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Fundamentals of Machining Cutting-Tool Materials and Cutting Fluids Machining Processes used to Produce Round Shapes: T

Fundamentals of Machining Cutting-Tool Materials and Cutting Fluids Machining Processes used to Produce Round Shapes: Turning and Hole Making Machining Processes used to Produced Various Shapes: Milling, Broaching, Sawing and Filing; Gear Manufacturing

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Fundamentals of Machining Cutting-Tool Materials and Cutting Fluids Machining Processes used to Produce Round Shapes: T

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  1. Fundamentals of Machining • Cutting-Tool Materials and Cutting Fluids • Machining Processes used to Produce Round Shapes: Turning and Hole Making • Machining Processes used to Produced Various Shapes: Milling, Broaching, Sawing and Filing; Gear Manufacturing • Machining Centers, Advanced Machining Concepts and Structures and Machining Economics

  2. Abrasive Machining and Finishing Operations • (719-758pp) • Advanced Machining Processes

  3. Chapter Objectives • Fundamentals and capabilities of the grinding process. • Characteristics of typical abrasives. • Structure of grinding wheels and various abrasive tools. • Grinding operations and their capabilities. • Finishing processes that are based on abrasive action. • How abrasive machining can compete with other processes.

  4. Chapter Outline • Introduction • Abrasives and Bonded Abrasives • The Grinding Process • Grinding Operations and Machines • Design Considerations for Grinding • Ultrasonic Machining • Finishing Operations • Deburring Operations • Economics of Abrasive Machining and Finishing Operations

  5. 26.1 Introduction • Accurate and fine finishing • Abrasives are small, hard, sharp, & irregular, soremove small amounts of material, producing tiny chips. FIGURE 26.1 A variety of bonded abrasives used in abrasive machining processes. Source: Courtesy of Norton Company.

  6. 26.1 Introduction • A wide variety of workpiece geometries with very fine dimensional accuracy and surface finishes. FIGURE 26.2 The types of workpieces and operations typical of grinding: (a) cylindrical surfaces, (b) conical surfaces, (c) fillets on a shaft, (d) helical profiles, (e) concave shape, (f) cutting off or slotting with thin wheels, and (g) internal grinding.

  7. 26.2 Abrasives and Bonded Abrasives • Conventional abrasives Super-abrasives Aluminum oxide Cubic boron nitride Silicon carbide Diamond

  8. 26.2 Abrasives and Bonded Abrasives

  9. 26.2 Abrasives and Bonded Abrasives • Friability—the ability of abrasive grains to fracture (break down) into smaller pieces. • Self-sharpeningcharacteristics, which are essential in maintaining their sharpness during use. • The shape and size of the abrasive grain also affect its friability.

  10. 26.2 Abrasives and Bonded Abrasives Abrasive types • Emery, corundum (alumina), quartz, garnet, and diamondfound in nature are inconsistent and unreliable due to impurities and nonuniformity. • Aluminum oxide (1893) • Seeded gel (unfused aluminum oxide,1987) • Silicon carbide (1891) • Cubic-boron nitride (1970s) • Diamond (synthetic or industrial diamond) (1955)

  11. 26.2 Abrasives and Bonded Abrasives Abrasive grain size • Abrasives are much smaller than the size of cutting tools and inserts, allowing a very fine surface finish and dimensional accuracy. Abrasive workpiece-material compatibility • Low reactivity between an abrasive grain and workpiece material produces less wear and dulling.

  12. 26.2 Abrasives and Bonded Abrasives Abrasive workpiece-material compatibility • Generally, the following recommendations are made for abrasive selection: • Aluminum oxide: Carbon steels, ferrous alloys, and alloy steels. • Silicon carbide: Nonferrous metals, cast irons, carbides, ceramics, glass, and marble. • Cubic boron nitride: Steels and cast irons above 50 HRC hardness and high temperature alloys. • Diamond: Ceramics, cemented carbides, and some hardened steels.

  13. 26.2.1 Grinding wheels FIGURE 26.3 Schematic illustration of a physical model of a grinding wheel, showing its structure and its wear and fracture patterns.

  14. 26.2.1 Grinding wheels FIGURE 26.4 Common types of grinding wheels made with conventional abrasives. Note that each wheel has a specific grinding face; grinding on other surfaces is improper and unsafe.

  15. 26.2.1 Grinding wheels FIGURE 26.5 Examples of superabrasive wheel configurations. The annular regions (rims) are superabrasive grinding surfaces, and the wheel itself (core) generally is made of metal or composites. The bonding materials for the superabrasives are (a), (d), and (e) resinoid, metal, or vitrified; (b) metal; (c) vitrified; and (f) resinoid.

  16. 26.2.1 Grinding wheels FIGURE 26.6 Standard marking system for aluminum-oxide and silicon-carbide bonded abrasives.

  17. 26.2.1 Grinding wheels FIGURE 26.7 Standard marking system for cubic boron nitride and diamond bonded abrasives.

  18. 26.3 The Grinding Process FIGURE 26.8 (a) Grinding chip being produced by a single abrasive grain. Note the large negative rake angle of the grain. The inscribed circle is 0.065 mm (0.0025 in.) in diameter. (b) Schematic illustration of chip formation by an abrasive grain with a wear flat. Note the negative rake angle of the grain and the small shear angle. Source: (a) After M.E. Merchant.

  19. 26.3 The Grinding Process • The major differences between the action of an abrasive grain and that of a single-point cutting tool can be summarized as follows: • The individual abrasive grains have irregular shapes and are spaced randomlyalong the periphery of the wheel. • The average rake angle of the grains is highly negative, typically or even lower. • The radial positions of the grains over the peripheral surface of a wheel vary, thus not all grains are active during grinding. • Surface speeds (that is, cutting speeds) in grinding are very high.

  20. 26.3 The Grinding Process FIGURE 26.9 The surface of a grinding wheel (A46-J8V), showing abrasive grains, wheel porosity, wear flats on grains, and metal chips from the workpiece adhering to the grains. Note the random distribution and shape of the abrasive grains. Magnification: 50. Source: S. Kalpakjian.

  21. 26.3 The Grinding Process FIGURE 26.10 Schematic illustration of the surface-grinding process, showing various process variables. The figure depicts conventional (up) grinding.

  22. 26.3 The Grinding Process Grinding forces • It can be shown that the grain force (which is tangential to the wheel) is proportional to the process variables.

  23. 26.3 The Grinding Process Specific energy (energy per unit volume) • Wear flat, • negative rake angles, • Smaller chip size • Lubrication can reduce more than 4 times

  24. 26.3 The Grinding Process Temperature • The temperature rise in grinding is an important consideration because of • Surface properties & metallurgical changes • Residual stresses on the workpiece • Distortions

  25. 26.3 The Grinding Process Temperature • The surface temperature rise in grinding: Thus, temperature increases with increasing depth of cut, d, wheel diameter, D, and wheel speed, V, and decreases with increasing workpiece speed, v. Note from this equation that the depth of cut has the largest exponent, hence it has the greatest influence on temperature.

  26. 26.3 The Grinding Process Sparks • The sparks produced when grinding metals are actually chips that glow, due to the exothermic (heat producing) reaction of the hot chips with oxygen in the atmosphere.

  27. 26.3 The Grinding Process Tempering • Excessive temperature rise in grinding can cause tempering and softening of the workpiece surface. • Process variables must be selected carefully in order to avoid excessive temperature rise. • The use of grinding fluids is an effective means of controlling temperature.

  28. 26.3 The Grinding Process Burning • Excessive temperature during grinding may burn the surface being ground. • A burn is characterized by a bluish color on ground steel surfaces—an indication that high temperature caused oxidation. • It can be detected by etching and metallurgical techniques.

  29. 26.3 The Grinding Process Heat Checking • High temperatures in grinding may cause the workpiece surface to develop cracks; this condition is known as heat checking. • The cracks usually are perpendicular to the grinding direction.

  30. 26.3 The Grinding Process Residual stresses • Temperature gradients within the workpiece during grinding are primarily responsible for residual stresses. • Grinding fluids, their method of application, as well as process parameters such as depth of cut and speeds significantly influence the magnitude and type of residual stresses developed (tension or compression).

  31. 26.3.1 Grinding-wheel wear • Grinding-wheel wear is caused by three different mechanisms: (1) attritiousgrain wear, (2) grain fracture, and (3) bond fracture.

  32. 26.3.1 Grinding-wheel wear Attritious grain wear • In attritious wear, the cutting edges become dull and develop a wear flat. • Attritious wear is low when the two materials (grain and workpiece) are chemically inert with respect to each other, much like what has been observed with cutting tools.

  33. 26.3.1 Grinding-wheel wear Grain Fracture • Grain becomes dull and grinding becomes inefficient and produces undesirably high temperatures. • A grain–workpiece material combination • Surface damage (such as burning) is likely to occur.

  34. 26.3.1 Grinding-wheel wear Bond Fracture • In general, softer bonds are recommended for harder materials and for reducing residual stresses and thermal damage to the workpiece. • If the bond is too strong, dull grains cannot be dislodged. • This prevents other sharp grains along the circumference of the grinding wheel from contacting the workpiece to remove chips, and the grinding process becomes inefficient.

  35. 26.3.2 Grinding Ratio • Grinding-wheel wear is measured by a parameter called the grinding ratio, G, and defined as • Grinding ratios range from 2 to 200 and even higher. • A high grinding ratio in practice (so as to extend wheel life) isn’t always desirable, because high ratios may indicate grain dulling and, thus, possible surface damage of the workpiece. • A lower ratio may be acceptable.

  36. 26.3.3 Design, truing and shaping of grinding wheels • Dressing is the process of • Conditioning worn grains on the surface of a grinding wheel by producing sharp new edges on grains so that they cut more effectively. • Truing, which is producing a true circle on a wheel that has become out of round.

  37. 26.3.3 Design, truing and shaping of grinding wheels FIGURE 26.12 (a) Forms of grinding-wheel dressing. (b) Shaping the grinding face of a wheel by dressing it by computer control. Note that the diamond dressing tool is normal to the surface at the point of contact with the wheel. Source: Courtesy of Okuma Machinery Works, Ltd.

  38. 26.3.4 Grindability of materials and wheel selection • Grindability: how easy to grind a material and it includes surface finish, surface integrity, wheel wear, cycle time, and overall economics of the operation. • Table 26.3 shows the range of speeds and feeds for abrasive processes. • Wheel selection involves not only the shape of the wheel and the shape of the part to be produced but the characteristics of the workpiece material as well.

  39. 26.3.4 Grindability of materials and wheel selection

  40. 26.4 Grinding Operations and Machines

  41. 26.4 Grinding Operations and Machines Surface grinding • Surface grinding is one of the most common operations generally involving the grinding of flat surfaces. FIGURE 26.13 Schematic illustrations of various surface-grinding operations. (a) Traverse grinding with a horizontal-spindle surface grinder. (b) Plunge grinding with a horizontal spindle surface grinder, producing a groove in the workpiece. (c) A vertical-spindle rotary table grinder (also known as the Blanchard type).

  42. 26.4 Grinding Operations and Machines Surface grinding FIGURE 26.14 Schematic illustration of a horizontal spindle surface grinder. FIGURE 26.15 (a) Rough grinding of steel balls on a vertical-spindle grinder. The balls are guided by a special rotary fixture. (b) Finish grinding of balls in a multiple-groove fixture. The balls are ground to within 0.013 mm (0.0005 in.) of their final size.

  43. 26.4 Grinding Operations and Machines Cylindrical grinding FIGURE 26.16 Examples of various cylindrical-grinding operations: (a) traverse grinding, (b) plunge grinding, and (c) profile grinding. Source: Courtesy of Okuma Machinery Works, Ltd.

  44. 26.4 Grinding Operations and Machines Cylindrical grinding FIGURE 26.17 Plunge grinding of a workpiece on a cylindrical grinder with the wheel dressed to a stepped shape. FIGURE 26.18 Schematic illustration of grinding a noncylindrical part on a cylindrical grinder with computer controls to produce the shape. The part rotation and the distance x between centers are varied and synchronized to grind the particular workpiece shape. FIGURE 26.19 Thread grinding by (a) traverse and (b) plunge grinding.

  45. 26.4 Grinding Operations and Machines Internal grinding • In internal grinding a small wheel is used to grind the inside diameter of the part, such as in bushings and bearing races. FIGURE 26.21 Schematic illustrations of internal grinding operations: (a) traverse grinding, (b) plunge grinding, and (c) profile grinding.

  46. 26.4 Grinding Operations and Machines Centerless grinding • High-production process • Workpiece is supported by a blade. FIGURE 26.22 Schematic illustrations of centerless grinding operations: (a) through-feed grinding, (b) plunge grinding, (c) and internal grinding; (d) a computer numerical-control cylindrical-grinding machine. Source: Courtesy of Cincinnati Milacron, Inc.

  47. 26.4 Grinding Operations and Machines Creep-feed grinding FIGURE 26.23 (a) Schematic illustration of the creep-feed grinding process. Note the large wheel depth of cut, d. (b) A shaped groove produced on a flat surface by creep-feed grinding in one pass. Groove depth is typically on the order of a few mm. (c) An example of creep-feed grinding with a shaped wheel. This operation also can be performed by some of the processes described in Chapter 27. Source: Courtesy of Blohm, Inc.

  48. 26.4 Grinding Operations and Machines Grinding fluids • Prevents temperature rise in the workpiece. • Improves part surface finish and dimensional accuracy. • Reduces wheel wear and power consumption.

  49. 26.4 Grinding Operations and Machines Grinding chatter • Chatter adversely affects surface finish. • Studying chatter marks can identify their source: (a) the bearings, (b) nonuniformities in the grinding wheel, (c) uneven wheel wear, (d) poor dressing, (e) not balanced properly, and (f) external sources (such as nearby machinery). The grinding operation itself can cause regenerative chatter, as it does in machining. Safety in grinding operations • Surface speed at which a freely rotating wheel bursts (explodes). • Damage to a grinding wheel can reduce its bursting speed severely.

  50. 26.6 Ultrasonic Machining • In ultrasonic machining (UM), material is removed by microchipping and erosion. • Ultrasonic machining is best suited for materials that are hard and brittle, such as ceramics, carbides, precious stones, and hardened steels. FIGURE 26.24 (a) Schematic illustration of the ultrasonic machining process. (b) and (c) Types of parts made by this process. Note the small size of the holes produced.

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