How do Grinding machines work

how are most grinding machines constructed, what are grinding machines used for and what is grinding machine pdf
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Published Date:03-08-2017
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12 Grinding machines and processes When you have read this chapter you should understand: • The main features of typical surface grinding machines. • The main movements of typical surface grinding machines. • How to care for surface grinding machines in order to maintain their accuracy and alignments. • The selection of a surface grinding machine appropriate to the work in hand. • The setting and securing of a workpiece on a magnetic chuck. • The correct setting and operation of a surface grinding machine. 12.1 Safety when The abrasive wheels used in grinding processes are relatively fragile and grinding can be easily broken. If an abrasive wheel breaks whilst rotating at high speeds it can do considerable damage and cause serious accidents. For this reason great care must be taken in: • Storing and handling abrasive wheels. • Mounting and balancing abrasive wheels. • Guarding abrasive wheels (burst containment). • Truing and dressing abrasive wheels. • Using abrasive wheels. Because of their potential danger there are additional regulations that apply specifically to the use of abrasive wheels and grinding processes. We will now examine some of the more important provisions of the Abrasive Wheel Regulations. 12.1.1 Training and appointment of persons to mount wheels Abrasive Wheel Regulation 9 states that no person shall mount an abra- sive wheel unless that person: • Has been trained in accordance with the training schedule of these regulations.Grinding machines and processes 377 • Is competent to carry out that duty. • Has been properly appointed and that the appointment has been con- firmed by a signed and dated entry in the appropriate register. This entry must carry particulars of the class or description of the abrasive wheels that person is appointed to mount. Any such appointment can be revoked by the company by a signed and dated entry in the register. • A copy of that entry or a certificate has been given to the appointed person and this must also indicate the particulars of the class or description of the abrasive wheels that person is appointed to mount. The above comments do not apply to a person undergoing training in the work of mounting abrasive wheels, providing they are working directly under the supervision of a competent person (instructor) who has himself or herself been trained and appointed under these regulations. A trainee must be certified as soon as the training module has been satisfactorily completed. 12.1.2 Guards Regulation 10 requires that a guard shall be provided and kept in position at every abrasive wheel unless the nature of the work absolutely precludes its use. An abrasive wheel guard has two main functions. • To contain the broken pieces of the wheel in the event of it bursting. • To prevent the operator, as far as possible, from coming into contact with the rapidly rotating wheel. To achieve these aims, the wheel should be enclosed to the greatest possible extent, the opening being as small as possible consistent with the nature of the work being performed. Apart from certain guards for portable grinding machines, all abrasive wheel guards should be capa- ble of adjustment. This is so that the whole wheel, except for that part necessarily exposed, can be enclosed. As the wheel wears down, the guard should be adjusted from time to time so as to maintain maximum protection. The guard should be securely bolted or otherwise attached to the frame or body of the machine. On portable machines the guard should be attached by a clamp of unit construction, the clamp to be closed on the machine frame by a single high tensile bolt. Except for very small machines, cast iron or similar brittle materials should not be used for abrasive wheel guards. Because of the magnitude of the forces involved when a wheel bursts, the sheet metal used for most cutter guards is unsuitable, and abrasive wheel guards should be fabricated by welding from substantial steel plate. 12.1.3 Wheel speeds The overspeeding of abrasive wheels is a common cause of failure by bursting. For this reason the manufacturer’s specified maximum permiss- ible speed must never be exceeded. Regulation 6 requires that every378 Engineering Fundamentals abrasive wheel having a diameter of more than 55 mm shall be marked with the maximum permissible speed at which it can safely be used, the speed, as specified by the manufacturer, to be stated in revs/min. The speed of smaller wheels shall be stated in a notice. In the case of mounted wheels and points, the overhang at the specified speed must also be stated in the notice. 12.1.4 Spindle speeds Regulation 7 requires that the maximum working speed or speeds of every grinding machine shall be specified in a notice attached to the machine. This enables the person who is mounting an abrasive wheel on the machine to check that the speed of the spindle does not exceed the maximum permissible speed of the wheel. 12.1.5 Selection of wheels Regulation 13 requires that in selecting a wheel, due account shall be taken of the factors that affect safety. Selecting the correct wheel for the workpiece is equally important for both safety and efficient production. As a general rule, soft wheels are selected for grinding hard workpiece materials. Similarly hard wheels are usually selected for the grinding of soft workpiece materials. The selection of abrasive wheels will be considered more fully in Section 12.4. 12.1.6 Misuse of the abrasive wheel Wheel breakage can occur if the operator presses the workpiece against the abrasive wheel with excessive pressure. This may occur if the: • Wheel is running slower than its recommended speed and is not cut- ting satisfactorily. • Wrong wheel has been selected for the job in hand. • Wheel has become loaded or glazed (see below and also Section 12.5). Particular care must be taken when grinding on the sides of straight sided wheels. Such a technique is dangerous when the wheel is appreci- ably worn or if a sudden or excessive pressure is applied. 12.1.7 Truing and dressing The wheel should be dressed when it becomes loaded or glazed. Loading and glazing prevent the wheel from cutting satisfactorily and can cause overheating of the work and also overheating of the abrasive wheel. Over- heating of the wheel results in weakening of the bond and failure of the wheel. It also tempts the operator into pressing the work harder onto the wheel which can result in wheel failure (bursting). Correct dressing of the abrasive wheel keeps the wheel running con- centric with the spindle axis. This is essential: • For maintaining wheel balance and preventing vibration patterns on the surface of the workpiece.Grinding machines and processes 379 • For preventing vibration damage to the machine bearings. • For accurate dimensional control. • When off-hand grinding since it allows the workrest to be kept close to the periphery of the wheel. This prevents the work from being dragged down between the wheel and the workrest. 12.1.8 Eye protection Persons carrying out dry grinding operations (no cutting fluid being used) and truing or dressing abrasive wheels are required to be provided with and to wear approved eye protectors (goggles) and dust masks. 12.2 Fundamental Grinding is the name given to those processes which use abrasive particles principles of grinding for material removal. The abrasive particles are made by crushing hard, crystalline solids such as aluminium oxide (emery) and silicon carbide. Grinding wheels consist of large numbers of abrasive particles, called grains, held together by a bond to form a multi-tooth cutter similar in its action to a milling cutter. Since the grinding wheel has many more ‘teeth’ than a milling cutter and, because this reduces the ‘chip clearance’ between the teeth, it produces a vastly improved surface finish at the expense of a slower rate of material removal. The fact that the cutting points are irregularly shaped and randomly distributed over the active face of the tool enhances the surface finish produced by a grinding process. Figure 12.1 shows the dross from a grinding wheel highly magnified. It will be seen that the dross consists of particles of abrasive material stripped from the grinding wheel together with metallic chips that are remarkably similar in appearance to the chips produced by the milling process. Figure 12.1 Grinding wheel dross380 Engineering Fundamentals The grains at the surface of the wheel are called active grains because they are the ones that actually perform the cutting operation. In peripheral grinding, each active grain removes a short chip of gradually increasing thickness in a similar way to the tooth of a milling cutter as shown in Fig. 12.2. As grinding proceeds, the cutting edges of the grains become dulled and the forces acting on the grains increase until either the dulled grains fracture and expose new cutting surfaces, or the whole of the dulled grains are ripped from the wheel exposing new active grains. Therefore, grinding wheels have self-sharpening characteristics. Rotation Milling Rotation Bond cutter tooth Abrasive grit (magnified) Work Work Chip Chip Feed Feed Figure 12.2 Cutting action of abrasive wheel grains 12.3 Grinding wheel A grinding wheel consists of two constituents: specification • The abrasive grains that do the cutting. • The bond that holds the grains together. The specification of a grinding wheel indicates its construction and its suitability for a particular operation. For example, let’s consider a wheel carrying the marking: 38A60-J5V This is interpreted as follows: • 38A is the abrasive type (see Table 12.1). • 60 is the grit size (see Table 12.2). • Jisthe grade (see Table 12.3). • 5isthe structure (see Table 12.4). • Visthe bond material (see Table 12.5). Therefore a wheel carrying the marking 38A60-J5V has an aluminium oxide type abrasive, the abrasive grit has a medium to fine grain size, the grade of the wheel is soft, the structure has a medium spacing, and the grains are held together by a vitrified bond.Grinding machines and processes 381 12.3.1 Abrasive This must be chosen to suit the material being cut. As a general classifi- cation: • ‘Brown’ aluminium oxide is used for grinding tough materials. • ‘White’ aluminium oxide is used for grinding hard die steels and high-speed steel cutting tools. • Silicon carbide (green grit) is used for very hard materials such as tungsten carbide tool tips. Table 12.1 indicates how the abrasive type may be coded using the British Standard (BSI) marking system. The British Standard marking system calls only for ‘A’ for aluminium oxide abrasives or ‘C’ for silicon carbide abrasives. However, it does permit the use of a prefix to the A or the C so that specific abrasives can be identified within each broad classification. Table 12.2 compares the British Standard marking system with that of the Norton Abrasive Company. TABLE 12.1 British Standard abrasive marking system Abrasive Aluminium oxide Silicon carbide Aloxite A Silicon carbide C Alundum A Black crystolon 37C Bauxilite A Unirundum C Blue aloxite BA Green silicon carbide GC Mixed bauxilite MA Green crystolon No. 39 39C Pink aloxite PA White aloxite AA White alundum 38A White bauxilite WA TABLE 12.2 Abrasive types (Norton abrasives) Manufacturer’s BS code Abrasive Application type code A A Aluminium oxide A high strength abrasive for hard, tough materials 32A A Aluminium oxide Cool; fast cutting, for rapid stock removal 38A A Aluminium oxide Light grinding of very hard steels 19A A Aluminium oxide A milder abrasive than 38A used for cylindrical grinding 37C C Silicon carbide For hard, brittle materials of high density such as cast iron 39C C (green) Silicon carbide For very hard, brittle materials such as tungsten carbide382 Engineering Fundamentals 12.3.2 Grain size (grit size) The number indicating the grain or grit size represents the number of openings per linear 25 mm in the sieve used to size the grains. The larger the grain size number, the finer the grain. Table 12.3 gives a general classification. The sizes listed as very fine are referred to as ‘flours’ and are used for polishing and super-finishing processes. TABLE 12.3 Grit size Classification Grit sizes Coarse 10, 12, 14, 16, 20, 24 Medium 30, 36, 40, 46, 54, 60 Fine 70, 80, 90, 100, 120, 150, 180 Very fine 220, 240, 280, 320, 400, 500, 600 12.3.3 Grade This indicates the strength of the bond and therefore the ‘hardness’ of the wheel. In a hard wheel the bond is strong and securely anchors the grit in place, thus reducing the rate of wear. In a soft wheel the bond is weak and the grit is easily detached, resulting in a high rate of wear. The bond must be carefully related to the use for which the wheel is intended. Too hard a wheel will result in dull, blunt grains being retained in the periphery of the wheel causing the generation of excessive heat at the tool/wheel interface with the resultant softening (blueing) of the tool being ground. Too soft a wheel would be uneconomical due to rapid wear and would also result in lack of control of dimensional accuracy in the workpiece when precision grinding. Table 12.4 gives a general classification of hardness using a letter code. TABLE 12.4 Grade Classification Letter codes Very soft E, F, G Soft H, I, J, K Medium L, M, N, O Hard P, Q, R, S Very hard T, U, W, Z 12.3.4 Structure This indicates the amount of bond between the grains and the closeness of adjacent grains. In milling cutter parlance it indicates the ‘chip clearance’. An open structured wheel cuts freely and tends to generate less heat in the cutting zone. Therefore an open structured wheel has ‘free-cutting’ and rapid material removal characteristics. However, it will not produceGrinding machines and processes 383 such a good finish as a closer structured wheel. Table 12.5 gives a general classification of structure. TABLE 12.5 Structure Classification Structure numbers Close spacing 0, 1, 2, 3 Medium spacing 4, 5, 6 Wide spacing 7, 8, 9, 10, 11, 12 12.3.5 Bond There is a wide range of bonds available and care must be taken to ensure that the bond is suitable for a given application, as the safe use of the wheel is very largely dependent upon this selection. • Vitrified bond. This is the most widely used bond and is similar to glass in composition. It has a high porosity and strength, producing a wheel suitable for high rates of material removal. It is not adversely affected by water, acid, oils or ordinary temperature conditions. • Rubber bond. This is used where a small amount of flexibility is required in the wheel, such as in thin cutting-off wheels and centreless grinding control wheels. • Resinoid (bakelite) bond. This is used for high-speed wheels where the bursting forces are great. Such wheels are used in foundries for dressing castings. Resinoid bond wheels are also used for the larger sizes of cutting-off wheels. They are strong enough to withstand con- siderable abuse. • Shellac bond. This is used for heavy duty, large diameter wheels, where a fine finish and cool cutting is required. Such wheels are used for grinding mill rolls. • Silicate bond. This is little used for precision grinding. It is mainly used for finishing cutlery (knives) and edge tools such as carpenters’ chisels. Table 12.6 lists the literal code used to specify the bonding materials discussed above. TABLE 12.6 Bond Classification BS code Vitrified bond V Resinoid bond B Rubber bond R Shellac bond E Silicate bond S384 Engineering Fundamentals 12.4 Grinding wheel The correct selection of a grinding wheel depends upon many factors and selection in this section of the chapter it is possible to give only general ‘guide- lines’. Manufacturers’ literature should be consulted for more precise information. 12.4.1 Material to be ground • Aluminium oxide abrasives should be used on materials with relatively high tensile strengths. • Silicon carbide abrasives should be used on materials with relatively low tensile strengths. • A fine grain wheel can be used on hard, brittle materials. • A coarser grain wheel should be used on soft, ductile materials. • When considering the grade, a general guide is to use a soft grade of wheel for a hard workpiece, and a hard grade of wheel for a soft workpiece. • When considering the structure, it is permissible to use a close struc- tured wheel on hard, brittle materials, but a more open structured wheel should be used for soft, ductile materials. • The bond is seldom influenced by the material being ground. It is usually selected to suit the process. 12.4.2 Rate of stock removal • A coarse grain wheel should be used for rapid stock removal, but it will give a comparatively rough finish. A fine grain wheel should be used for finishing operations requiring low rates of stock removal. • The structure of the wheel has a major effect on the rate of stock removal; an open structured wheel with a wide grain spacing being used for maximum stock removal whilst providing cool cutting conditions. • It should be noted that the performance of a grinding wheel can be appreciably modified by the method of dressing (see Section 12.5) and the operating speed. 12.4.3 Bond As explained in Section 12.3, the bond is selected for its mechanical properties. It must achieve a balance between: • sufficient strength to resist the rotational, bursting forces and the applied cutting forces; and • the requirements of cool cutting together with the controlled release of dulled grains and the exposure of fresh cutting edges.Grinding machines and processes 385 12.4.4 Type of grinding machine A heavy, rigidly constructed machine can produce accurate work using softer grade wheels. This reduces the possibility of overheating the work- piece and ‘drawing’ its temper (i.e. reducing its hardness) or, in extreme cases, causing surface cracking of the workpiece. Furthermore, broader wheels can be used and this increases the rate of metal removal without loss of accuracy. 12.4.5 Wheel speed Variation in the surface speed of a grinding wheel has a profound effect upon its performance. Increasing the speed of the wheel causes it to behave as though it were of a harder grade than that marked upon it. Conversely, reducing the surface speed of a grinding wheel causes it to behave as though it were of a softer grade than that marked upon it. Care must be taken when selecting a wheel to ensure that the bond has sufficient strength to resist the bursting effect of the rotational forces. Table 12.7 lists the recommended speeds for off-hand, toolroom, and light production grinding. Never exceed the safe working speed marked on the wheel. TABLE 12.7 Recommended wheel speeds Wheel speed Surface coverage range (m/s) range (feet/min) Cylindrical grinding (vitrified or 33–25 6500–5000 silicate bond) Internal grinding 25–20 5000–4000 Surface grinding 33–20 6500–4000 Tool and cutter grinding 30–23 6000–4500 12.5 Grinding wheel A clear understanding of grinding wheel defects is essential for safe work- defects ing. A wheel that is loaded or glazed will not cut freely and if excess force is used the wheel may shatter. This is extremely dangerous. 12.5.1 Loading When a soft material, such as a non-ferrous metal, is ground with an unsuitable wheel, the spaces between the grains become clogged with metal particles. Under such circumstances the particles of metal can often be seen embedded in the wheel. This condition is referred to as loading and is detrimental to the cutting action of the grinding wheel. Loading destroys the clearance between the grains, causing them to rub rather than to cut. This results in excessive force having to be used to press the work against the wheel in an attempt to make the wheel cut. This in itself386 Engineering Fundamentals may be sufficient to fracture the wheel. In addition, considerable heat is generated by the wheel rubbing instead of cutting and this may not only adversely affect the hardness of the component, but it may cause the wheel to overheat, the bond to weaken, and the wheel to burst. 12.5.2 Glazing A wheel consisting of relatively tough grains, strongly bonded together, will exhibit the self-sharpening action (see Section 12.2) only to a small degree and will quickly develop a shiny, or glazed, appearance. This is due to the active grains becoming blunt and shiny over a large area. Like any other blunt cutting tool, a glazed wheel will not cut properly and this will lead to overheating of the workpiece and the wheel. Grinding under these conditions is inefficient and the force required to make the wheel cut may be sufficiently excessive to cause the wheel to burst. The only permanent remedy for glazing is the use of a softer grade of wheel. 12.5.3 Damage If you find the abrasive wheel of a grinding machine you are about to use is damaged or defective in any way do not attempt to start up the machine. Report the damage immediately to your instructor or your supervisor. Damage may consist of the wheel being chipped, cracked, worn unevenly or dressed on the side until it is dangerously thin. Vibration caused by lack of balance or worn spindle bearings should also be reported. In addition to wheel faults, report any missing or faulty guards or incorrectly adjusted work rests. These must be corrected or replaced by a qualified person before you use the machine. 12.6 Grinding wheel To make a ‘glazed’ or ‘loaded’ abrasive wheel serviceable or to ‘true’ the dressing and truing wheel so that its circumference is concentric with the spindle axis, the wheel must be dressed. There are various devices used to dress grinding wheels but they all have the same aims. These are: • To remove blunt grains from the matrix of the bond. • To fracture the blunt grains so that they exhibit fresh, sharp cut- ting edges. • To remove any foreign matter that may be embedded in the wheel. • To ensure the periphery of the wheel is concentric (running true) with the spindle axis. 12.6.1 Huntington type wheel dresser Lugs hook over workrest This is shown in Fig. 12.3. The star wheels dig into the wheel and break Figure 12.3 Huntington wheel out the blunt grains and any foreign matter that may be clogging the dresser wheel. Since the star wheels rotate with the grinding wheel little abrasiveGrinding machines and processes 387 action takes place and wear of the star wheels is minimal. This type of wheel dressing device is widely used for pedestal type, off-hand grinding Wheel machines, but it is not suitable for dressing and truing the wheels of precision grinding machines. Diamond 12.6.2 The diamond wheel dresser This is shown in Fig. 12.4. Generally, Brown Burt stones from Africa are Holder used since these are useless as gem stones and are, therefore, relatively cheap. The diamond cuts the wheel to shape and is used for dressing Chuck and truing the wheels on precision grinding machines, such as surface and cylindrical grinding machines. The diamond holder should be rotated (a) from time to time to maintain the shape of the stone and prevent it from becoming blunt. Wheel 2.0 mm • Figure 12.4(a) shows the diamond being used incorrectly. Used in this lead way, the diamond will develop a ‘flat’, and this will blunt the new grains as they are exposed. • Figure 12.4(b) shows the correct way to use the diamond. It should ◦ ◦ 5°−15° trail the direction of rotation of the wheel by an angle of 5 to 15 , but lead the centre of rotation slightly. This will maintain the shape of the diamond so that it will keep sharp and dress cleanly. Chuck The effective structure of the wheel can also be controlled to some extent by the way in which the wheel is dressed. Traversing the diamond rapidly (b) across the face of the wheel has the effect of opening the structure, whilst a slow traverse has the effect of making the wheel cut as though it had a Figure 12.4 Diamond wheel close structure. dresser: (a) incorrect – tip of dia- mond will wear flat, this will blunt the new abrasive grains as 12.6.3 The dressing stick they are exposed; (b) correct – This consists of a stick of coarse abrasive crystals bonded together. It diamond leading wheel centre is used for removing the sharp corners from grinding wheels and for and trailing direction of rotation, dressing small, mounted wheels. It is also used for relieving the sides of the diamond will keep sharp and grinding wheels when working up to a shoulder. dress cleanly 12.7 Grinding wheel Precision grinding machines make provision for balancing the grinding balancing wheel and its hub. An out-of-balance wheel produces vibration, causing a ‘chessboard’ pattern on the finished surface and, if allowed to continue, causing wear and damage to the spindle bearings. Large and heavy grind- ing wheels also need to be balanced, since the out-of-balance forces can be very considerable and may cause the wheel to burst. Unlike pedestal and bench type off-hand grinding machines where the wheel has a lead bush and is mounted directly onto the spindle of the machine, the abrasive wheels of precision grinding machines do not have a lead bush but are mounted directly onto a separate hub. This, in turn, is mounted on the machine spindle. To effectively carry out the balancing of the grinding wheel a balancing stand of the type shown in Fig. 12.5388 Engineering Fundamentals Figure 12.5 Grinding wheel balancing stand should be used. The hardened steel knife edges can be levelled by means of two adjusting screws. A levelling plate with a sensitive bubble level is positioned temporarily across the knife edges and indicates when they are level in all directions. The hub usually contains adjustable balance weights and the proce- dure for the static balancing of a grinding wheel and hub assembly is as follows. • After mounting, the grinding wheel should be trued on the machine before balancing and it may require rebalancing from time to time as it wears down. • Position the three balance segments (weights) equidistant around the face of the flange as shown in Fig. 12.6(a). Grub screws are provided Mark here (a) (b) (c) (d) Figure 12.6 Balancing procedure (reproduced courtesy of Jones and Shipman plc)Grinding machines and processes 389 for clamping the balance weight in position when the balance point is reached. • To balance the wheel and hub they are first mounted on a mandrel which, in turn, is supported on the knife edges of the balancing stand. Allow the wheel and hub to turn freely. The wheel will roll back and forth until it stops with the heaviest part of the assembly at the bottom. When stationary, mark the top centre of the wheel with chalk as shown in Fig. 12.6(b). • Move the segments equally round the flange until one segment is aligned with the mark as shown in Fig. 12.6(c). • If movement still occurs, move the other two segments gradually towards the mark, as shown in Fig. 12.6(d) until the wheel and hub remain stationary in any position. The grinding wheel and hub are removed from the mandrel and are care- fully mounted on the grinding machine spindle, where the wheel is retrued ready for use. 12.8 The double-ended Figure 12.7(a) shows a typical double-ended, off-hand grinding machine off-hand grinding machine widely used in workshops for sharpening single point cutting tools. It uses plain cylindrical grinding wheels of the type shown in Fig. 12.7(b). Figure 12.7 Double-ended, off-hand grinding machine390 Engineering Fundamentals Because of its apparent simplicity, this type of grinding machine comes in for more than its fair share of abuse. For safe and efficient cutting the grinding wheel must be correctly mounted and correctly used. Let’s now consider the correct way to mount a grinding wheel on this type of machine. Remember that under the Abrasive Wheel Regulations already discussed in this chapter, only certificated personnel and trainees under the direct supervision of a certificated person may change a grinding wheel. For the following notes on mounting a new grinding wheel, refer mainly to Figs 12.8, 12.9 and 12.10. • For the names of the parts of the wheel mounting assembly see Fig. 12.8(a). 3. 1. OPER. SPEED TESTED TO NOT TO 6445 RPM EXCEED 3600 RPM 1 1 7 X /2 X 1 /4 38A46-5VBE 2. 1. Grinding wheel Blotter Flange clearance Fixed flange Lead bush Key Grinding wheel marking 38A 46-K5VBE Retaining nut ‘38’ Alundum Type of Spindle vitrified bond brand of adhesive Loose flange Grain size Vitrified bond Grade (hardness) Structure or bond strength (grain spacing) (a) (b) Figure 12.8 Mounting a grinding wheel (stage 1): (a) the wheel mounting; (b) checking the new wheel (repro- duced courtesy of Norton Grinding Wheel Co.) • Remove the securing nut. Viewed from the front of the machine, the left-hand wheel nut will have a left-hand thread. The right-hand wheel nut will have a right-hand thread. • Remove the outer (loose) flange and the wheel that is to be discarded. • Clean the spindle and wheel flanges to remove any trace of the old wheel and any burrs that may be present. • Check that the new wheel is of suitable size and type for the machine and the work it is to perform. This information is printed on the ‘blotters’ on each side of the wheel as shown in Fig. 12.8(b). • Check particularly that the operating speed is correct. Remember that the spindle speed must be marked on the machine. • Check that the wheel is not cracked or faulty by ‘ringing’ it as shown in Fig. 12.9(a). To do this the wheel is freely suspended on stout twineGrinding machines and processes 391 Wheel Bush Spindle Twine Grinding wheel Tightening Wooden ‘striker’ up flange will bend wheel (a) (b) Figure 12.9 Mounting a grinding wheel (stage 2): (a) ‘ringing’ a grinding wheel; (b) fitting the bush: incor- rect – if the lead bush in the centre of the wheel is too tight a fit on the spindle, there is a danger that the wheel will crack as the flanges are tightened up; correct – the bush is eased out with a three-square scraper until the wheel can float on the spindle, it will then pull up square with the fixed flange without cracking. (Note: The misalignment of the bush has been exaggerated for clarity) and lightly tapped with a wooden rod. If the wheel is free from cracks or manufacturing faults, such as voids, it will ‘ring’ with a clear note. • Slip the wheel onto the spindle. The lead bush in the centre of the wheel should be an easy fit on the spindle. If it is tight the abrasive wheel may twist and crack as the flanges are tightened up. Tight bushes should be opened up with a three-square scraper so that the wheel can float into position as shown in Fig. 12.9(b). The error in the bush has been exaggerated for clarity. • Replace the ‘loose’ flange and check that the ‘blotters’ on the sides of the abrasive wheel are slightly larger than the flanges. The blotters Switch for light source built into visor Safety glass visor provides eye protection The clearance between the workrest and the wheel must be kept to a minimum Substantial guard encloses as much of the wheel as possible Dangerous Safe Figure 12.10 Setting the wheel guard and workrest adjustment392 Engineering Fundamentals prevent the sharp edges of the flanges from biting into the wheel and starting a crack. The diameter of the flanges should be at least half the diameter of the wheel to give it adequate support. • Replace the securing nut on the spindle and tighten it up. Use only the minimum of force to secure the wheel. Excessive tightening will crush and crack the wheel. • Replace the wheel guard and adjust the visor and workrest as shown in Fig. 12.10. • Test the wheel by running it up to speed. DO NOT stand in front of the wheel whilst testing it in case it shatters. • Finally, true the wheel ready for use. 12.9 Resharpening hand The off-hand grinding machine just described is used mainly for resharp- tools and single point ening workshop hand tools and single point tools such as lathe tools and cutting tools shaping machine tools. We will now look at some examples of how this should be done. 12.9.1 Chisels These are ground as shown in Fig. 12.11(a). The cutting edge should be slightly radiused by rocking the chisel from side to side as shown. Correct Incorrect direction direction of grinding of grinding Tool marks marks rest (a) (b) (c) (d) Figure 12.11 Sharpening bench tools: (a) sharpening a cold chisel – the cutting edge should be slightly radiused by rocking the chisel from side to side as indicated; (b) removing a ‘mushroomed’ head; (c) sharpening a centre punch; (d) correct grinding of centre punch point X 90° Radius of grinding wheel increases the clearance angle B a The tool should be moved back and a immediately below the cutting forth across the face of the grinding edge; this weakens the tool wheel to even out the wear; profile angles are only controlled by hand and eye Tool B X Work Face of Grinding rest grinding wheel wheel (a) (b) (c) (d) Figure 12.12 Sharpening drills and lathe tools: (a) off-hand grinding a drill point; (b) twist drill point angle and lip length gauge; (c) grinding the clearance angle; (d) grinding the plan profile20 20 15 15 10 10 5 5 Grinding machines and processes 393 Protractor scale Take care not to overheat the chisel so that the cutting edge becomes discoloured. This will indicate that the chisel edge has become soft and 0 useless. Any ‘mushrooming’ of the chisel head must be removed as shown in Fig. 12.11(b). 12.9.2 Centre punches and dot punches Centre punches and dot punches are sharpened as shown in Fig. 12.11(c). The punch is held against the grinding wheel at the required angle and rotated between the thumb and forefinger to generate the conical point. Again, care must be taken not to soften the point of the punch by over- Pointer heating it. The grinding marks must run from the point and not around it. This is shown in Fig. 12.11(d). Incorrect grinding will weaken the point causing it to crumble away. 12.9.3 Twist drills These are most easily ground against the flat side of the grinding wheel (a) as shown in Fig. 12.12(a). The straight cutting lip of the drill should lie vertically against the side of the wheel, and the drill should be gently 0 rocked against the wheel about the axis XX to produce the point clear- ance. When the drill has been ground it should be checked on a drill point gauge as shown in Fig. 12.12(b). This ensures that the angles are equal and correct. It also ensures that the lips of the drill are of equal length. This is essential for efficient cutting and the production of accu- rately sized holes. Again care must be taken not to overheat the tool and soften it. 12.9.4 Single point tools Lathe and shaping machine tools can also be ground on the off-hand grinding machine. Figure 12.12(c) shows the front clearance angle being ground, and Fig. 12.12(d) shows the plan trailing angle being ground. The tool shown is a straight nosed roughing tool for a lathe. Again care must be taken not to overheat the tool and soften it. Experienced centre-lathe operators usually judge the cutting angles (b) by ‘eye’ based on years of experience. However, more consistent results can be obtained by using a lathe tool protractor as shown Figure 12.13 Lathe tool pro- in Fig. 12.13. tractor: (a) checking the clear- ance angle; (b) checking the rake angle 12.10 Surface grinding Surface grinding machines can be divided into four categories: machine • Horizontal spindle – reciprocating table. • Horizontal spindle – rotary table. 5 5 10 10 15 15 20 20394 Engineering Fundamentals • Vertical spindle – reciprocating table. • Vertical spindle – rotary table. 12.10.1 Horizontal spindle – reciprocating table In this book we are concerned only with the horizontal spindle, recipro- cating table type of machine as used in toolrooms for precision grinding. A typical example is shown in Fig. 12.14, and names its main features. As well as manual table traverse, it has a powered table traverse that is infinitely variable from zero to 25 m/min. The cross-feed may be adjusted manually or automatically. The automatic cross-feed rate is variable from about 0.2 mm to about 5 mm per pass of the wheel. The vertical in-feed of the wheel is very precisely controlled in increments of 0.005 mm. This type of machine uses grinding wheels that cut mainly on the periphery and, if wheel wear is to be kept to a minimum in the interest of dimen- sional accuracy, stock removal is rather limited. Precision surface grinding wheels are normally mounted on hubs containing balance weights as described in Section 12.7. Usually the surface grinder operator keeps a number of wheels of different specifications ready mounted on spare hubs so that they can be quickly interchanged as required. Figure 12.14 Typical toolroom type surface grinding machine Figure 12.15 shows the relative geometrical movements and alignments for a horizontal spindle, reciprocating table grinding machine. You can seeGrinding machines and processes 395 Wheel head slides Wheel head 90° movement Wheel Worktable rotation Spindle axis = == = 90° 90° Saddle Table slides (longitudinal) movement (transverse) Table movement Saddle Column Base Cross-slides for saddle Figure 12.15 Surface grinding machine: movements and alignments that they have a close similarity to those of a horizontal milling machine. This is not surprising since both machines are designed to produce plain surfaces using a cylindrical, rotating cutter whose axis is horizontal. The important difference is that whereas the milling machine is concerned with high rates of material removal, the surface grinding machine is designed to produce a surface of high dimensional accuracy and to a high standard of surface finish but with a low rate of material removal. Surface grinding is essentially a finishing process. Most cutting takes place on the periphery of the wheel but shallow steps can be ground using the side of the wheel as well. Since the grinding wheel is relatively weak with respect to side forces, great care must be taken when working on the side of the wheel. Special attachments are available for dressing the grinding wheel to different shapes so that it can grind radii or other profiles. 12.11 Workholding Workholding on surface grinding machines is usually effected by means of a magnetic chuck. However, this is possible only if a workpiece made from a ferromagnetic material such as steel is to be ground. Alternatively any of the techniques associated with the milling machine may be used. For example, direct clamping to the machine table, the use of vices of various types and also grinding fixtures. 12.11.1 Magnetic chucks Most chucks employ permanent magnets. Figure 12.16(a) shows the con- struction of a standard chuck, and Fig. 12.16(b) shows the construction of a fine pole chuck for holding thinner components. Figure 12.17(a) shows a section through a standard chuck in the ‘ON’ position. It will be seen that the lines of magnetic flux pass through the workpiece which must be made of a magnetic material. The magnets are mounted in a grid which can be offset by the operating handle. When this

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