ABRASIVEABRASIVE CUTTINGCUTTING I. INTRODUCTION II. ABRASIVE MATERIALS Abrasive have been used as cutting tools since In the early stages of abrasive cutting, the the dawn of civilisation. In the early stages of products were made with natural materials like industrialisation, there was a tendency to move sand, emery, corundum etc. A great impetus to development was the manufacture of synthetic towards other cutting materials, but this process abrasive – aluminium oxide and silicon carbide. has recently stopped and, in fact, there is now These two abrasive materials were gradually a trend to replace many conventional cutting refined to achieve the optimum characteristics operations with abrasive methods. In advanced in terms of hardness, friability, sharpness, industrial countries, almost 25% of all machining thermal resistance etc. The increasing use of operations are done with abrasives and this high alloy steels, aero-space alloys, carbide percentage is expected to rise to 50% in the next tools and ceramics led to the development of decade. This fantastic growth is due to the on- new abrasives like synthetic diamond, boron going research in the abrasives industry which carbide and boron nitride. The growth of the has resulted in the development of sophisticated metallurgical industry and increasing popularity abrasive products and processes catering to of grinding for metal conditioning led to the enhanced requirements in terms of productivity, development of Zirconium Oxide as an abrasive accuracy and quality. The chart shown in table material suitable for extremely heavy duty below gives an idea of the vast scope of operations. abrasive grinding processes. These range from III. ABRASIVE PRODUCTS traditional precision finishing operations through The numerous abrasive cutting operations fettling and cutting off, to the latest primary stock require a wide variety of abrasive products removal processes used in steel plants. Most differing in size, shape and composition. The engineers are aware of the extreme precision shape and size are easily chosen from the which can be achieved by grinding, lapping or machine manufacturer’s recommendations but superfinishing, but few may be acquainted with the selection of specifications (composition) the fact that automatic grinding machine can pose considerable problems. Unlike other remove as much as 200kg/hour of material. operations, abrasive tools have to be selected

SCOPE OF ABRASVIE CUTTING PROCESSES VERY ROUGH ROUGH PRECISION FORM OTHER ABRASIVE GRINDING GRINDING GRINDING GRINDING PROCESSES 1) Automatic 1) Pedestal 1) Cylindrical 1) Thread 1) Polishing Steel Conditioning 2) Bench 2) Surface 2) Gear 2) Lapping 2) Swingframe 3) Portable 3) Internal 3) Profile 3) Honing 4) Cutting-Off 4) Centreless 5) Drill Fluting 4) Superfinishing 5) Floor Polishing 5) Tool & Cutter 6) Spline 5) Sanding 6) Saw Gumming 6) Roll 7) Track 6) Tumbling 7) Crankshaft 8) Cylindrical 7) Electro-Chemical 8) Disc Angulare grinding 9) Knife (Wheal head) 10) Camshaft

1 ABRASIVE CUTTING very carefully from a somewhat bewildering Fig. 1 variety which are available. The five main para- meters in abrasive product specifications are : 1. The type of abrasive 2. The grit size 3. The grade (or hardness) 4. The structure 5. The bond Each of these parameters can be varied over a wide range as shown in table 2. The initial choice, based on operating factors, require specialised knowledge and experience and is best left to the abrasive specialist. However, unlike other tools, the performance of abrasive time, the point‘B’ on the job will advance to the products tends to be more susceptible to point ‘C’ along the arc ‘BC’. Thus a portion of variations in operating conditions. As such, there material equal to the shaded triangle ‘ABC’ will is a definite need to optimise speci-fications on be removed by the grain. The arc length ‘BD’ the shop floor by conducting proper trials. The can be equated to the chord length ‘BD’ since shop engineer should be in a position to the segment is very small. Then the arc-of- understand grinding problems and arrive at contact ‘AB’ is roughly equal to the length ‘BE’. optimum solutions. This requires a basic From elementary geometry it can be seen that: knowledge of the grinding process. BE2 = BF2 – FE2 = (FA)2 – (FA – EA)2 2 IV. MECHANICS OF THE GRINDING = 2.FA.EA – EA PROCESS = EA (2FA – EA) = EA.D 2 Basic research in metal cutting invariably starts By analogy BE = EH.d with the mechanics of chip information, because \ t = EA + EH = BE2 + BE2 this helps to clarify the environment in which Dd the cutting tool functions and the significance BE = tDd = AB ... Equation (1) of various process parameters. The grinding ÖD+d wheel is a composite tool consisting of many thousands of cutting edges (grains) oriented in The above approximate expression shows that haphazard directions and therefore, does not arc-of-contact increases with the increase in any lend itself to easy analysis. However, a simple of the values ‘t’,’D’ or’d’, i.e. with an increase in analysis is possible if the grinding wheel is the depth-of-cut, diameter of wheel or job. considered as a milling cutter and the abrasive Each individual grain removes a triangular grains as the individual teeth. This may be an shaped chip and the depth-of-cut, increases over-simplification, but it does help in from zero to a maximum of ‘CJ’ along the arc- understanding the importance of the process of-contact. With sufficient accuracy the points parameters at least in qualitative terms. ‘C’ and ‘J’ can be shifted on to the tangents at the point ‘B’. Then we have: A simple mathematical analysis for the case of CJ = BC Sin ( + ) plunge cut cylindrical grinding is given in Fig. q f 1. Let us assume that the grinding wheel is fed Each grain could have a maximum depth-of-cut into the job to a radial depth of ‘t’. An abrasive equal to ‘CJ’ only if individual grains on the grain on the wheel periphery will take time ‘T’ to wheel periphery, were spaced at a distance cover the arc-of-contact ‘AB’. During the same equal to the arc-of-contact. In actual conditions

2 ABRASIVE CUTTING the grains are much closer together and the increased without increasing ‘a’ to a level at individual depth-of-cut is much less than ‘CJ’. If which the cutting forces would be we assume for simplicity that the grains are prohibitively high. The obvious way to equally spaced and that there are ‘m’ grains per achieve this result is to compensate units length, then the number of grains along increase in ‘t’ by increasing the wheel the arc-of-contact will be ‘m’. ‘AB’. The average diameter and speed. This is why there is a depth-of-cut of each grain ‘a’ is therefore, trend in modern grinding practice to continuously increase wheel size and speed. a = CJ = BC sin (q + f) m.AB AB m For example, crankshaft wheel sizes have steadily risen from 600mm to 1,200 mm The arc lengths ‘BC’ and ‘AB’ are the distances diameter over the last few decades. Again covered by points on the job and wheel surface vitrified wheel speeds in the ball bearing in the same time ‘T’ substituting for the arcs in industry have risen from 30 m/sec to 80 m/ terms of diameter and r.p.m. we have: sec. and in some cases upto 120 m/sec. a = p dnT sin (q + f) b. Use greater depth-of-cut and higher work- p DNT m speed to compensate for ‘hard cutting or action’. The so-called 'hard cutting action' occurs when the operating parameters are a = dn sin (q + f) DN m too mild to enable abrasive fracture and shedding of dull grains. Increase in depth- ...Equation (2) of-cut and job speed enhances the cutting Using the expression for ‘BE’ from (1) and forces and facilitates normal wheel wear. For neglecting small quantities of the second order example automatic grinding cycles are we can derive an expression for sin (q + f). specifically selected in such a way that the The equation (2) can be brought to the form roughing feed is high enough to cause wheel wear (and expose fresh sharp cutting a = 2 dn Ö1 + 1 Ö t edges) while the finishing feed is low enough m DN D d to cause a slight dulling of the cutting edges ...Equation (3) which improves surface finish. An obvious corollary of the above rule is to decrease the The above expression for the average grain depth-of-cut and work-speed when the depth-of-cut ‘a’ is extremely useful in wheels act ‘soft’. understanding the effect of various process parameters. It is important to appreciate that the c) Use finer grit sizes for better surface grain depth-of-cut characterises the peak to finish and harder job materials. The finer valley height (i.e. surface finish) of the ground the grit size of the wheel, the more are the surface and also the magnitude of cutting forces number of abrasive grains and bigger the acting on the grain. The magnitude of cutting value of ‘m’. It is quite clear from the equation forces has a crucial bearing on the break down that this decreases the value of ‘a’ and of the abrasive grains and bonding material and results in a better surface finish. At the same thus, determines the extent of the wheel wear, time, the cutting forces are prevented from glazing, heat generation, etc. If these points are increasing because the lower the depth-of- kept in view, one can arrive at the following cut compensates the increased sheer important conclusions on process parameter resistance of harder materials. It should be selection and on ways of resolving various noted that at an increase in bonding strength grinding problems: (hardness) can also help to withstand the higher cutting forces on harder materials but a) Use larger wheels and higher speeds for there is a danger of retaining blunt grains faster production. It is evident that faster too long and burning the job. production is possible only if ‘t’ can be

3 ABRASIVE CUTTING

Thus, it will be no exaggeration to say that the b. Cutting Forces mathematical analysis of grain depth-of-cut is The question of cutting forces is particularly an invaluable aid in understanding the day-to- important in precision grinding operations day problems in the Correct Selection and Use since it is linked to the problem of achieving of Grinding Wheels. the desired accuracy and surface quality. V. CHARACTERISTIC FEATURES OF THE Various theoretical methods have been GRINDING PROCESS suggested to determine the magnitude of cutting forces based on the mechanics of a. Heat and residual stresses chip removal and properties of the material A characteristic feature of the grinding being ground. However, these analytical process is the extremely high temperature methods are perforce, approximate as the in the cutting zone. This fact will be tool geometry and other parameters are not immediately evident even to a casual constant as in the case of milling, turning observer from the shower of sparks (which etc. Investigations have shown that the are actually molten globules of metal) flying abrasive grains in a grinding wheel act as off from the job. The reason for this miniature cutting tools with an average phenomenon lies in the nature of chip negative rake angle of 45o. This factor in formation as discussed earlier. The abrasive conjunction with the unfavourable grains usually have a large negative rake orientation of the grains and low depth of angle and a considerable radius at the cut, results in high normal (radial) forces cutting edge when compared to the depth- during grinding. Thus, for example, the ratio of-cut. Therefore there is a tendency for the of the normal ‘Py' to the tangential grains to initially slide over the metal (before component ‘Pz’ of the cutting force is usually ‘biting’ into it), and in the process to generate in the vicinity of Py/Pz=2 during grinding intense frictional heat at the point-of-contact. with a free cutting wheel. This ratio increases Actual measurements have shown that as the wheel glazes and may reach a value instantaneous temperature of 1,200o K and of 4 to 5. In comparison turning with a tool higher are reached. The poor thermal having 15o positive rake angle and 45o plan conductivity of the wheel and the short angle would give a ratio Py/Pz-0.4. In other duration of heating combine to concentrate words, the nature of distribution of cutting the heat generated in a thin surface layer of forces in grinding differs radically from other the work piece. The high local temperatures metal-removal processes. This poses and quenching action of the coolant result various problems in designing the machines in changes in the metal microstructure on and achieving the necessary levels of the surface and leads to the formation of accuracy. large residual stresses. In extreme cases c. Power and energy requirements these stresses can cause metal failure in the form of cracks, etc. On the other hand, it is The material removed in grinding is in the important to appreciate that grinding form of a very large number of minute parameters can be varied to induce particles, as opposed to the substantial large moderate residual streees, which are chips formed during other machining beneficial in terms of the wear resistance processes. This results in considerably and the static and the fatigue strength of the higher specific energy requirements. parts. This aspect of the process which can Moreover, the frictional losses are also much help in eliminating subsequent strain higher. As a consequence, the consumption hardening operations (like burnishing etc.) of energy in grinding is much higher than in needs to be further investigated and applied other processes like turning and milling. in practice. Since there is a definite tendency in India to use low-power motors on grinding

4 ABRASIVE CUTTING

machines, it may be worthwhile mentioning Mechanical attrition and adhesion can be the basis of power selection for various reduced by increasing the high-temperature operations. The power of the wheel-head hardness of the abrasive grains, while the motor for operations involving line contact diffusion wear process can be minimised by can be calculated from the equation: the suitable selection of abrasives having the W = Pz.V.B least chemical affinity to the material being 102 cut. This is the reason why aluminium oxide Where abrasives are used on steels and silicon carbide abrasives on non-ferrous material. W – Power in KW Wear can also be reduced by using cutting Pz – tangential cutting force in Kg.per.cm fluids with high pressure and chemically width of wheel face active additives which reduce friction and V – Wheel speed in m/sec. form a protective coating on the interacting B – Wheel width in cm. surfaces. Thus, the power needed is a function of the ii. Chipping of the edges. wheel size and speed as well as the unit cutting forces. For conventional grinding This can be minimised by the use of tougher applications, the wheel speed are 30-35m/ grains having stronger shapes (blocky as sec. (for vitrified wheels) and 48m/sec. (for against acicular). Considerable research resinoid wheels). The following table will give work in this area has led to the development the idea of the value of ‘Pz’ for various types of a large variety of synthetic abrasive of machines: products tailor-made to suit specific grinding operational requirements. Type of machine Pz (Kg/cm) iii. Failure of the bonding material and Portable Bench and tool release of abrasive grains. grinding 1 This happens due to the erosion of the bond Cylindrical, Surface Grinding 2-3 material between the grains; softening of Pedestal Grinding 3-4 organic bonds due to heat in the cutting zone; fracture of the bonding material under Swingframe Grinding 5-6 repetitive shock loads; and cyclic stresses An important corollary of the above set-up in the wheel body due to elastic expression for power is that their higher deformation of the wheel, etc. These forces speeds can be advantageously used to are being combated by the use of improved reduce the cutting forces while increasing bonding materials which have better the power and material removal rate. adhesion and compatibility with the grains, and higher strength and heat resistance. d. Wheel Wear Since grinding wheels are consumed rather Tool wear is an universal feature common rapidly, a correct selection of specifications to all machining processes, but nowhere is possible only if wheel wear can be does it reach such high levels as in grinding. quantitatively evaluated. For this purpose, it As a consequence, wheel cost is a major is conventional to determine the grinding factor in the overall picture of grinding costs. ratio which is the ratio between material Grinding wheel wear occurs mainly as a removed and wheel-wear. This ratio is a result of the following phenomenon : valuable aid for the comparison of wheels provided due care is taken to keep all the i. Wear of the cutting edges. other operating parameters constant during This is due to mechanical attrition as well as the test. due to adhesion and diffusion processes. This provision is important because changes

5 ABRASIVE CUTTING in operating conditions can drastically alter and exposing a layer of fresh, sharp the grinding ratio and lead to erroneous cutting edges. conclusions. 2. removing the surface layer which has e. Cutting efficiency been clogged with chips; and The cutting efficiency of a grinding wheel is 3. restoring the wheel form. the ratio of the material-removal-rate to the The conventional dressing methods using normal (radial) component of the cutting crush dressers, diamond tools, crusher rolls force. The necessity of considering the and grinding wheels or sticks are too well material-removal-rate is obvious whereas the known to require elaboration. However, it is reason for considering forces may not be worth mentioning that there is a widespread so evident. Actually the force factor is equally tendency to remove excessive material important since it is correlated with such during dressing, even though a cut of parameters like dimensional, geometrical 0.05mm to 0.1mm (total for all passes) is accuracy, surface quality, power usually sufficient for restoring cutting consumption etc. It has been found efficiency. An interesting development in the experimentally that the above ratio (cutting dressing field is the introduction of diamond efficiency) decreases exponentially between rollers for the continuous dressing of wheels dressings of the grinding wheel. This, of on automatic machines. The economics of course, mainly applies to precision grinding this method are under investigation, but operations and not to snagging operations there is no doubt as to its advantage in high- where self dressing wheel is used. The precision grinding. decrease in cutting efficiency usually results in increased cutting forces as the feeds, VI. JOB QUALITY IN PRECISION which govern the material-removal-rate, are GRINDING. usually maintained at the constant level. Increase in the cutting forces, in turn, results The quality of jobs produced by precision in increased power consumption, greater grinding operations are characterised by deflections of the machine spindle and work- accuracy, surface finish and surface quality. piece (causing errors of form), higher heat Increasing sophistication in the ball-bearing, generation (causing burns, cracks and automobile, instrumentation, aero-space and dimensional inaccuracy due to thermal other high-tech industries has resulted in a expansion), and vibrations (causing chatter reduction of production tolerances to a few marks and poor surface finish), etc. microns and fractions of microns. It is evident that such accuracy is unattainable with ordinary It is usually difficult or impossible to manually operated machines. Therefore, the determine grinding forces, except in trend abroad is to use in-process gauging to laboratory conditions. Therefore, for all ensure accuracy under mass-production practical purposes, it is preferable to conditions. It is worth emphasizing that evaluate cutting efficiency as the ratio of the numerous models of simple mechanical and material-removal-rate to the power pneumatic in-process gauging systems are consumed. This is simple and merely readily available at low cost and can be requires the use of a watt meter in the motor advantageously used for production grinding. circuit. These systems offer a viable alternative in many f. Wheel Dressing. cases to the imported electronic systems whose high cost seems to have acted as a deterrent to The wheel dressing operation during wide-spread use. Surface finish and quality are grinding is the equivalent of tool sharpening particularly important in grinding. It is worth and has the objectives of: discussing here the factors which affect surface 1. removing the blunt grains from the wheel finish in grinding.

6 ABRASIVE CUTTING

i. Spark-Out dressing operation is the feed. In one During grinding, the technological system experiment, for example, change in the comprising o the work-holding fixture, job dressing feed from 500 mm / min to 100 mm and wheel are subjected to deformations / min while grinding with a 46 grit wheel under the action of the cutting forces. Thus, resulted in improving the surface finish from for example, the job deflects to a 1.2 microns to 0.3 microns Ra. All other considerable extent in cylindrical grinding parameters were, of course, kept constant. while the wheel spindle deflects appreciably iii. Grit Size in internal grinding, etc. Even if the wheel The influence of grit-size on surface finish is feed is stopped at a given moment, material well-known. However, the popular belief that removal continues due to the gradual ‘fine’ grit-size is the solution to all surface decrease in the value of deflection of the finish problems is not justified. The earlier various parts. This process of material analysis of the chip-removal process shows removal without in-feed of the wheel is that a finer grit-size does result in a smaller referred to as ‘spark-out’. Research in grain depth-of-cut and therefore, in an grinding has shown that the rate of material improved finish. But it should also be noted removal and depth-of-cut decrease that a fine grit wheel removes material slowly gradually (exponentially) during spark-out. and increases in the feed-rate can cause The final value of the depth-of-cut prohibitively high cutting forces. approaches zero. It is evident, therefore, that the depth of scratches left by individual iv. Cutting Parameters grains is extremely small, resulting in a very Cutting parameters influence surface finish good surface finish. in the following ways: A point which is often overlooked is that a) Traverse cylindrical grinding gives a spark-out should not be continued too long. better finish than plunge grinding. This It has been established that the surface finish is because the leading edge of the wheel improves during spark-out upto a maximum during traverse grinding removes a point and then starts deteriorating. This is greater portion of the material and the because low-amplitude vibrations due to trailing portion works with a very small imperfect balancing etc., begin to have a depth-of-cut and produces a good finish. detrimental effect when the depth-of-cut falls Moreover, reversal of the job movement below a certain value (at greater depths of at the end of each pass, results in cut the effect is negligible). intersecting scratch-lines which ii. Method of Dressing facilitates the elimination of peaks on the profile. For the same reason modern The influence of the dressing operation on machines have a wheel oscillating device the surface finish is extremely high. Even a for use during plunge grinding. fine grit wheel if coarsely dressed, can result in a very poor finish. The dressing operation b) Increase in wheel speed improves helps to produce an even surface on the surface finish because each unit area of wheel and precludes the possibility of the job surface comes in contact with a individual grains protruding excessively from greater number of grains. Besides this, the surface. Moreover, diamond dressing at the depth-of-cut of each grain decreases very slow feeds results in a generation of as the speed increases and this also flat surfaces on the individual abrasive contributes to a better finish. grains. Both these characteristics of c) Decrease in the work-speed has an effect diamond dressing result in better finish. similar to increase in wheel-speed and The most important parameter of the improves surface finish.

7 ABRASIVE CUTTING v. Bond Material VII.AUTOMATION OF THE GRINDING During grinding, the bond material between PROCESS the abrasive grains rubs on the job surface. The grinding process has been traditionally Organic bonds, being non-crystalline and used as a means of achieving mass- relatively soft, have a polishing effect, which production scales with high accuracy and improves surface finish. Hard crystalline surface finish on precision components. vitrified bonds on the other hand tend to These operations have for long been the scratch the job surface and impart a poorer exclusive domain of highly skilled operators. finish. The polishing action is best with However, the trend towards mass- rubber bonds. In this case the resilience production and sophistication in industry has (compressibility) of the bond prevents the necessitated the development of automatic individual grains from penetrating too deeply machines which do not depend on skilled into the work and eliminates the possibility labour. Development work has been of deep scratches. concentrated on in-process gauging vi.Cutting Fluids systems and automatic cycles, which are peculiar to the grinding process, as well as The influence of cutting fluids on surface on the mechanisation of loading and finish is comparatively small. It has been unloading operations. In-process gauging found that fluids with higher lubricating is perhaps, the most important prerequisite properties (e.g. cutting oils) give a better for automation as it eliminates the frequent finish. The reason lies in a reduction of the measurements (and consequent loss of plastic deformation taking place in the time) which are necessary during manual cutting-zone and other complicated operations. The enormous variety of phenomena. A point to note is that the electrical, pneumatic, electronic and digital cleanliness (absence of suspended read-out in-process gauging equipment particles) of the fluid is extremely important. which are presently available, precludes the Poor filteration of the fluid can lead to fishtail possibility of any serious discussion within marks, scratches and bad finish, even in the the scope of a short article. It should, best of conditions. This can also stick and however, be mentioned that the purpose of clog up the pipe resulting in an inadequate in-process gauging is not merely to achieve quantum of coolant flow. the required work-piece size, but what is The above analysis should be considered equally important, to also give intermediate in its proper perspective. The importance signals for controlling the multi-step attributed to any one particular factor, is only automatic grinding cycles. relative and can change depending on the a. Automatic Grinding Cycles operating conditions. Thus, it would be feasible to use a medium grit wheel with a The accuracy and productivity of the long spark-out for getting good finish on a grinding process depends to a great extent general purpose machine. For mass on the use of automatic grinding cycles. The production it would be advisable to split logical considerations which have formed operations into roughing and finishing, the basis for development of grinding cycles carrying out the latter operation with a fine are as follows : grit wheel. Operations requiring extreme i. Minimum time should be wasted on non- precision would necessitate critical attention productive cycle elements like approach to all the factors mentioned above. A and initial contact between work-piece balanced approach to surface finish and wheel. This is extremely important problems is preferable to any rule-of-thumb. as the low feed-rates used in grinding

8 ABRASIVE CUTTING

can lead to considerable time-loss if the iii. The material-removal-rate is theoretically rapid feed motion of the wheel-head is limited by the necessity of avoiding a not corelated to the blank size. For defective surface layer (burns, cracks example, the radical machining etc.) on the finished work-piece and allowance within a batch may vary ensuring the requisite geometric between 0.2 and 0.5 mm, and accuracy and surface finish. Since the consequently, at a roughing feed of 1.5 depth of penetration of surface defects mm/min. there would be an extra depends on the material-removal-rate, unproductive approach time of 12 secs. the optimum cycle would be one where for the smaller blanks as compared to the material-removal-rate is built-up the larger blanks. Such time losses can rapidly to a high value and then be minimised by using rapid plunge decreased gradually in such a way that feeds of 5-10 mm/min., to cover the gap the instantaneous depth of surface between the moment when the fast defects equals the instantaneous value hydraulic approach stops and cutting of the grinding allowance. The rate of starts. The change to roughing feeds is material removal at the end of the cycle actuated by a current relay in the motor should ensure the desired surface circuit. quality. ii. The technological system comprising the iv. The technological system also imposes machine-fixture-tool-workpiece has a certain restrictions on the material- finite rigidity and tends to deflect removal-rate as the horse-power and gradually till stable cutting conditions are deflections cannot be increased established. Thus, for example, the indefinitely to meet the theoretically cutting forces and material-removal-rate optimum, productivity limits. This increase gradually (exponentially) from necessitates the use of stepped cycles the moment when the wheel (at constant which are optimum within practical in-feed) touches the work-piece. There constraints (Fig. 3, curve II). is a considerable period of low-intensity Fig. 3 material removal before the wheel can remove metal at a rate commensurate with the selected roughing feed. This period can also be reduced by using rapid plunge feeds for the fast build-up of force and material removal and then switching over to the appropriate roughing feeds (See Fig. 2). Fig. 2

v. There are further constraints in designing mechanisms and feed-back control systems for varying the feed continuously on the basis of values of instantaneous machining allowances. Therefore, in the interests of simplicity and reliability, a ‘spark-out’ stage is sometimes preferred (Fig. 3, curve III).

9 ABRASIVE CUTTING

vi. During initial development work of consumed and inversely proportional to the grinding cycles, the last cycle element cutting speed. Therefore, the obvious way was usually ‘spark-out’, i.e.: in-feed, was to increase material-removal-rate is to stopped (or reduced) and further increase the available power and cutting material removal occurred due to a speed simultaneously so that cutting forces spring-back action of the system. are kept at a relatively low value. This However, research showed that the approach which is termed as “Abrasive ‘spark-out’ process was unstable Machining” has been successfully adopted because the magnitude of cutting forces in the latest range of precision grinding (and material-removal-rate) fluctuated at machines used in the ball bearing, the moment of wheel retraction due to a automotive and other high-tech industries. variation in cutting efficiency, etc. This Some typical examples include the new results in size and surface quality models of Newall crank-shaft grinders, variations. The development of ‘Famir’ track grinding machines, Cincinnati sophisticated mechanisms which reduce Twin-Grip centreless grinders, Norton High- the ‘stick-slip effect’ has made it possible Speed cylindrical grinders, etc. These to achieve the controlled low feed-rates machines can finish grind parts from blanks which are necessary for getting a fine made of bar stock, forgings or castings surface finish. Therefore, modern without any preliminary machining. automatic machines usually incorporate In the semi-precision grinding field there are multi-step feed cycles without spark-out numerous jobs which are rough machined (Fig. 3, curve IV). This has the advantage and ground to obtain accurate locating of using relatively simple electrical relay surfaces. A significant proportion of such control systems. A reverse-feed stage is jobs are cast housings which pose incorporated before the finishing-feed considerable problems in conventional stage so as to reduce the deflections in machining because of the hard cast surface the system and enable a constant and sand inclusions, etc. This is an area in material-removal-rate immediately prior which ‘abrasive machining’ concepts have to the retraction of the wheel. This paid significant dividends. A large number precaution eliminates any size variation of segmental surface grinding machines with due to a delayed response of the power ratings of 100-150 HP and more are gauging device and also results in stable available which can effectively replace values for surface finish. milling and planing machines. The advantages gained from this changeover are VIII. RECENT TRENDS IN THE both in terms of unit costs and productivity. DEVELOPMENT OF ABRASIVE The grinding process has gradually attained CUTTING a pre-eminent position in the steel The basic trend in the development of conditioning field. Concerted efforts by abrasive cutting processes has been to find machine and abrasive manufacturers ways of improving the rate of stock-removal (mainly Norton Co., USA and Centromaskin so that abrasive processes can replace Co., Sweden) have resulted in the conventional machining methods. development of automatic steel conditioning equipment incorporating such features as a In the precision grinding area, the main 300 HP motor, wheel speed of 80m/sec. limitation on the rate of stock-removal is the undesirability of excessive cutting forces. automatic control of speed, power etc. The From the equation for power mentioned wheels used for this process are made with earlier, it is evident that the cutting forces the sophisticated zirconium oxide abrasive and a special hot-pressed resinoid bond. are directly proportional to the power

10 ABRASIVE CUTTING

A significant trend in the recent years has IX. CONCLUSION been the introduction of diamond, cubic Abrasive cutting processes have reached a boron nitride (popularly known as Borazon) stage of development where they offer a abrasive product for a wide range of viable alternative to most other conventional precision applications. These two new cutting methods. Therefore, it is imperative abrasives deliver a dramatic improvement that production engineers keep abreast of in performance vis-a-vis conventional developments in this field. Needless to say, abrasives when grinding some special the abrasives industry in India is making materials like carbides, ceramics, high-alloy every effort to supply quality products tool steels, granite, marble etc. It is expected conforming to the best in international that these two abrasives will gradually standards. replace conventional abrasives in many precision grinding operations.

11