Grinding

Abrasive Machining is usually the among the last operations performed on manufactured parts. Abrasives – conventional –aluminum oxide, silicon carbide. Super abrasives: Cubic boron nitride, diamond

Aluminum oxide – white ( very friable), dark, (less friable), monocrystalline (single crystals) Silicon Carbide – green (more friable) black (less friable) FRIABILITY is the self sharpening characteristics of the abrasive

Grain sizes are very small. Smaller the size, the smoother the finish Grain sizes are very small. Smaller the size, the smoother the finish. Grain size identified by GRIT number which is based upon the mesh of the sieve that the grains can pass through. For a cutting wheel, abrasives are bonded with an adhesisve: Vitrified- essentially a glass Resinoid-a thermoset plastic Rubber- used as cut-off blades like a saw. Metal bonds-embedded in a metal matix

Grinding Wheel FIGURE 9.1 Schematic illustration of a physical model of a grinding wheel, showing its structure and grain wear and fracture patterns. TABLE 9.1 Knoop hardness range for various materials and abrasives. Manufacturing Processes for Engineering Materials, 5th ed. Kalpakjian • Schmid © 2008, Pearson Education ISBN No. 0-13-227271-7

Grinding Wheel Types FIGURE 9.2 Some common types of grinding wheels made with conventional abrasives (aluminum oxide and silicon carbide). Note that each wheel has a specific grinding face; grinding on other surfaces is improper and unsafe.

Grinding Wheel Marking System FIGURE 9.4 Standard marking system for aluminum-oxide and silicon-carbide bonded abrasives.

Diamond and cBN Marking System FIGURE 9.5 Standard marking system for diamond and cubic-boron-nitride bonded abrasives.

Mechanics Each grain is irregular and spaced randomly Rake angle is negative typically highly negative (-60 deg), shear angles (Ch 8 are low) Cutting grains (on periphery) have different radial positions. Cutting speeds are very high.6000 ft/min

Abrasive Grains FIGURE 9.6 The grinding surface of an abrasive wheel (A46-J8V), showing grains, porosity, wear flats on grains (see also Fig. 9.7b), and metal chips from the workpiece adhering to the grains. Note the random distribution and shape of the abrasive grains. FIGURE 9.7 (a) Grinding chip being produced by a single abrasive grain. Note the large negative rake angle of the grain. Source: After M.E. Merchant. (b) Schematic illustration of chip formation by an abrasive grain. Note the negative rake angle, the small shear angle, and the wear flat on the grain.

Grinding Variables Chip vol. Chip length, external grinding Chip length, internal grinding FIGURE 9.8 Basic variables in surface grinding. In actual grinding operations, the wheel depth of cut, d, and contact length, l, are much smaller than the wheel diameter, D. The dimension t is called the grain depth of cut. Chip vol. Chip length, surface grinding

Undeformed chip thickness-surface grinding. v=surface speed of the work piece V=surface speed of the wheel C=#of cutting points per unit area (100-1000 per in2) w=chip width r=w / avg. chip width d=wheel depth of cut D=Dia of wheel

Relative Grain Forces Specific Energy Note that grains develop a wear flat and just causes friction Plowing – plastic deformation without chip removal

Grinding Parameters FIGURE 9.9 Chip formation and plowing (plastic deformation without chip removal) of the workpiece surface by an abrasive grain. TABLE 9.2 Typical ranges of speeds and feeds for abrasive processes.

Specific Energy in Grinding TABLE 9.3 Approximate Specific-Energy Requirements for Surface Grinding. Temperature rise:

Residual Stresses FIGURE 9.10 Residual stresses developed on the workpiece surface in grinding tungsten: (a) effect of wheel speed and (b) effect of type of grinding fluid. Tensile residual stresses on a surface are detrimental to the fatigue life of ground components. The variables in grinding can be controlled to minimize residual stresses, a process known as low-stress grinding. Source: After N. Zlatin.

Dressing FIGURE 9.11 (a) Methods of grinding wheel dressing. (b) Shaping the grinding face of a wheel by dressing it with computer-controlled shaping features. Note that the diamond dressing tool is normal to the wheel surface at point of contact. Source: OKUMA America Corporation.

Surface Grinding FIGURE 9.12 Schematic illustrations of 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) Vertical-spindle rotary-table grinder (also known as the Blanchard-type grinder). FIGURE 9.12 Schematic illustration of a horizontal-spindle surface grinder.

Thread and Internal Grinding FIGURE 9.14 Threads produced by (a) traverse and (b) plunge grinding. FIGURE 9.15 Schematic illustrations of internal-grinding operations.

Centerless Grinding FIGURE 9.16 (a-c) Schematic illustrations of centerless-grinding operations. (d) A computer-numerical-control centerless grinding machine. Source: Cincinnati Milacron, Inc.

Creep-Feed Grinding FIGURE 9.17 (a) Schematic illustration of the creep-feed grinding process. Note the large wheel depth of cut. (b) A groove produced on a flat surface in one pass by creep-feed grinding using a shaped wheel. Groove depth can be on the order of a few mm. (c) An example of creep-feed grinding with a shaped wheel. Source: Courtesy of Blohm, Inc. and Society of Manufacturing Engineers.

Finishing Operations FIGURE 9.18 Schematic illustration of the structure of a coated abrasive. Sandpaper, developed in the 16th century, and emery cloth are common examples of coated abrasives. FIGURE 9.19 Schematic illustration of a honing tool to improve the surface finish of bored or ground holes. FIGURE 9.20 Schematic illustration of the superfinishing process for a cylindrical part: (a) cylindrical microhoning; (b) centerless microhoning.

Lapping FIGURE 9.21 (a) Schematic illustration of the lapping process. (b) Production lapping on flat surfaces. (c) Production lapping on cylindrical surfaces.

Chemical-Mechanical Polishing FIGURE 9.22 Schematic illustration of the chemical-mechanical polishing process. This process is widely used in the manufacture of silicon wafers and integrated circuits, where it is known as chemical-mechanical planarization. Additional carriers and more disks per carrier also are possible.

Polishing Using Magnetic Fields FIGURE 9.23 Schematic illustration of the use of magnetic fields to polish balls and rollers: (a) magnetic float polishing of ceramic balls and (b) magnetic-field-assisted polishing of rollers. Source: After R. Komanduri, M. Doc, and M. Fox.

Ultrasonic Machining Contact force: Contact time: FIGURE 9.24 (a) Schematic illustration of the ultrasonic-machining process; material is removed through microchipping and erosion. (b) and (c) Typical examples of cavities produced by ultrasonic machining. Note the dimensions of cut and the types of workpiece materials. Contact force: Contact time:

Advanced Machining Processes TABLE 9.4 General characteristics of advanced machining processes.

Chemical Milling FIGURE 9.25 (a) Missile skin-panel section contoured by chemical milling to improve the stiffness-to-weight ratio of the part. (b) Weight reduction of space launch vehicles by chemical milling of aluminum-alloy plates. These panels are chemically milled after the plates have first been formed into shape, such as by roll forming or stretch forming. Source: ASM International.

Chemical Machining FIGURE 9.26 (a) Schematic illustration of the chemical machining process. Note that no forces are involved in this process. (b) Stages in producing a profiled cavity by chemical machining.

Roughness and Tolerance Capabilities FIGURE 9.27 Surface roughness and dimensional tolerance capabilities of various machining processes. Note the wide range within each process. (See also Fig. 8.26.) Source: Machining Data Handbook, 3rd ed., ©1980. Used by permission of Metcut Research Associates, Inc.

Chemical Blanking FIGURE 9.28 Typical parts made by chemical blanking; note the fine detail. Source: Courtesy of Buckabee-Mears St. Paul.

Electrochemical Machining FIGURE 9.30 Typical parts made by electrochemical machining. (a) Turbine blade made of a nickel alloy, 360 HB; the part on the right is the shaped electrode. Source: ASM International. (b) Thin slots on a 4340-steel roller-bearing cage. (c) Integral airfoils on a compressor disk. FIGURE 9.29 Schematic illustration of the electrochemical-machining process. This process is the reverse of electroplating, described in Section 4.5.1.

Electrochemical Grinding FIGURE 9.31 (a) Schematic illustration of the electrochemical grinding process. (b) Thin slot produced on a round nickel-alloy tube by this process.

Electrical Discharge Machining FIGURE 9.32 Schematic illustration of the electrical-discharge-machining process.

EDM Examples FIGURE 9.33 (a) Examples of shapes produced by the electrical-discharge machining process, using shaped electrodes. The two round parts in the rear are a set of dies for extruding the aluminum piece shown in front; see also Section 6.4. Source: Courtesy of AGIE USA Ltd. (b) A spiral cavity produced using a shaped rotating electrode. Source: American Machinist. (c) Holes in a fuel-injection nozzle produced by electrical-discharge machining. FIGURE 9.34 Stepped cavities produced with a square electrode by EDM. In this operation, the workpiece moves in the two principal horizontal directions, and its motion is synchronized with the downward movement of the electrode to produce these cavities. Also shown is a round electrode capable of producing round or elliptical cavities. Source: Courtesy of AGIE USA Ltd.

Wire EDM FIGURE 9.35 Schematic illustration of the wire EDM process. As much as 50 hours of machining can be performed with one reel of wire, which is then recycled.

Laser Machining FIGURE 9.36 (a) Schematic illustration of the laser-beam machining process. (b) Cutting sheet metal with a laser beam. Source: (b) Courtesy of Rofin-Sinat, Inc. TABLE 9.5 General applications of lasers in manufacturing.

Electron-Beam Machining FIGURE 9.37 Schematic illustration of the electron-beam machining process. Unlike LBM, this process requires a vacuum, and hence workpiece size is limited by the chamber size.

Water-Jet Machining FIGURE 9.38 (a) Schematic illustration of water-jet machining. (b) A computer-controlled water-jet cutting machine. (c) Examples of various nonmetallic parts machined by the water-jet cutting process. Source: Courtesy of OMAX Corporation.

Abrasive-Jet Machining FIGURE 9.39 (a) Schematic illustration of the abrasive-jet machining process. (b) Examples of parts produced by abrasive-jet machining; the parts are 50 mm (2 in.) thick and are made of 304 stainless steel. Source: Courtesy of OMAX Corporation.

Design Considerations FIGURE 9.40 Design guidelines for internal features, especially as applied to holes. (a) Guidelines for grinding the internal surfaces of holes. These guidelines generally hold for honing as well. (b) The use of a backing plate for producing high-quality through-holes by ultrasonic machining. Source: After J. Bralla.

Economic Considerations FIGURE 9.41 Increase in the cost of machining and finishing operations as a function of the surface finish required. Note the rapid increase associated with finishing operations.

Case Study: Stent Manufacture FIGURE 9.42 The Guidant MULTI-LINK TETRATM coronary stent system. FIGURE 9.43 Detail of the 3-3-3 MULTI-LINK TETRATM pattern. FIGURE 9.44 Evolution of the stent surface. (a) MULTI-LINK TETRATM after lasing. Note that a metal slug is still attached. (b) After removal of slug. (c) After electropolishing.