy and is easily controlled and automated. In addition, the open tubs allow for direct observation during the process, which can also deburr or smooth internal recesses or holes. MEDIA The success of any of the mass-finishing processes depends greatly on media selection and the ratio of media to parts, as presented in Table 35-2. The media may prevent the parts from impinging upon one another as it simultaneously cleans and finishes. Fillers, such as scrap punchings, minerals, leather scraps, and sawdust, are often added to provide additional bulk and cushioning. Natural abrasives include slag, cinders, sand, corundum, granite chips, limestone, and hardwood shapes, such as pegs, cylinders, and cubes. Synthetic media typically contain 50 to 70 wt% of abrasives, such as alumina (Al2O3), emery, flint, and silicon carbide.This material is embedded in a matrix of ceramic, polyester, or resin plastic, which is softer than the abrasive and erodes, allowing the exposed abrasive to perform the work.The synthetics are generally produced by some form of casting operation, so their sizes and shapes are consistent and reproducible (as opposed to the random sizes and shapes of the natural media). Steel media with no added abrasive are frequently specified for burnishing and light deburring.

Media selection should also be correlated with part geometry, since the abrasives should be able to contact all critical surfaces without becoming lodged in recesses or holes.This requirement has resulted in a wide variety of sizes and shapes, including those presented in Figure 35-9. The different abrasives, sizes, and shapes can be selected or combined to perform tasks ranging from light deburring with a very fine finish to heavy cutting with a rough surface. COMPOUNDS A variety of functions are performed by the compounds that are added in addition to the media and workpieces.These compounds can be liquid or dry, abrasive or nonabrasive, and acid, neutral, or alkaline. They are often designed to assist in deburring, burnishing, and abrasive cutting, as well as to provide cleaning, descaling, or corrosion inhibition. In deburring and finishing, many small particles are abraded from both the media and the workpieces, and these must be suspended in the compound solution to prevent them from adhering to the parts. Deburring compounds also act to keep the parts and media clean and to inhibit corrosion. Burnishing compounds are often selected for their ability to develop desired colors and enhance brightness. Cleaning compounds such as dilute acids and soaps are designed to remove excessive soils from both the parts and media and are often specified when the incoming materials contain heavy oil or grease. Corrosion inhibitors can be selected for both ferrous and nonferrous metals and are particularly important when steel media are being used. Another function of the compounds may be to condition the water when aqueous solutions are being used. Consistent water quality, in terms of "hardness" and metal ion content, is important to ensure uniform and repeatable finishing results. Liquid compounds may also provide cooling to both the workpieces and the media. SUMMARY OF MASS-FINISHING METHODS The barrel and vibratory finishing processes are really quite simple and economical and can process large numbers of parts in a batch procedure. Soft, nonferrous parts can be finished in as little as 10 minutes, while the harder steels may require 2 hours or more. Sometimes the operations are sequenced, using progressively finer abrasives. Figure 35-11 shows a variety of parts before and after the mass-finishing operation, using the triangular abrasive shown with each component. Despite the high volume and apparent success, these processes may still be as much art as science. The key factors of workpiece, equipment, media, and compound are all interrelated, and the effect of changes can be quite complex. Media, equipment, and compounds are often selected by trial and error, with various approaches being tested until the desired result is achieved. Even then, maintenance of consistent results may still be difficult.

Surface engineering is a multidisciplinary activity intended to tailor the properties of the surfaces of manufactured components so that their function and serviceability can be improved. Processes include solidification treatments such as hot-dip coatings, weld-overlay coatings, and thermal spray surfaces; deposition surface treatments such as electrodeposition, chemical vapor deposition, and physical vapor deposition; and heat treatment coatings such as diffusion coatings and surface hardening. Electroplating means the electrodeposition of an adherent metallic coating onto an object that serves as the cathode in an electrochemical reaction. The resulting surface provides wear resistance, corrosion resistance, high-temperature resistance, or electrical properties different from those in the bulk material. Many manufacturing processes influence surface properties, which in turn may significantly affect the way the component functions in service.The demands for greater strength and longer life in components often depend on changes in the surface properties rather than the bulk properties. These changes may be mechanical, thermal, chemical, and/or physical and therefore are difficult to describe in general terms. For example, two different surface finishes on Inconel 718 can have a marked effect on the fatigue life, changing the fatigue limit from 60 ksi after gentle grinding to as low as 22 ksi using electrical discharge machining (Figure 35-1). Many metalcutting processes specified by the manufacturing engineer to produce a specific geometry can often have the effect of producing alterations in the surface material of the component, which, in turn, produces changes in performance. The term surface integrity was coined by Field and Kahles in 1964 in reference to the nature of the surface condition that is produced by the manufacturing process. If we view the process as having five main components (workpiece, tool, machine tool, environment, and process variables), we see that surface properties can be altered by all of these parameters (see Table 35-1) by producing the following: • High temperatures involved in the machining process • Plastic deformation of the work material (residual stress) • Surface geometry (roughness, cracks, distortion) • Chemical reactions, particularly between the tool and the workpiece

Surface integrity should be a joint concern of manufacturing and engineering. Manufacturing must balance cost and producibility with design requirements. It bears repeating to say that engineering must design components with knowledge of manufacturing processes.A reduction in fatigue life resulting from processing can be reversed with a posttreatment.This is another example of design for manufacturing. It is important to understand that the various manufacturing and surface-finishing processes each impart distinct properties to the materials that will influence the performance of the product.The achievement of satisfactory product performance obviously depends on a good design, high-quality manufacturing (including surface treatment), and proper assembly. The failure of parts in service, however, is usually the result of a combination of factors. A brief survey of features associated with surfaces and surface processing follows. Each of the various machining processes produces characteristic surface textures (roughness, waviness, and lay) on the workpieces. In addition, the various processes tend to produce changes in the chemical, physical, mechanical, and metallurgical properties on or near the surfaces that are created. For the most part, these changes are limited to a depth of 0.005 to 0.050 in. below the surface. The effects can be beneficial or detrimental, depending on the process, material, and function of the product. Machining processes (both chip forming and chipless) induce plastic deformation into the surface layer, as shown in Figure 35-20. The cut surfaces are generally left with tensile residual stresses, microcracks, and a hardness that is different from the bulk material. Processes such as EDM and laser machining leave a layer of hard, recast metal on the surface that usually contains microcracks. Ground surfaces can have either residual tension or residual compression, depending on the mix between chip formation and plowing or rubbing during the grinding operation. If sufficient heat is generated, phase transformations can occur in the surface and subsurface regions.

The range of surface roughnesses that are typically produced by various manufacturing processes is indicated in Figure 35-7, which is a very general picture of typical ranges associated with these processes. However, one can usually count on its being more expensive to generate a fine finish (low roughness). To aid designers, metal samples with various levels of surface roughness are available. All of the processes used to manufacture components are important if their effects are present in the finished part. It is convenient to divide processes that are used to manufacture parts into three categories: traditional, nontraditional, and finishing treatments. In traditional processesthe tool contacts the workpiece. Examples are grinding, milling, and turning. These material removal processes will inflict damage to the surface if improper parameters are used. Examples of improper parameters are dull tools, excessive infeed, inadequate coolant, and improper grinding wheel hardness. The nontraditional processes have intrinsic characteristics that, even if well controlled, will change the surface. In these processes the workpiece does not touch the tool. Electrochemical machining (ECM), electrical discharge machining (EDM), laser machining, and chemical milling are examples of

beads, and even CO2 are mechanically impelled against the surface to be cleaned.When sand is used, it should be clean, sharp-edged silica sand. Steel grit tends to clean more rapidly and generates much less dust, but it is more expensive and less flexible. When the parts are large, it may be easier to bring the cleaner to the part rather than the part to the cleaner.A common technique for such applications is sand blasting or shot blasting, where the abrasive particles are carried by a high-velocity blast of air emerging from a nozzle with about a 3 /8-in . opening.Air pressures between 60 and 100 psi, producing particle speeds of 400 mph, are common when cleaning ferrous metals, and 10 to 60 psi is common for nonferrous metals.The abrasive may be sand or shot, or materials such as walnut shells, dry-ice pellets, or even baking soda. Pressurized water can also be used as a carrier medium. When production quantities are large or the parts are small, the operation can be conducted in an enclosed hood, with the parts traveling past stationary nozzles. For large parts or small quantities, the blast may be delivered manually. Protective clothing and breathing apparatus must be provided and precautions taken to control the spread of the resulting dust. The process may even require a dedicated room or booth that is equipped with integrated air pollution control devices. From a manufacturing perspective, these processes are limited to surfaces that can be reached by the moving abrasive (line-of-sight) and cannot be used when sharp edges or corners must be maintained (since the abrasive tends to round the edges). BARREL FINISHING OR TUMBLING Barrel finishing or tumbling is an effective means of finishing large numbers of small parts. In the Middle Ages, wooden casks were filled with abrasive stones and metal parts and were rolled about until the desired finish was obtained.Today, modifications of this technique can be used to deburr, radius, descale, remove rust, polish, brighten, surfaceharden, or prepare parts for further finishing or assembly.The amount of stock removal can vary from as little as 0.0001 to as much as 0.005 in. In the typical operation, the parts are loaded into a special barrel or drum until a predetermined level is reached. Occasionally, no other additions are made, and the parts are simply tumbled against one another. In most cases, however, additional media of metal slugs or abrasives (such as sand, granite chips, slag, or ceramic pellets) are added. Rotation of the barrel causes the material to rise until gravity causes the uppermost layer to cascade downward in a "landslide" movement, as depicted in Figure 35-8. The sliding produces abrasive cutting that can effectively remove fins, flash, scale, and adhered sand. Since only a small portion of the load is exposed to the abrasive action, long times may be required to process the entire contents

■ 35.7 TEXTURED SURFACES While technically not the result of a surface finishing process or operation, textured surfaces can be used to impart a number of desirable properties or characteristics.The types of textures that are often rolled onto the sheets used for refrigerator panels serve to conceal dirt, smudges, and fingerprints. Embossed or coined protrusions can enhance the grip of metal stair treads and walkways. Corrugations provide enhanced strength and rigidity. Still other textures can be used to modify the optical or acoustical characteristics of a material. ■ 35.8 COIL-COATED SHEETS Traditionally, sheet metal components, such as panels for appliance cabinets, have been fabricated from bare-metal sheets. Pans are blanked and shaped by the traditional metalforming operations, and the shaped panels are then finished on an individual basis. This requires individual handling and the painting or plating of geometries that contain holes, bends, and contours. In addition, there is the time required to harden, dry, or cure the applied surface finish. An alternative approach is to apply the finish to the sheet material after rolling but before coiling. Coatings can be applied continuously to one or both sides of the material while it is in the form of a flat sheet.Thus the coiled material is effectively prefinished, and efforts need to be taken to protect the surface during the blanking and forming operations used to produce the final shape. Various paints have been applied successfully, as well as a full spectrum of metal coatings and platings.The sheared edges will not be coated, but if this feature can be tolerated, the additional measures to protect the surface may be an attractive alternative to the finishing of individual components. A second sequence that has some advantages takes the coils of steel that have been cut to length and stamps the holes and notches into them to create blanks. The blanks are pretreated, dried, powder coated, cured, and restacked. Then they are postformed to shape them into the back, side, and front panels of appliance cabinets. The manufacturers call this blank coating. The coating thickness is about 1.5 mils 0.2 mil versus 2 mils 0.5 mil (less powder, better quality), and rusting at the corners of the holes is eliminated. ■ 35.9 EDGE FINISHING AND BURRS Burrs are the small, sometimes flexible projections of material that adhere to the edges of workpieces that are formed by cutting, punching, or grinding, like the exit-side burrs formed in the milled slot of Figure 35-18. Dimensionally, they are typically only 0.003 in. thick and 0.001 to 0.005 in. in height, but if not removed, they can lead to assembly failures, short circuits, injuries to workers, or even fatigue failures.

Manufacturing 35

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