Grinding PDF

CHAPTER 12 GRINDING 12.1 Definition Grinding is a metal cutting procedure in which a multi-edged tool, whose cutting edges are geometrically undefined, removes the chips. During grinding, the tool carries out the cutting motion. The...

CHAPTER 12 GRINDING 12.1 Definition Grinding is a metal cutting procedure in which a multi-edged tool, whose cutting edges are geometrically undefined, removes the chips. During grinding, the tool carries out the cutting motion. The cutting speeds commonly used in grinding are approximately 20 times those used in turning (25 to 45, sometimes up to 120 m/s). The feed movement is executed as a function of the cutting technique, the tool or the workpiece. The grinding techniques are categorised according to the workpiece shape - in face- and cylindrical grinding, or according to component mounting - as grinding between centres or centreless grinding. It would also make sense to further subdivide these methods according to their ranges of application, such as grinding of slide ways or tools. At the point where the cutting takes place, grinding is very similar to other machining operations, the difference being that the workpiece is cut by the sharp edges of small pieces of abrasive material, rather than the edge of a hardened steel or carbide cutting tool. The irregularly- shaped abrasive particles may be bonded to a wheel or coated belt, or may be used loose. The particles commonly consist of aluminum oxide, silicon carbide, cubic boron nitride, diamond, or other hard materials. The individual abrasive grains are each smaller than a conventional metalworking cutting tool, and the grains on a typical wheel make a multitude of minute cuts. Fig.12.1 illustrates the grinding process schematically (Some grains, depending on their shape, do not cut but instead rub or slightly deform the surface of the workpiece.) Cutting speeds are high but the depth of cut from each grain is shallow. A water or water-oil emulsion is often sprayed on the wheel and workpiece to control the dust that otherwise arises and to overcome the heating effect of the operation. Grinding wheels are often porous, especially those designed for use with softer materials. Figure 12.1. The grinding process. Sharp edges of individual abrasive grains act as minute cutting tools, removing small amounts of material from the workpiece. Chapter 12 1  As the wheel cuts, it wears, causing some abrasive particles to become smooth but causing others to fracture, exposing new sharp edges. New wheels, and those that have become worn, are dressed with a diamond tool that removes some of the abrasive material and bonding agent, exposing sharp edges of new abrasive grains and providing a straighter, more uniform, cutting surface. Grinding is most commonly a finish-machining operation to provide a smoother surface or greater dimensional accuracy, particularly with hardened materials. When used as the primary metal removal method, the term, abrasive machining is often used. Figure 12.2. (a) Grinding chip being produced by a single abrasive grain. (A) chip, (B) workpiece, (C) abrasive grain. Note the large negative rake angle of the grain. The inscribed circle is 0.065 mm 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. The grinding techniques are categorised according to the workpiece shape - in face- and cylindrical grinding, or according to component mounting - as grinding between centres or centreless grinding. It would also make sense to further subdivide these methods according to their ranges of application, such as grinding of slide ways or tools. 12.2 Grinding techniques 12.2.1 Flat grinding Plane or flat grinding is the grinding of plane surfaces. During flat grinding, the tool performs the cutting motion, whereas the workpiece executes the feed motion. The grinding procedure can be performed by the circumference or face of the grinding tool. Consequently, the following types are distinguished: 12.2.1.1 Circumferential grinding In circumferential grinding (Figure 12.3), the wheel spindle is in horizontal position. The machine table with the workpiece travels back and forth in a straight line. As a rule, the lateral feed per stroke is carried out by the table. Machines with a rotary table are an alternative. In these machines, the workpiece moves in a circle on a face chuck, and the lateral feed is performed by the grinding tool. Since the grinding wheel contacts the workpiece only on a small portion of its circumference during circumferential grinding, the metal removal rate is limited for these methods. Using special wheels and appropriate machines, the full-width grinding method is competitive with milling. Chapter 12 2  Figure 12.3. Face - and profile grinding Figure 12.4. Face grinding principle machine with vertical wheel spindle 12.2.1.2 Face grinding In face grinding, the grinding procedure is carried out with the front end of the grinding wheel (Figure 12.4). During face grinding, the grinding wheel (designed as segmented grinding wheel or as a ring wheel) performs the cutting motion, whereas In contrast to circumferential grinding, the contact area between workpiece and tool is much greater in face grinding. Consequently, this method makes it possible to achieve higher metal removal rates. In face grinding, the tool axis may be vertical (Figure 12.4) or horizontal (in case of larger machines, see Figure 12.5). Due to their compact design and great cutting capacity, machines with vertical wheel spindle axis are predominantly used for face grinding. Machines with horizontal wheel spindle axis are used only if the surface pattern is decisive, usually just for appearance’s sake, such as in profile grinding operations (Figure 12.5). Figure 12.5. Segmented- surface grinding machine with horizontal wheel spindle axis The face grinding procedures are distinguished according to the surface pattern generated (Figure 12.6): In cross grinding K, the grinding contours cross each other, whereas in arc grinding S, the grinding contours are allocated radially at one side. The mutually crossing grinding contours in cross grinding are generated if the wheel spindle axis is located normally to the workpiece. The radial allocation in arc grinding is created if the wheel spindle axis is inclined towards the workpiece. Chapter 12 3  Figure 12.6. Grinding patterns in face grinding a) Cross grinding K if wheel spindle axis is normal to workpiece. b) Arc grinding S if spindle axis is inclined towards the workpiece. 12.2.1.3 Profile grinding Profile grinding is a circumferential grinding method carried out with profiled grinding wheels. In this procedure, as a rule, lateral feed is inapplicable. There are two common methods used to profile grinding wheels. Simple profiles like radiuses, angles and grooves are generated with the common dressing attachments. Complicated profiles are created with the so-called diaform attachment. This attachment is used to profile the grinding wheel along a template following the copying principle. Making use of CNC equipment, dressing and profiling are more and more being implemented by means of controlled motions. Figure 12.7. 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). 12.2.2 Cylindrical grinding Cylindrical grinding refers to the grinding of rotary parts. In machining, a distinction is made between grinding from the outside (grinding the outer diameter of a shaft) and from the inside (grinding of a hole). Another distinctive feature is the type of workpiece mounting, for example, whether the workpiece is held with or without a centre. Centreless grinding is explained in Chapter 12.2.4. 12.2.2.1 External cylindrical grinding During external cylindrical grinding, the wheel performs both the cutting- and die infeed motion. The workpiece, which is fixed between centres or clamped in the chuck, is brought into rotation by a driving plate. Grinding wheel and workpiece have the same direction of rotation. Chapter 12 4  Figure 12.8. 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. 12.2.2.1.1 External cylindrical grinding with longitudinal feed In grinding with longitudinal feed (Figure 12.9), as a rule, the table of the cylindrical grinding machine, and thus the workpiece, performs the longitudinal feed. It is necessary to harmonize longitudinal feed and workpiece speed. If selecting longitudinal feed is set too high, the result is spiral-like markings on the workpiece. A clean grinding pattern is obtained if feed s per workpiece rotation is less than grinding wheel width B. Thin shafts may only be ground with small depths of cut due to the risk of deflection. For thick shafts, infeed is limited by the machine’s input power. Too high depths of cut lead to greater contact areas between workpiece and wheel. Consequently, they result in increased cutting forces. For this reason, extreme infeed values may result in grinding wheel fracture. To work with greater depths of cut, decrease longitudinal feed. Figure 12.9. External cylindrical Figure 12.10. Plunge grinding – grinding with longitudinal feed – working principle. working principle. 12.2.2.1.2 Plunge grinding In plunge grinding (also plunge-cut grinding, see Figure 12.10), there is no longitudinal feed. The grinding wheel only performs the motion for depth setting. This method is needed, for example, to grind chamfers of shafts. For the infeed amount, the same criteria as for external cylindrical grinding with longitudinal feed are valid. Chapter 12 5  12.2.2.1.3 Thread grinding Thread grinding is defined as cylindrical grinding with profiled grinding wheels. In this method as well, a distinction is made between longitudinal grinding (grinding with longitudinal feed of the workpiece) and plunge grinding. During thread grinding with longitudinal feed, the thread can be generated with a “single- edged” wheel or a “multi-edged” wheel. The narrow single-edged wheel, which has the profile of the thread to be generated (Figure 12.11), has a width of 6 to 8 mm. The width of the multi-edged wheel is about 40 mm. This wheel is dressed conically. The threads (grooves) of the grinding wheel that first come into contact with the profile rough-grind it, while the two threads at the end (Figure 12.12) finish-grind it. This way, the entire chip removal is distributed over several grooves of the grinding wheel. This reduces the load per groove. For this reason, multi-edged wheels have a longer tool life than single-edged wheels. Since the multi-edged wheel (Figure 12.12) is dressed conically, it is impossible to grind a thread directly on a shoulder with this wheel. As a result, this wheel can only be used for through threads. Single-edged wheels are preferred to generate exact threads, since with these wheels one can achieve accuracy values of ± 2 µm for the effective diameter and ± 10 angular minutes for the thread angle. Figure 12.11. Longitudinal grinding Figure 12.12. Longitudinal grinding of a thread with singleedged wheel. of a thread with multiedged wheel. During thread -plunge grinding (Figure 12.13), the thread is generated with a multiedge grinding wheel. Here, the grinding wheel is dressed in parallel. During plunge grinding, as in the milling of short threads, the workpiece performs only 11/6 rotation. On each side, the grinding wheel should be about 2 mm wider than the thread to be generated. For internal thread grinding, the same conditions as external thread grinding are valid; however, the grinding wheel diameters are correspondingly smaller in this case. Depending on workpiece size, they range from 20 to 150 mm. During thread grinding, workpiece and grinding wheel have the same rotation direction. A motion for depth setting, which is executed by the grinding wheel, only exists in plunge grinding. For thread grinding, the grinding result depends to a great extent on selecting an adequate wheel. The range of grain sizes (80 to 600) is the same for all leads, and the choice depends only on the minor thread radius. Chapter 12 6  Figure 12.13. Plunge-thread grinding with multi-edged wheel 12.2.2.2 Internal cylindrical grinding Internal cylindrical grinding (Figure 12.14) corresponds to external cylindrical grinding in terms of its main criteria. Figure 12.14. Internal cylindrical Figure 12.15. Contact length l of the grinding –principle view 1 grinding grinding wheel in workpiece d wheel, 2 workpiece, 3 three-jaw workpiece diameter in mm, D chuck. grinding wheel diameter in mm The contact area between workpiece and wheel (Figure 12.15) is greater. The contact length l depends on depth of cut a and the diameter ratio between grinding wheel and workpiece. Cutting motion, longitudinal feed and the motion for depth setting are carried out by the workpiece. In internal grinding, the cutting speeds that are optimal for grinding can generally not be reached due to the small grinding wheel diameter. Optimal conditions are obtained when the following are selected D ≈ 0,8 d D in mm wheel diameter d in mm diameter of the workpiece hole. Chapter 12 7  12.2.3 Cutting data for flat grinding and cylindrical grinding with clamped workpiece The depth of cut a (infeed e of the grinding wheel) chosen depends on the wheel’s grain size and the dimensions of the workpiece that is to be ground. Coarse-grained wheels allow greater depths of cut than fine-grained ones. Also, when fine-grained wheels are used, the pores clog more quickly. When this occurs, the wheel no longer cuts, but rather squeezes and lubricates. The general rule for common grinding is: “Depth of cut must be less than the height of the abrasive grains protruding out of the bonding.” In full-width grinding, this rule is broken. This is made possible by open-porous wheels of special design. In finishing, the following must be observed: 1. The speed of the grinding wheel must be kept high and that of the workpiece low, if excellent surface quality is required; 2. Sparking emanating from the grinding wheel means that the wheel needs to be guided over the workpiece without infeed until no sparking no longer appears; 3. Reversal of the longitudinal feed must be adjusted so that the grinding wheel travel exceeds the workpiece only by one third of its width (1/3 B); otherwise the workpiece dimensions will be smaller than specified. 12.2.3.1 Grinding wheel speed, workpiece speed Both speeds v and vw should be in a predefined mutual speed ratio q. vc q= vw q speed ratio vc in m/s cutting speed of the grinding wheel (peripheral speed) vw in m/s peripheral speed of the workpiece For the corresponding q values of different materials, see Table 12.1. Table 12.1 Speed ratio q for different materials Material q Steel 125 Grey cast iron 100 Ms and Al 60 12.2.4 Centreless grinding Centreless grinding is a grinding procedure in which the workpiece is located freely on a guide bar, in contrast to external- or internal cylindrical grinding in which the workpiece is between centres or clamped in a chuck (Figure 12.16). The workpiece rotation is generated through frictional resistance between the grinding- and regulating wheels. The axes of both wheels are located horizontally in one plane. The workpiece centre is situated above the connecting line of grinding- and regulating wheel centre. The 3 major elements for centreless grinding are: grinding wheel regulating wheel workpiece seat The workpiece seat is made of steel. It is hardened or equipped with a cemented carbide bar. Chapter 12 8  Figure 12.16. Centreless grinding principle 12.3 Application of grinding techniques 12.3.1 Flat grinding Flat grinding is applied to generate plane-parallel and profiled surfaces. Typical parts with plane-parallel surfaces are die blocks for cutting dies, die shoes for pressand draw dies, clutch lamellae, rings of different design (Figure 12.17) and many other machine elements. Grinding of external splines and punches with gear-tooth profiles, as well as grinding of profiled tools with complicated profiles from solids, are examples of uses of profile grinding. Figure 12.17. Flat grinding machine with rotary table. 12.3.2 Cylindrical grinding Both external- and internal cylindrical grinding are used to machine rotary parts of any design (Figure 12.18). Figure 12.18. Examples of external cylindrical grinding Chapter 12 9  12.4 Achievable accuracy values and allowances during grinding Table 12.2 Allowances and achievable accuracy values As a general rule: The greater the machining diameter or the machining thickness and the longer the workpiece, the higher the allowance. The allowances are valid for unhardened workpieces. For hardened workpieces, increase the values from the tables by 20–40%. Chapter 12 10