Volume 16 - Machining Part 15 doc

Figure 18(a) shows a turning operation on a lathe. The work rotates at a certain speed, V C , and the tool has a depth of cut (DOC) and axial feed per revolution along the part. Figure 18(b) shows how these are related. The three forces acting on the tool are shown in Fig. 18(a): • Cutting force, F C • The force radial or perpendicular to the work surface, F R • Axially, the feed force, F F Figure 18(c) shows how these three forces are resolved into F N , a normal force. Drilling, boring, and milling operations can be similarly modeled. Fig. 18 Nomenclature for turning operations on a lathe. (a) Schematic of workpiece and tool setup. (b) Relation of depth of cut to axial feed per revolution and workpiece speed, V C . (c) Vector diagrams of the cutting (F C ), radial (F R ), and feed-force (F F ) components that form the resultant normal force (F N ) Figure 19 shows an external grinding system. There are two forces: F N in the normal direction and F T in the tangential direction. As the slide is fed into the part at the rate F , it induces F N , which causes the workpiece to be ground at a radial rate W and the wheel to wear at a radial rate S . Fig. 19 Schematic of an external grinding system. The terms in the schematic are used to calculate the metal removal rate (Z W ), the wheel wear rate (Z S ), and the power consumption. In Fig. 18 and 19, Z is a volumetric removal rate. In cutting, Z is the metal removal rate calculated as shown in Fig. 18(b). For grinding, Z W is the metal removal rate, and Z S is the wheel wear rate. In both cases, the tangential force, F C or F T , multiplied by the work speed in cutting, V C , or wheel speed in grinding, V S , is the power used. System Graphs Turning. Figure 20 shows turning data on 4130 steel tubing with four tools having different rake angles. Figure 20(a) plots metal removal rate against the normal force, and four linear relationships describe the slope. The tool with the 45° rake had a small threshold force and the steepest slope, while the 25° rake tool had the largest threshold force and the shallowest metal removal parameter (MRP) slope (metal removal rate per unit force). This means that for any Z value (any horizontal line from the vertical axis) the 45° tool would always use the least force and that the force would increase as the rake angle is reduced to 40, 35, and 25°, which would always use the highest force. Fig. 20 Turning data for 4130 steel tubing using four tool bits having different rake angles at V C of 0.45 m/s (90 sfm). (a) Metal removal rate plotted against normal force yields MRP slope (mm 3 /s, kgf): A, 3 6.3; B, 22.0; C, 13.7; D, 5.73. (b) Power plotted against metal removal rate yields SP slope (W, s/mm 3 ): A, 0.846; B, 0.98; C, 1.037; D, 1.47 Figure 20(b) plots power against metal removal rate. Again, there are four linear relationships, with the slopes designated specific power, which is the power required to cut at some removal rate. The worst tool for cutting efficiency, the 25° rake, used the most power at any Z value; the best cutting tool, the 45° rake, used the least power. This method provides two values, the metal removal parameter and the specific power, which describe the cutting and power requirements of each system. Usually, for cutting, the normal forces are ignored (but sometimes measured); each power is divided by its own metal removal rate, and different values of power/metal removal rate are obtained from five individual tests. These values are usually plotted against metal removal rate, showing that specific energy decreases with higher removal rates. In fact, Fig. 20(b) would simply show that a threshold power exists. If there were no threshold power, all five power/metal removal rate values would be nearly identical. Figure 21 shows graphs of the hard turning of 58 HRC high-speed tool steel with a cubic boron nitride tool. Again, there are two numbers describing this system: the MRP slope (Fig. 21a) and the SP slope (Fig. 21b). Fig. 21 Turning data for 58 HRC high-speed tool steel cut with a CBN tool having a - 6° rake. Depth of cut was 2 mm (0.08 in.), and V C was 1.33 m/s (260 sfm). (a) Metal removal rate plotted against normal force yields the metal removal parameter of 7.0 mm 3 /s, kgf. (b) Power consumption plotted against metal removal rate yields the specific power of 2.7 W, s/mm 3 . External Grinding. Figure 22 shows external grinding results on soft steel using CBN-electroplated wheels (one layer of abrasive on a steel hub). The 36-grit abrasive wheel cut freer (higher MRP slope) and used less power (shallower SP slope) than the 80-grit abrasive wheel. Fig. 22 External cylindrical grinding of 4150 steel at 23 HRC using CBN- electroplated wheels. An oil coolant was used with V S of 57 m/s (11,200 sfm). Wheel grit sizes: A, 36 grit; B, 80 grit. (a) Workpiece metal removal rate plo tted against normal force to obtain metal removal parameter. (b) Power consumption plotted against workpiece metal removal rate to obtain specific power These and other tests, including turning with negative-rake tools to simulate grinding, are shown in Fig. 23, which is a plot of the SP slope versus the MRP slope. The best direction on Fig. 23 is down (low power) and to the right (low normal force). All cutting results are in the best direction in the lower-right section, but the negative-rake cutting (simulating grinding) is in the upper-left or worst position. The upper-right section, which offers higher power but low normal force, contains the grinding results, although there was a 6:1 difference between the worst and the best grinding results. This means that ordinary cutting uses power much more efficiently than grinding, except where negative-rake tools are used. Therefore, the low-power aspects of cutting influence the ability to send a chip sliding up an inclined tool plane (positive- rake tools). However, cutting uses normal forces as high as those in grinding because both sets of data are generally in the same area on the right of the graph. This means that the normal or deflection forces in cutting can be as large as those in grinding. There was an 8:1 change in normal force in both the cutting and grinding data. Fig. 23 Specific power plotted against m etal removal parameter to forecast optimum operating conditions for turning and grinding. CCC, ceramic-coated carbide References 1. R.P. Lindsay, The Effects of Grinding Fluids on the Performance of CBN and a New High Alumina Abrasive, Japan Society of Precision Engineers, 1987 2. R.S. Hahn and R.P. Lindsay, Principles of Grinding, Mach. Mag., July-Nov 1971 3. R. Snoeys and J. Peters, The Significance of Chip Thickness in Grinding, Ann. CIRP, Vol 23 (No. 2), 1974, p 227-237 4. G. Pahlitzsch and R. Schmitt, Abrichten Von Schleifscheiben Mit Diamantstucken Rollen, Ann. CIRP, Vol 17 (No. 2), 1969 5. R.S. Hahn and R.P. Lindsay, "The Production of Fine Surface Finishes While Maintaining Good Surface Integrity by Grinding," Paper presented at the International Grinding Conference, Carnegie- Mellon University, 1972 6. R. Snoeys, M. Maris, and J. Peters, Thermally Induced Damage in Grinding, Ann. CIRP, Vol 27 (No. 1), 1978, p 571-581; C.P. Bhateja and R.P. Lindsay, Ed., Grinding: Theory, Techniques and Troubleshooting, Society of Manufacturing Engineers, 1982 Selected References • J.R. Besse, "Practical Creep Feed Grinding," Paper MR87- 820 SME, Society of Manufacturing Engineers, 1987 • Injury in Ground Surfaces, Norton Company, 1973 • R. Komanduri, Some Aspects of Machining With Negative Rake Tools Simulating Grinding, Int. J. Mach. Tool Des. Res., Vol 11, 1971, p 223-233 • R. Komanduri, W. Konig, and H. Tonshoff, Machining of Hard Materials, Ann. CIRP, 1984 • R.P. Lindsay, "On the Metal Removal and Wheel Removal Parameters Surface Finish, Geometry and Thermal Damage in Precision Grinding," PhD. thesis, Worcester Polytechnic Institute, 1971 • R.P. Lindsay, "On the Surface Finish Metal Removal Relationship in Precision Grinding," ASME Paper 72-WA/Prod-13, American Society of Mechanical Engineers, 1972 • R.P. Lindsay, "Sparkout Behavior in Precision Grinding," Paper 72- 205, Society of Manufacturing Engineers, 1972 • R.P. Lindsay, The Effect of Wheelwear Rate on the Grinding Performance of Three Wheel Grade s, Ann. CIRP, Vol 32 (No. 1), 1983, p 247-249 • R.P. Lindsay, "The Effect of Contact Time on Forces, Power and Metal Removal Rate in Precision Grinding," Paper presented at the International Grinding Conference, Lake Geneva, WI, Society of Manufacturing Engineers, 1984 • R.P. Lindsay, System Parameters for Cutting and Grinding, in Proceedings of the Ninth Annual Conference on Composites and Advanced Ceramic Materials, American Ceramic Society, 1985 • S. Malkin and N. Joseph, Minimum Energy in Abrasive Processes, Wear Mag., Vol 32, 1975, p 15- 23 • R.F. Pugh and R.F. Pohl, "High-Speed Metal Removal," Special Publication ARLOD-SP- 82004, U.S. Army Armament Research and Development Command, 1982 Grinding Equipment and Processes William N. Ault, Norton Company Introduction METAL IS REMOVED from the workpiece by the mechanical action of irregularly shaped abrasive grains in all grinding operations. This article will discuss grinding wheels and disks, coated abrasives, and grinding machines and processes. These processes include: • Rough grinding • Precision grinding • Surface grinding • Cylindrical grinding • Centerless grinding • Internal grinding • Tool grinding The quantitative relationships among important grinding process variables are described in the preceding article in this Section. In addition, the grinding fluids described briefly in this article are discussed in greater detail in the article "Metal Cutting and Grinding Fluids" in this Volume. Grinding Wheels and Disks In their simplest form, grinding wheels can be thought of as multitooth cutters. They consist of three primary components: • Abrasive (the cutting tool) • Bond (the toolholder) • Porosity or air for chip clearance and/or the introduction of coolant Each of these components has a profound effect on the grinding process. Standard Marking Systems for Grinding Wheels. Abrasives can be classified as conventional abrasives or superabrasives (see the article "Superabrasives" in this Volume). Figure 1 shows the standard marking system for conventional abrasive products (aluminum oxide or silicon carbide abrasives). Figure 2 shows the standard marking system for superabrasive products (diamond or cubic boron nitride, CBN, abrasives). Although standard marking systems are available, many parts of the markings have no standard of measurement. Fig. 1 Standard marking system for conventional aluminum oxide and silicon carbide abrasive grinding wheels Fig. 2 Standard marking system for diamond (a) and cubic boron nitride (b) superabrasive grinding wheels Abrasive Type and Grit Size. In both marking systems, the first two components of the marking deal with the abrasive type. The second component defines the chemistry of the abrasive, while the first defines the specific type of that abrasive. The second two components deal with the size of the abrasive particle. The third position defines the sieve spacing to which the particle corresponds. In other words, the grit size is roughly equal to the linear holes per inch of a sieve that the particle would just pass through. A 60-grit particle, for example, would pass through a 56-mesh screen but would be caught on a 64-mesh screen. Table 1 lists the mean particle sizes for various grit sizes. The grit size varies indirectly with the particle size. The fourth position further describes the particle size distribution by defining the combination of grit sizes that has been used to manufacture the grinding wheel. There is no industry standard for grit size combination. Table 1 Mean particle sizes for grits used in conventional abrasive grinding wheels Particle size (mean) Grit size m in. 4 6848 0.2577 6 5630 0.2117 8 4620 0.1817 10 3460 0.1366 12 2550 0.1003 14 2100 0.0830 16 1660 0.0655 20 1340 0.0528 24 1035 0.0408 30 930 0.0365 36 710 0.0280 46 508 0.0200 54 430 0.0170 60 406 0.0160 70 328 0.0131 80 266 0.0105 90 216 0.0085 100 173 0.0068 120 142 0.0056 150 122 0.0048 180 86 0.0034 220 66 0.0026 240 63 0.0024 280 44 0.0017 320 32 0.0012 400 23 0.0009 500 16 0.0006 600 8 0.0003 900 6 0.0002 Levigated alumina 3 0.0001 Note: Grit size varies indirectly with particle size. Bond Designation and Grain Spacing. The latter part of the marking deals mainly with the bond and the spacing of the grain in the bond. For conventional abrasive wheels, the fifth, sixth, and seventh positions are the bond hardness (or the amount of bond), the porosity or grain spacing, and the bond type. For superabrasive wheels, those positions are again the bond hardness, the concentration or the amount of grain in the abrasive section (and actually, by inference, the grain spacing), and the bond type. The bond hardness or grade of a wheel is defined by an alpha character: the letter A being soft or having very little bond and the letter Z being hard and holding the abrasive tightly into the grinding wheel. The definition of what constitutes a given grade letter for hardness or harshness of the grinding action varies by bond and by manufacturer because there are no industry standards. Porosity. There are no standards for structure or porosity in conventional abrasive grinding wheels or for concentration in superabrasive wheels other than the relative scales. In conventional abrasive wheels, low structure numbers are dense and have little porosity (consider that the number is the distance between the abrasive grains), while high numbers denote porous products. In superabrasive wheels, higher concentrations have more superabrasive particles. Bond Types. Bond markings are generally standardized throughout the abrasives industry. The eighth position in conventional and superabrasive marking systems further defines the bond type and is vendor specific. In superabrasive product markings, the ninth position denotes abrasive layer thickness. Superabrasive wheels often contain thin rims of expensive abrasive in a bond matrix or a nonabrasive preform or holder. Abrasives In addition to particle or grit size, abrasives have a number of properties that determine their efficacy in the grinding process. These properties include chemistry, crystal structure, hardness, durability, friability, and sharpness. Properties of Abrasives The chemistry of an abrasive can affect its ability to cut at grinding interface temperatures. Diamond and silicon carbide are harder than aluminum oxide, but when steel is ground under high pressures, a chemical reaction occurs that degrades these abrasives compared to the relatively chemically inert aluminum oxide. In a different vein, the chemical purity of an abrasive is often an indicator of crystal structure. Crystal Structure. There are basically three types of abrasive particle: monocrystalline, multicrystalline, and microcrystalline. Monocrystalline grains contain a single crystal. They tend to be relatively durable, and they wear along crystal planes. Multicrystalline grains, usually made up of two to ten crystals, vary greatly with regard to durability and sharpness. These abrasives fracture along crystal boundaries (often catastrophically) and along crystal planes. Microcrystalline abrasives (which may contain crystals smaller than 1 m) tend to retain their sharpness during grinding, and they fracture along crystal boundaries. In some applications, microcrystalline abrasives may have more usable volume than mono- or multicrystalline grains before being shed by the bond. Hardness. Figure 3 shows the relative hardness of various abrasives on the Knoop hardness scale. Hardness is an advantage; it is inefficient to abrade a material with an abrasive that is not significantly harder than the material. The ability of an abrasive wheel to grind a material is normally measured by the G ratio (see the article "Principles of Grinding" in this Volume), usually defined as the volume of metal removed per volume of wheel used. Under optimum conditions, superabrasive wheels, primarily because of their hardness, will yield G ratios hundreds of times larger than those of conventional abrasive products. Fig. 3 Knoop hardness ratings of various abrasives Durability. Durable grains tend to withstand heavy grinding pressures without catastrophic wear. Under light grinding pressures, they tend to dull, drawing higher power and giving better surface finishes unless there is vibration due to lack of material penetration. Friability. Friable grains fracture to expose new sharp cutting points and may do so a number of times before the bond sheds the grain (Fig. 4). Friable grains tend to remain sharp and tend to draw lower power, often giving rougher finishes because they do not dull as readily. Very friable grains may not be efficient at high power levels and heavy grinding pressures because of premature wear. With regard to conventional abrasives, friable grains are normally used on heat- sensitive and hardened steels to ensure material penetration at relatively low power and low frictional heat levels in precision grinding. [...]... contamination System life Health and safety Disposal Fire hazard Grinding fluids Petroleum-base and mineral-base cutting oils D A A D A A-B A C-D B-C D Water-soluble oils Synthetic fluids Semisynthetic fluids Water plus additives B-C B-C B-C C-D C-D C-D B-C C C A-B A B-C B-C A A A-B A A-B A-C A B B-C B-C B B-C B-C B-C B B-C A A D A A C C C B C A Note: A, excellent: B, very good; C, good; D, poor Source:... creep-feed surface grinding (b), resulting in decreased aircutting time (c) Air-cutting time in horizontal-spindle reciprocating-table grinding, with its many light passes over the workpiece, is eliminated in creep-feed grinding because the full depth of cut is accomplished in a single pass Fig 25 Creep-feed grinders with ball-screw drives to ensure rigidity There are three subtypes of creep-feed... line or in an arcuate path (a) Parts to be machined are pushed in and retracted by the drawer-like movement of a feeding slide (b) Parts to be ground move in an arcuate path while being transported in the nests of a rotating feed wheel (c) Parts to be machined move diagonally while advancing along a rail Peripheral Surface Grinding Horizontal-spindle reciprocating- or rotary-table machines typically use... required for higher-speed, higher heat producing grinding situations Other advantages of water-soluble coolants over oils include: • • • Higher heat removal and better cooling of the workpiece Improved operator acceptance due to improved cleanliness Reduced fire and health hazards Water-soluble coolants can be used in most light-, moderate-, and heavy-duty grinding operations Water-soluble coolants... Floorstand (Fig 15) or offhand (Fig 16) and swingframe operations tend to split upon the subsequent use of the ground part If the part is subsequently polished (such as a hand tool, plumbing fixture, or turbine blade), it is usually offhand ground with a coated abrasive belt If the part is subsequently machined, ground, and so on, it is usually ground on a floorstand machine with a grinding wheel Fig 15 Floorstand... grinding wheel shown in background Creep-Feed Grinding A significant subset of horizontal-spindle reciprocating-table grinding is creep-feed grinding In conventional reciprocating-table grinding, many light passes are taken at rapid traverse rates (typically 0.025 to 0.05 mm, or 0.001 to 0.002 in., deep at 1270 to 2540 mm/min, or 50 to 100 in./min); in creep-feed grinding, full depth of cut is ground... remove material Table 3 Specifications for several typical surface grinders Type (table-spindle) Reciprocating horizontal Reciprocating horizontal Fixed, horizontal Rotary horizontal Rotary vertical Size mm 205 × 635 610 × 3050 150 × 915 610 915 in 8 × 25 24 × 120 6 × 36 24 36 Main motor kW hp 4.1 5.5 150 200 4.3 5.8 15 20 26 35 Weight Mg lb 2.2 4,840 18.6 41,000 5.5 12,200 6.4 14,000 Fig 21 Surface... Floorstand rough grinding of a casting using a zirconia-alumina resin bond wheel Note the pressure bar used to increase the grinding rate Fig 16 Front view (a) and side view (b) of a backstand grinder having coated abrasive belts for use in offhand rough grinding operations Steel conditioning is done on high-speed, high-horsepower grinders typically using zirconia-alumina reinforced resin bond wheels (8 to 12... be mulled to a dull but even more durable shape They are normally used in medium- to high-pressure, relatively heat insensitive operations on medium-to-soft materials White aluminum oxides are the most pure of the fused aluminas They are multicrystalline, very friable, not durable, and relatively sharp They are used in heat-sensitive operations on hard, ferrous materials Monocrystalline fused aluminum... abrasive is seeded gel alumina abrasive Seeded gel is the purest of the aluminum oxides and the hardest ( 2150 HK) It is also durable, friable, and inherently sharp Seeded gel is made by a ceramic process in which submicron particles are sintered to form microcrystalline abrasive grit particles A 60-grit particle of seeded gel contains billions of individual crystals Seeded gel is purer, harder (because . B-C B-C B-C A Cleanliness D C-D A B A Stability A C-D A B-C C Tolerance to contamination A-B C-D A-B B-C C System life A B-C A B-C C Health and safety C-D C A-B B B Disposal B-C. Petroleum-base and mineral-base cutting oils Water-soluble oils Synthetic fluids Semisynthetic fluids Water plus additives Cooling D B-C A B A Lubricity A B-C B-C B-C D Rust. cleanliness • Reduced fire and health hazards Water-soluble coolants can be used in most light-, moderate-, and heavy-duty grinding operations. Water-soluble coolants with EP additives are replacing