Volume 16 - Machining Part 16 pps

Fig. 12 Superabrasive wheel configurations and their designations Construction. Because of their long wheel life and the higher cost of the superabrasives, these wheels are generally used in a rim-type construction. Exceptions are extremely small inside diameter or very thin grinding wheels. The annular region of the wheel containing the superabrasives, called the rim, is integrally bonded to the core or structural part of the superabrasive grinding wheel (Fig. 13). The core is generally made of composites, aluminum, bronze, steel, or ceramic, depending on such performance requirements as strength, stiffness, and dimensional stability. Fig. 13 Construction of a typical superabrasive wheel Concentration. The rim, or grinding face, consists of a bond, or matrix, that contains the superabrasive grains. The volume fraction of the abrasive grains in the rim is known as the concentration. This often determines the performance or behavior of superabrasive wheels. Bond Systems. Four bond systems are typically used in superabrasive wheels: • Resin • Vitrified • Metal • Layered product The details of the constituents of the bond and the processing techniques result in specific bond types or bond designation and determine the response of the bond to the applications. The manufacturer should be consulted for optimum bond selection. Each bond system has its own unique properties, as follows (Table 4). Table 4 Advantages of diamond abrasive bond types Resin bond • Readily available • Easy to true and dress • Moderate freeness of cut • Applicable for a range of operations • First selection for learning the use of diamond wheels Vitrified bond • Free cutting • Easy to true • Does not need dressing (if selected and trued properly) • Controlled porosity to enable coolant flow to the grinding zone and chip removal • Intricate forms can be crush formed on the wheels • Suitable for creep-feed or deep grinding, inside diameter grinding, or high- conformity grinding • Potential for longer wheel life than resin bond • Excellent under oil as coolant Metal bond • Very durable • Excellent for thin slot, groove, cutoff, simple form, or slot grinding • High stiffness • Good form holding • Good thermal conductivity • Potential for high-speed operation • Generally requires high grinding forces and power • Difficult to true and dress Layered products • Single abrasive layer plated on a premachined steel preform • Extremely free cutting • High unit-width metal removal rates • Form wheels, easily produced • Form accuracy dependent on preform and plating accuracy • High abrasive density • Generally not truable • Generally poorer surface finish than bonded abrasive wheels Resin bond wheels provide good resilience and vibration-absorbing characteristics, which reduce chatter at the grinding zone. Wheels with resin bonds are easy to true and dress and are commonly selected for a wide range of applications. Vitrified bond wheels offer controlled porosity, which facilitates chip removal and coolant flow to the grinding zone. They generally last longer than resin bond wheels and are suitable for producing accurate and complex forms. Metal bond wheels, although difficult to true and dress, offer long life, good form-holding characteristics, and good thermal conductivity. They are excellent for simple form grinding, but usually require greater grinding forces and more power than resin or vitrified bond wheels. Layered product wheels utilize a single abrasive layer plated or brazed to a premachined preform. They usually produce a poorer surface finish than bonded abrasive wheels. Layered product wheels are used for small production runs or where tolerance and surface finish are not very critical. Superabrasive Wheel Applications The proper use of superabrasive wheels often requires careful evaluation of all factors of the grinding system, such as: • Machine tool • Work material • Wheel selection • Operational factors Table 5 lists some of the key variables that affect each of these factors. Table 5 Variables influencing grinding operations with superabrasives Machine tool • Design o Rigidity o Precision o Dynamic stability • Features o Controls o Power, speed, and so on o Slide movements o Truing and dressing equipment • Coolant o Type o Pressure o Flow o Filtration systems • Wheel selection • Core material • Wheel design • Abrasive o Type, properties o Particle size o Size distribution o Content • Bond o Type o Hardness/grade o Porosity o Thermal conduction • Operational factors • Fixtures • Wheel balancing • Truing, dressing, and conditioning techniques and devices • Grinding cycle optimization • Inspection methods Work material • Properties o Mechanical o Thermal o Microstructural • Geometry o Wheel/part conformity o Access to coolant o Shape/profile required • Part quality o Geometry o Consistency • Figure 14 shows a schematic of the grinding process. In a superabrasive wheel, the abrasive grit serves as a wear-resistant cutting edge for long periods of time, unless it is pulled out of the bond prematurely. Therefore, it is imperative to maximize the abrasive/work interactions leading to chip generation and grinding efficiency and to minimize the rubbing or interaction at the bond/work, chip/bond, or chip/work interfaces. Fig. 14 Schematic illustrating interactions in the grinding zone of a grinding wheel/workpiece interface. 1, abrasive/work interface; 2, chip/bond interface; 3, chip/work interface; 4, bond/work interface Machine Tool Variables Machine tool developments in the past 20 years have contributed to innovative superabrasive applications. Precision spindles and slides, rigid machine frames, accurate positioning methods, multiaxis computer numerical control (CNC) movement to achieve complex geometries with a high degree of accuracy, high-speed spindles, and high-pressure flow coolant systems are some of the features incorporated into grinding machines using superabrasive wheels. High-speed tool steel end mills are produced in the conventional method by milling the flutes on a cylindrical rod and then heat treating (Fig. 15a). During heat treating, the flutes become distorted and require finish grinding to restore flute geometry and to generate other features of the cutting geometry. It is also expensive to maintain an in-process inventory of the premachined blanks. Capital and labor costs for this process are high. The conventional process has been significantly improved upon by heat treating the rod and then grinding in the flutes with a CBN wheel (Fig. 15b), thus eliminating the milling operation used in the conventional process. The improved process utilizes the high material removal rate capability of CBN superabrasive wheels while maintaining the form or geometry of the wheel face for a relatively long time. However, the improved process requires the following: • Multiaxial CNC machines of high rigidity that have the flexib ility to be programmed for a range of part geometries • Oil coolant systems able to withstand high pressures and high fluid flows • Suitable enclosure to ensure operator safety • High-precision truing and dressing equipment that lends itself to automation sho uld production quantities warrant such an investment Figure 16 shows the setup used to grind in flutes in an end mill and illustrates the array of coolant lines required for the machining operation. Fig. 15 Methods of producing high-speed tool steel end mills. (a) Conventional process in which flutes are milled in prior to hardening. (b) Improved process in which flutes are ground in with a CBN wheel after hardening. The new process proved to be more cost effective and produced an end mill with more exact tool geometry. Fig. 16 Schematic of tool setup for grinding in flutes on an end mill (the improved process) showi ng positions of coolant lines Wheel Selection The shape, size, configuration, and features of superabrasive bond types are described in the section "Bond Systems" in this article. A typical example of superabrasive wheel designation is shown in Fig. 17. Wheel manufacturers should be consulted on the details of each specification and their influence on grinding results. Additional information on wheel selection is available in the article "Grinding Equipment and Processes" in this Volume. The principles of superabrasive wheel applications are discussed in the sections "Diamond Grinding Wheels" and "Cubic Boron Nitride Grinding Wheels" in this article. Fig. 17 Typical specifications used for superabrasive wheels Diamond Grinding Wheels. Diamond grinding wheels are used for a wide variety of work materials, such as carbide, glass, industrial ceramics, plastics, electronic ceramics, and composites and high-density/structural ceramics. Specification data for the materials are extensive and readily available. Grinding of Carbide Materials. Figure 18 shows the effect of diamond abrasive particle size, concentration, and type on surface finish, metal removal rate, and G ratio (volume of work removal/volume of wheel worn). These qualitative curves are based on data compiled from grinding carbide with diamond abrasives. Fig. 18 Plots of surface finish (curve A), metal removal rate (curve B), and G ratio (curve C) against particle size (a), concent ration (b), and diamond type (c) to show the relative properties of diamond abrasives in the grinding of carbides Figure 19 shows the effect of work material structure (toughness), chip size produced, and abrasion resistance of the bond used on surface finish, metal removal rate, and G ratio. Figure 20 shows the effect of wheel speed, V s , and grinding pressure on surface finish, metal removal rate, and G ratio. Grinding pressure is a control variable commonly used in toolroom or insert grinding operations. Fig. 19 Plots of surface finish (curve A), metal removal rate (curve B), and G ratio (curve C) against mat erial (a), chip type (b), and bond type (c), to show the relative properties of diamond abrasives in the grinding of carbides Fig. 20 Plots of surface finish (curve A), metal removal rate (curve B), and G ratio (curve C) against wheel speed (a), and normal force (b), to illustrate the relative effect of operating conditions on carbide grinding Grinding of Ceramics. The advent of high-strength structural ceramics and their possible use in a variety of high- performance applications offer the potential for even wider use of diamond abrasive wheels. Some recent results are discussed in this section. Figure 21 shows the influence of grit size, bond type, and material removal rate in the grinding of hot-pressed silicon nitride (HPSN). Figure 21(a) shows the grinding forces measured normal to the workpiece surface (F N ), and Fig. 21(b) shows the tangential grinding forces measured parallel to the work surface or in the direction of table traverse (F T ). Figures 21(a) and 21(b) show the forces in a normalized scale; therefore, a direct comparison is possible between the bonds used and the grit sizes. The normal and tangential forces are generally higher for the finer grit, within the experimental conditions of 50 to 255 mm/min (2 to 10 in./min) of table speed, 2.5 mm (0.100 in.) downfeed, and the corresponding normalized (unit-width) metal removal rate. Fig. 21 Effect of bond type and grit size on normal (a) and tangential (b) forces in the grinding of hot- pressed silicon nitride. Wheel speed was 28 m/s (5500 sfm) at both low (2 mm 3 /s, mm; or 0.2 in. 3 /min, in.) and high (10 mm 3 /s, mm; or 1.0 in. 3 /min, in.) unit- width metal removal rates. M2 indicates a modification of the original metal bond (M1). Grit sizes are 180 and 320. Higher forces were generally observed at finer grit sizes for the three bond systems evaluated. This conclusion may be more suitable for the grinding of structural components (such as cutting tools), than for electronic applications, in which the normalized metal removal rates used are generally well below the values tested in this example. Among the bond systems evaluated, the vitrified bond generated the lowest normal forces, while the resin bond generated the lowest tangential forces (or lowest grinding power). The metal bonds evaluated required higher normal and tangential forces. The modification to the metal bond (M2) to improve the free cutting action of the wheel appeared to be significantly beneficial at the higher normalized metal removal rate. Figure 22 shows the wheel wear measured as G ratio for the test conditions shown in Fig. 21. In general, finer-grit diamond that required higher forces exhibited lower G ratios (within the experimental conditions evaluated). However, the metal bond wheels that required higher forces also exhibited higher G ratios. High tangential forces and lower G ratios (despite the lower normal forces) are measured for the vitrified bond wheel. This highlights the significance of coolant application in the grinding of ceramic materials, particularly with vitrified bond diamond wheels. When the test was conducted in a machine setup with better coolant application conditions, the vitrified bond wheel showed relatively high G ratios at the higher unit-width metal removal rate (Fig. 22). [...]... unit-width volumetric removal rate plotted against unit-width normal force (b) Unit-width power plotted against workpiece unit-width volumetric removal rate to obtain slope, which equals specific power (c) G ratio and average surface finish plotted against workpiece unit-width volumetric removal rate The fatigue life of parts ground with CBN wheels is reported to be higher than that of unground case-hardened... utilized in higher-accuracy grinding to achieve better-toleranced parts or in higher-productivity grinding to obtain more parts per hour without sacrificing tolerance requirements However, high-speed grinding is always associated with higher grinding power (for a given unit-width metal removal rate) and therefore requires better coolant systems to prevent workpiece burn or damage to the part High-speed grinding... 178 mm (7.0 in.) and VS is 60 m/s (12,000 sfm) (a) Workpiece unit-width volumetric removal rate plotted against unit-width normal force (b) Unit-width power plotted against workpiece unit-width volumetric removal rate to obtain slope, which equals specific power (c) G ratio and average surface finish plotted against workpiece unit-width volumetric removal rate The surface finish produced by CBN grinding... almost any length-to-diameter ratio can be honed In oil-well applications, holes 32 mm (1 in.) in diameter and 9.8 m (32 ft) long (a length-to-diameter ratio of 307:1) are honed At the opposite extreme, the process has been used for 38 mm (1 in.) diam arbor holes as short as 0.4 mm ( in.) (a length-to-diameter ratio of 1:96) Bore Shape Although most internal honing is done on simple, straight-through holes,... of coolant on grinding performance with CBN wheels The operation is the inside diameter grinding of M7 high-speed tool steel using a B180J100V wheel A, 5% water-soluble oil; B, 100% oil coolant (a) Unit-width power plotted against unit-width metal removal rate (b) G ratio plotted against unit-width metal removal rate Effect of Wheel Speed The optimum wheel speed for CBN grinding is approximately 45... Publication MR 8 5-2 68, Society of Manufacturing Engineering, 1985, p 2-1 4 to 2-5 6 I Inasaki, Grinding of Hard and Brittle Materials, Ann CIRP, Vol 36 (No 2), 1987 Proceedings of Intersociety Symposium on Machining of Advanced Ceramic Materials and Components, The American Ceramic Society, 1987 J Jablonowski, What is New in Grinders, Special Report 783, Am Mach Auto Mfg., Feb 1986, p 11 0-1 24 H.O Juchem... 11 0-1 24 H.O Juchem and H Wapler, Machining of Ceramics, Ferrites, Carbon and Graphite With Diamond Tools, Ind Diamond Rev., Feb 1979, p 4 3-5 0 R Komanduri and D Maas, Ed., Proceedings of the Milton C Shaw Grinding Symposium, Vol 16, American Society of Mechanical Engineers, 1985 R.P Lindsay, "On the Surface Finish-Metal Removal Relationship in Precision Grinding," Paper 72WA/Prod-13, American Society of Mechanical... Ferrite, Al2O3-TiC, and zirconia require relatively lower forces The relative grinding power required for the ceramics evaluated is shown in Fig 23(b) The wheel wear measurements indicate that ferrite has the highest G ratio, followed by zirconia, Al2O3-TiC, hot-pressed silicon nitride, and tungsten carbide (in decreasing order) Fig 23 Relative unit-width normal force (a) and relative unit-width grinding... conditions (constant force, constant volume of abrasive used, and constant grinding conditions) The finer-grit CBN wheel produces workpiece surface roughness that is dependent on the abrasive grit size used for dressing Fig 40 Effect of dressing stick parameters on CBN wheels for constant infeed rate dressing Such is not the case with the coarser-grit CBN wheel The coarser-grit CBN produces finer work surface... (line A-B) An increase in grinding power or force (line B-C1) can be corrected at point B by precise truing of the wheel face (see Fig 32a) or slight dressing (see Fig 32c) A gradual decrease in grinding power caused by excessive bond wear (line B-C2) can be corrected by precision retruing (see Fig 32b) of the wheel at point B If the bond wear is excessive, then at point B the wheel could produce parts . (12,000 sfm). (a) Workpiece unit- width volumetric removal rate plotted against unit-width normal force. (b) Unit-width power plotted against workpiece unit- width volumetric removal rate to obtain. higher-accuracy grinding to achieve better-toleranced parts or in higher-productivity grinding to obtain more parts per hour without sacrificing tolerance requirements. However, high-speed. (6000 sfm). (a) Workpiece unit-width volumetric removal rate plotted against unit-width normal force. (b) Unit- width power plotted against workpiece unit-width volumetric removal rate to obtain