Requirements for the use of diamond roller dressers

Transcription

1 STEINMETZ Schleiftechnik An den Hirtenäckern 2 D Karlstein / M Germany Phone / / Fax / info@steinmetz-schleiftechnik.de Internet: Requirements for the use of diamond roller dressers Dressing and profiling of grinding bodies by means of diamond rollers is increasingly gaining in importance in the field of grinding technology. Due to the economics to be achieved, this process is used not only industrial scale manufacture, but also in small-lot production, where it is more and more accepted thanks to short retooling times and easy handling. This paper deals with the specific requirements to be taken into account regarding the grinding machines, in particular the diamond roller device, the various characteristic quantities as well as the functional operation of the dressing process, furthermore the design criteria of the dressing devices which influence these characteristic quantities and, consequently, the ultimate result of the dressing process. M. Steinmetz is the owner of a design office with engineering facilities for grinding and measuring technology in Karlstein. 1

2 1. Introduction Quite particular requirements must be met in order to ensure an optimal dressing result with diamond roller dressers. The major factors of influence to achieve an optimization of the dressing operation are as follows: speed ratio, feed rate, dwell time, direction of rotation, radial forces, type and quantity of diamonds used. These parameters and their effects on the dressing or grinding result respectively are described in detail, and recommendations are submitted to achieve an optimization. Naturally, particular requirements must be met in terms of the design criteria of diamond roller dresser devices which are initially described in a general way and, in conclusion, illustrated in detail by means of various designs. 2. Speed ratio The speed ratio between the diamond roller equipped with ist own drive and the grinding wheel to be dressed (qr = vr/vs) is a kinematic adjustable quantity which has a decisive influence on the maximum peak to valley height of the cutting area of the grinding wheel, in particular in the case of dressing operations with the roller dresser and the grinding wheel rotating in the same direction. In the case of a ratio qr = 1 (theoretical extreme case), that means when grinding wheel and diamond roller have contact with each other while rotating in the same direction at the same peripheral speed, the result is a crushing effect and, under pressure, a rolling-in effect of the roller profile into the grinding wheel. In this way, the diamond grit will cause, with acute effective path configurations and short meshing distances, a splintering and crushing of the grit structure of the grinding wheel, leading to a roughening effect over wide areas into great depth. Consequently, a maximum peak to valley height effect on the grinding wheel is achieved, ensuring a high rate of cutting. If the speed ratio is changed, for example to qr = 0,95, the result will be a relative speed between the contact surfaces of the roller dressers and the grinding wheels and, consequently, an abrasive effect. There is still some measure of a splintering and crushing effect of the grit structure of the wheel, this effect makes itself felt, however, only into slight depths and across minor zones. The maximum peak to valley height of the cutting surface of the grinding wheel is consequently reduced and, as a result, the surface quality of the workpiece will be improved. The more the circumferential speed of the diamond roller is reduced to the one of the grinding wheel, that means the smaller qr is chosen, the broader will the effective path configuration become with increased interacting overlapping effect; the abrasion effect becomes a grinding process. As a consequence, there will be a reduced maximum peak to valley of the cutting surface of the grinding wheel, a reduction of the grinding performance of the grinding wheel and an improvement in the surface quality of the workpieces. 2

3 Practical experience has shown, however, that the value qr should not become any smaller than approx. 0,25 if the grinding wheel and the roller dresser rotate in the same direction because, in the other case, there is a risk of burning. If the speed of rotation ratio is considerably reduced, this will, on the other hand, mean both in the case of rotation in the same and in the opposite direction with an almost stationary diamond roller with great meshing distance of the diamond grit that the path configurations or meshing trails respectively in the circumferential direction will become very prominent because there is only very slight overlapping or none at all. The cutting surface of the grinding wheel will show grooves and elevations. This critical range between qr = 0,25 with both devices running in the same direction and qr = 0,10 with the devices running in the opposite direction must, consequently, be avoided. When the grinding wheel and the roller dresser come into contact while rotating in the opposite direction, an overlapping effect of the meshing traces is achieved much faster. This leads, as opposed to the rotation in the same direction, within a shorter time to a smaller final maximum peak to valley height of the grinding wheel surface, and this fact can be made use of to achieve at a reduced cutting performance a higher workpiece surface quality. Basically, however, it must be stated that a major improvement of the maximum peak to valley height cannot be realized by altering the speed ratio qr, when roller dresser and grinding wheel rotate in the opposite direction. Tests carried out by Pahlitzsch and Schmitt, Technical University, Braunschweig, Federal Republic of Germany, have shown this very clearly. The above-mentioned statements give rise to the requirement that diamond roller dressing devices should be equipped with a drive the speed of which is continuous adjustable so that it is ensured that a corresponding alteration of the speed ratio qr can purposefully be utilized to influence the maximum peak to valley height of the grinding wheel surface to be dressed as well as the workpiece quality. In this connection, it is imperative that the speed of rotation must be all means remain constant, even while the device is subjected to different loads. 3. Feed The feed of the dressed off quantity for each rotation of the grinding wheel has an effect in a respective proportion on the meshing depth. This will consequently result in an interrelation between this parameter and the maximum peak to valley height of the grinding wheel which can be regarded without a dwelling period being taken into account as a proportional interdependent phenomenon. Since the feeding quantities per grinding wheel rotation should be in the range from 0.1 to 1.5 µm, the requirements in terms of precision for the dressing devices are extremely exacting. Another requirement is determined by the fact that the magnitudes of the feed involved should be adjustable if this function is not axecuted by the grinding machine, as for instance in the case of an arrangement of the dressing device on the gearless head of a surface grinding machine or cylindrical grinding machine. If the dressing device is, however, arranged on the machine table, an extra feeding mechanism of the machine is not required. 3

4 4. Dwell time The dwell time, too, has a considerable influence on the final maximum peak to valley height of the cutting area of the grinding wheel. If a diamond roller is retracted from the grinding wheel immediately after the dressing pass, the result will be due to the lower number of diamond grit meshing processes a smaller amount of overlapping and, consequently, a high degree of peak to valley height. In contrast, this maximum peak to valley height is reduced if more dwell rotations become effective, the diamonds will not penetrate deeper into the grinding wheel surface, but they will continuously level it due to the overlapping effect of several diamond grit mesh effects on the same cutting surface elements. The maximum peak to valley height can, in this case, fall off so heavily that during grinding a cutting performance no longer is achieved. On the other hand it is a fact that these phenomena will, in the course of grinding, naturally lead to smaller peak to valley heights and, consequently, to improved surface qualities at the workpiece which will however again result only up to the extreme condition where burns will appear. Particular caution is advisable, in this connection, when dressing with grinding wheel and roller dresser rotating in the opposite direction, which method serves to produce a low maximum peak to valley height within a considerably reduced time than is the case when dressing with grinding wheel and roller dresser rotating in the same direction. In most cases, therefore, the dwelling cycle will become unnecessary with rotation in the opposite direction. If dressing is to be carried out with grinding wheel and roller dresser rotating in the same direction, the dressing device is to be equipped in such a way that, subsequent to the dressing cycle having been carried out with preselected feeding rate, a dwelling period can be preselected. Such a control device which can be actuated electrically via a timing circuit or by hydraulic means is again to be intalled only for these devices if the machine is not equipped with such a control. It should be mentioned, furthermore, that the diamond roller should be retracted with fast speed as far as possible (approx. 10mm) and be brought back for the next dressing cycle, to save time, again with fast speed as close as possible to the grinding wheel. The feed-in phase can simply be incorporated in the automatic working cycle of a grinding operation, whereas the other dressing parameters are pre-selected. These adjustment criteria can, however, be corrected also during the grinding operation, of for example the surface quality of the workpieces to be ground or the machine performance is to be altered by means of the cutting speed of the grinding wheel. 5. Direction of ratation The significance of the direction of rotation of the roller dresser that means the different dressing conditions when grinding wheel and roller dresser rotate in the same or opposite direction has been dealt with above. 4

5 The dressing period when wheel and roller rotate in the opposite direction, is very short. Due to the wide effective path configurations and the small meshing distances, it is possible to quickly achieve a large overlapping effect of the meshing trails. This will lead to small final maximum peak to valley heights of the cutting surface of the grinding wheel, and thus to a high surface quality of the workpiece. The speed ratio qr has in this connection, as has been mentioned earlier, only a small influence. Due to the high relative speed, only a low rotational speed of the roller dresser amounting to 250 to approximately 700 rpm, is required to achieve a dressing effect. This result in various advantages for the method when grinding wheel and roller dresser rotate in the opposite direction, for example in the advantage that the low rotation speed has positive results regarding the necessary quietness of running of the dressing devices. Another advantage to be consideres is the fact that practically no dwelling period must be taken into account because due to the very fast overlapping effect of various diamond grit meshing effects the maximum peak to valley height is quickly reduced already in the course of the feed-in phase. In the extreme case, of course, this may lead to a burning of the grinding wheel. It is therefore advisable to retract the roller from the grinding wheel immediately after the feed-in phase has been terminated. Of great importance are also the radial forces involved. In the case of grinding wheel and dresser running in the opposite direction, these radial forces are considerably smaller than in the case of grinding wheel and dresser running in the same direction. An explanation and amplification of these phenomena will be given in Chapter 6. Good dressing results can still be achieved with machines, bearings and mounting devices whose stability is not quite 100 per cent, if grinding wheel and dresser run in the opposite direction. It should be attempted, though, to construct the machine, the bearings and the complete mounting devices as rigid as possible. What has been said so far may give the impression that dressing in the opposite direction might be easier, posing fewer problems than dressing in the same direction. Notwithstanding some facts have to be taken into consideration which entail substantial drawbacks for the first method (dressing in the opposite direction) compared to the second method (dressing in the same direction). Thus the maximum peak to valley height of the grinding wheel, for instance, can be influenced only to a limited extend when dressing in the opposite direction because due to the fast rate of mutual overlapping of the diamond grit meshing effects, the final maximum peak to valley height of the cutting surface of the grinding wheel to be achieved and consequently the resulting cutting performance will always be small. An increase in performance, that means an increase in the quantity of material removed by the grinding wheel in terms of time as would be desirable for instance for rough grinding is thus hardly possible. 5

6 The useful life of the diamond roller, too, is frequently considerably shortened when dressing with the grinding wheel and the roller dresser rotating in the opposite direction, in contrast to dressing with wheel and roller dresser rotating in the same direction. The effective path configurations of the individual diamond grits are similar to prolonged or shortened hypocycloids. When dressing with the grinding wheel and the roller dresser rotating in the opposite direction, the hypocycloids will be always very long, due to the high relative speed and the meshing effects involved. This will, though, entail the advantage of fast overlapping with wider effective path configurations, but on the other hand entail also the disadvantage of a heavy thermal load condition of the surface elements of the diamond grit which is additionally increased even by the high relative speed. This risk of a thermal load on the diamond roller must be distinctly recognized, so much the more as it is not at all easy to effectively counteract this risk. There is, it is true, a possibility to take remedial measures by reducing the feed per rotation of the grinding wheel, but this simultaneously lead to a further decrease in the maximum peak to valley height of the grinding wheel, and consequently to an increase in the burning risk at the workpiece. It follows that it is necessary, in most cases, to operate with large dressing feed rates, which automatically entails an increase in the thermal load on the diamond roller. In the final analysis, there is only one effective remedy: efficient coolant application. Quite different and considerably more efficient are the different factors when dressing with grinding wheel and roller dresser rotating in the same direction. Here, with smaller relative vrel, the overlapping effect of the meshing trails is smaller, due to the particular meshing kinematics of the individual diamond grit particles with pointed or acute effective path configurations and larger meshing distance. Furthermore, wider effective path configurations with smaller meshing distance can be achieved by an increase in the relative speed vrel, which leads to an increase in the mutual overlapping of the diamond meshing trails. Depending on the selection of the quantity qr, a larger or smaller final maximum peak to valley height of the cutting surface of the grinding wheel can be achieved. It follows from this that a continuous adjustable drive of the diamond roller is very useful for the dressing method with the grinding wheel and the roller dresser rotating in the same direction because it enables, in a simple way, a precise influence on the cutting surface of the grinding wheel and, consequently, of the subsequent operating result. A factor of particular influence is the dwell time in connection with dressing when grinding wheel and roller dresser rotate in the same direction. This dwell time is of importance only for this method, and it has, as has been mentioned before, a substantial influence on the final maximum peak to valley height of the cutting surface of the grinding wheel. Though the feed rate per rotation of the grinding wheel has about the same effect on the result of the dressing process both in the case of dressing with the grinding wheel and roller dresser rotating in the same and in the opposite direction it is easier to control the influence of this feeding rate on the final maximum peak to valley height in the case of dressing with wheel and roller dresser rotating in the same direction than in the case of dressing with their rotating in the opposite direction. The quantity of material removed by the diamond roller, measured in terms of time, when dressing with the grinding wheel and 6

7 the roller dresser rotating in the opposite, is larger than when dressing with their rotating in the same direction, due to the meshing kinematics mentioned earlier. As a consequence, no larger maximum peak to valley height results are achieved on the cutting surface of the grinding wheel when dressing with grinding wheel and roller dresser rotating in the opposite direction with larger feeding rates than when dressing with wheel and roller dresser rotating in the same direction with smaller feeding rates. That means when dressing in the opposite direction, the feeding rate per rotation of the grinding wheel has a smaller influence on the final maximum peak to valley height of the cutting surface of the grinding wheel than when dressing with the grinding wheel and the roller dresser rotating in the same direction, since here a longer period of time is available for the removal of the same quantity of material. This permits a much more differentiated adjustment. As regards the radial forces when dressing with the grinding wheel and the roller dresser rotating in the same direction, a detailed discussion will follew further below. In connection with this method, the radial forces are greater than in the case of dressing with the grinding wheel and the roller dresser rotating in the opposite direction, and they can be absorbed and borne only by a very rigid machine with a stable design and its sturdy means of attachment. In conclusion, a comparison between the two dressing methods with the grinding wheel and roller dresser running in the opposite and in the same direction, will lead to the following results: Dressing with the grinding wheel and roller dresser rotating in the opposite direction requires only short dressing periods and low rotation speeds of the roller dresser; the radial forces involved are small; dwell time can be eliminated; feeding rates must be selected relatively high in order to achieve sufficiently effective peak to valley heights although the influence of these and other adjustment parameters on the final condition of the cutting surface of the grinding wheel is rather insignificant; shorter tool life of the roller dresser due to the high thermal load on the diamond grit peaks; application of efficient coolant particularly imperative. Longer dressing periods when dressing with the grinding wheel and the roller dresser rotating in the same direction, depending on the speed ratio; this offers an opportunity to influence the final maximum peak to valley height more effectively, for example great maximum peak to valley height for rough grinding, or small maximum peak to valley height for finish grinding; the maximum peak to valley height can also additionally be reduced by smaller feeding rates per rotation of the grinding wheel as well as by prolonged dwell times; in the other hand, a very large maximum peak to valley height can be achieved by arranging a very short dwell time or by eliminating the dwell time altogether, as well as by a suitable selection of the speed ratio qr and the feed rate per rotation of the grinding wheel; the diamond roller dressing devices must be equipped with suitable adjustment mechanisms in order to enable an exact preselection of these parameters. On the basis of the conclusions to be drawn from this comparison, it becomes obvious that a mutual overlapping effect to the diamond meshing trails is achieved which is becoming increasingly faster if a roller dresser is used in the opposite direction method, when the diamonds of this roller dresser become increasingly levelled out in the course of 7

8 operating due to natural wear and tear. As from a given quantity of diamond grit levelling effect, a maximum peak to valley height on the cutting surface of the grinding wheel is no longer achieved, the roller dresser has become blunt. With the grinding wheel and the roller dresser rotating in the same direction, however, and with suitably selected parameters for dressing, still very satisfactory dressing results can be achieved with such a roller dresser on an identical grinding wheel. This may be demonstrated at the example of an extreme case. The speed ratio qr = 1, that means grinding wheel and diamond roller being in contact, rotating in the same direction, with the same circumferential speed. This will result in a crushing effect, in which connection profiling is achieved by breaking or crushing the grit particles out of the wheel body. This process results in a very gritty, coarse cutting surface of the grinding wheel. If the circumferential speed of the diamond roller is reduced, then the crushing effect between roller dresser and grinding wheel will be reduced with increasing relative speed and, consequently, the radial forces will also fall off. With increased relative speeds, the crushing effect is ultimately transformed into a grinding effect. It follows, that still very good dressing results can be achieved by means of dressing in the same direction if suitable dressing parameters are preselected, with a roller dresser which already has become unsuitable for the dressing method on the opposite direction. This will, however, necessitate a different diamond roller dressing device. 6. Radial forces The radial forces generated during diamond roller dressing operation are important parameters of the process, as has been mentioned before several times. They are decisive for the relative shifting of the diamond roller and the grinding wheel and can, subsequently, cause profiling defects. Furthermore, any unquietness of running on the part of the roller and consequently the resulting vibrations may become troublesome if the dressing device is not rigid enough. This means that both the grinding machine and the diamond roller dressing device must absorb the radial forces in order to ensure a statisfactory dressing result. It is of great importance to design both the bearings of the diamond roller and the complete assembly of the machine itself particularly rigid and robust. It becomes necessary, in this connection, to exceed partly the dynamic load bearing capacity of rolling bearings, as normally indicated by the rolling bearing industry, however not so much as regards the dynamic permissible values but rather regarding the plastic deformation of the rolling bearing bodies and their races. The radial forces becoming effective during the dressing process are varying in the course of the dressing operation and may amount up to 6 kp/cm width of meshing. With varying radial forces of this magnitude, any plastic deformation in the mirco range, due to the resilience involved, may have negative results on the cutting surface of the grinding wheel. It becomes necessary, for this reason, to pre-tighten the rolling bearings very well. Since rather high radial forces are generated particularly in the course of dressing with grinding wheel and roller dresser rotating in the same direction with the added necessity of having to operate simultaneously with higher rotational speed of the roller dresser it is advisable to use an increased number of rolling bearings instead of extremely hihg pretightening the bearings, in order to prevent in this way the effects of a plastic 8

9 deformation. The same principles apply for the complete design of the diamond roller dressing device, particularly as regards the guiding rails of the carriage and the feeding mechanism and, last but not least, the main body as well as the fastening device for the attachment to the grinding machine. In order to justify these requirements which are sometimes regarded as exaggerated, it may be advisable to discuss the origin of these forces in more detail: When dressing with the grinding wheel and roller dresser rotating in the same direction, the diamond particles mesh under an acute angle into the cutting surface of the grinding wheel. This meshing angle will achieve its maximum value at a speed ratio qr = 1, that means at the relative speed vrel = 0; the only result is an interrelated crushing effect. The diamond particles meet vertically the cutting surface of the grinding wheel and crush the grit structure of the grinding wheel. In such a condition of crushing, a maximum rate of pressure becomes necessary in order to achieve a removal of material. The radial forces required in this connection range around 60 kp/cm mutual meshing width. If the circumferential speed of the diamond roller is reduced, the forces involved are reduced analogously. Removal of material is no longer effected by means of the crushing pressure effect but, with a reduced relative speed, additionally by the abrasive effect on the grit particles, and with an increased relative speed by a grinding effect of these particles by means of the diamond grit of the roller dresser. The meshing angle of the diamonds becomes increasingly more blunt, which will result in a decrease in the radial forces, though these are still considerably larger than in the case of dressing with the grinding wheel and the roller dresser rotating in the opposite direction. In the case of dressing in the opposite direction, the radial forces involved are smaller, because the angles of contact result in a wide effective path configuration and, consequently, an overlapping effect at a faster rate, however at a lower rotational speed of the grinding wheel. Thus the performance of material removal is, in the case of dressing with the grinding wheel and the roller dresser rotating in the opposite direction, substantially larger, and a dressing pressure of a major magnitude cannot build up readily, even at elevated feeding rates. The radial force increases particularly heavily with the feeding motion of the roller dresser when dressing with the grinding wheel and the roller dresser rotating in the same direction because under these circumstances the diamond particles whose meshing effect, due to the acute effective path configuration, is again overlapped only after several rotations must chip off material cross sections from the grinding wheel which result from a corresponding number of rotations of the grinding wheel. This will not only prolong the time required for dressing, but also the feed-in pressure will quickly build up, and will fall off only slowly. Extremely large fluctations of the radial forces will be generated which the device must absorb as completely as possible. It is particularly for these reasons that a very rigid design is in all details required. It is obvious that the grinding machine must also meet these requirements. 7. Details on the type and quantity of diamonds used The influence of the particular diamonds used has been shown in earlier investigations carried out by Pahlitsch and Schmitt as well as in recent papers by Salje and 9

10 Scheidemann of the Technical University Braunschweig, Federal Republic of Germany. According to their findings, there is a close interrelation between the final maximum peak to valley height of the dressed grinding wheel and the profile bearing proportion of the diamond grit particles of the roller dresser. Not only will the initial grinding pass of the roller dresser which is carried out by the manufacturer in order to ensure a maximum truth of running and a profile fidelity have its influence on the profile bearing proportion, but it is also a fact that an increase in the cutting performance is achieved, together with an increase in the meshing surface of the grit. Extensive observations during the application of new diamond rollers have shown that the maximum peak to valley height as regards the Rt values of the workpieces will gradually improve. This readily confirms the fact that has been corroborated by tests that a diamond roller must be reground in order to achieve wider effective surfaces at the diamonds. Furthermore, a larger number of diamond grit cutting surfaces can become effective under identical conditions which again will lead to the fact that widened effective cross sections of the diamond grit particles also ensure an increased overlapping of the meshing effect in axial direction on the cutting surface of the grinding wheel. Such a regrinding, however, is in direct contrast to the age-old usages in connection with single point dressers.in this case it is endeavoured to carry out dressing with a diamond edge as sharp as possible to achieve a favourable dressing result. A blunt diamond will create on the grinding wheel so little of a maximum peak to valley height that the wheel can no longer achieve any removal performance it will burn. For this reason, the diamond is rotated in such a case, to provide a new sharp edge for dressing purposes. If this is no longer possible because all edges have been used up, then the diamond must be reset or replaced by a new one. In this respect, the use of disposable dressers has been very successful. What is being achieved by regrinding the diamond roller, is exactly what is not desired in the case of the single point dresser. The individual diamond grit particles are provided with surfaces by making the sharp edges blunt.these surfaces are particularly necessary in the case of diamond rollers which are used when dressing with the grinding wheel and the roller dresser in the same direction in order to achieve wider effective path configurations and, consequently, a faster mutual overlapping. When dressing with the grinding wheels and the roller dresser rotating in the opposite direction, the individual diamond grit particles can be acute because due to the meshing kinematics described wider effective paths of configuration will be achieved anyway and, consequently, the rapid overlapping effect of the meshing trails will automatically be ensured. It is to be noted, however, that the thermal load involved will lead to a rapid blunting of the intitially sharp edges, due to wear and tear. Not with standing, the diamond roller manufacturer the problem being regarded from this aspect may be well advised to separate the manufacture of diamond rollers on one hand for dressing in the opposite direction, and on the other hand for dressing in the same direction. In terms of diamond concentration, there may also be a difference. For the purposes of dressing in the opposite direction, only a limited number of diamonds is 10

11 needed on the circumference of the diamond roller that means a lower concentration than in the case of diamond rollers used for dressing in the same direction. This may entail even differences in the price structures involved. On account of the interrelations between the various factors such as diamonds used, maximum peak to valley height, removal forces and peak to valley height of the workpiece, a uniform distribution of the diamonds used must be ensured in the case of symmetric profiles. If the feed rate is small, due to the angular position in relation to the feed direction, then the fact must be taken into account in connection with the diamond distribution that with the same feed rate larger maximum peak to valley heights would be achieved. According to Meyer, Hamburg, the useful life of a diamond roller depends on several factors. Though a generally applicable, simple relation cannot be given, a rough classification can be made as follows: Rollers with fine-profiles or threaded rollers to dressing passes; Rollers with close tolerances and stringent requirements as regards waviness and peak to valley height of the workpiece, as well as rollers with complicated profiles - a proximately to dressing passes; Dressing rollers for workpieces with conventional tolerances, profiles and surface qualities Approximately to dressing passes. As a basis for these guiding values, a total feed rate of 0,02 mm per pass is applied. The figures concerning the life of the roller dressers cover large ranges because a multitude of factors influence the actual life of a roller dresser. This will include: rigidity of the grinding machine and the diamond roller dressing device, trueness of the arbour during the dressing operation, specification of the grinding wheel, dressing conditions, diamond grit size and dressing roller design. In conclusion it should be mentioned that the diamond roller should have a diameter as large as possible, because in this connection the same parameters apply as in the case of grinding wheels, that means the larger the circumferential area, the longer the useful life. Any higher purchasing costs are more than outweighed by the prolongation of the useful life. 8. Special requirements regarding diamond roller dressing devices With dressing operations by means of diamond dressing rollers, a trueness of shape + 0,002 mm and less at the workpieces must be warranted, and also very stringent surface quality requirements. It follows that the diamond roller dressing devices must be designed with an even higher precision. A concentric running of the arbour below 0,002 mm in radial and axial direction is a decisive condition precedent to achieving the workpiece surface finish required which may be characterized today, among other things, by a maximum profile deviation of 0,004 mm. Extremely high quietness of running and vibration-free operation, in co-application with high-quality ball bearings and most careful assembly work must be achieved by careful balancing the engine shaft and the roller arbour. It is also important to maintain a constant uniformity of the circumferential speed, even at different directions and magnitudes of load applied. 11

12 With a view to the rigidity required, the diameter of the arbour should, consequently, be selected as large as possibly permitted by the design of the device. As an optimal ratio between diamond roller and arbour, the combination H3/h3 has been successful, by means of which on one hand difficulties with assembly and disassembly of the diamond roller can be avoided, and on the other hand smallest possible deviations of concentricity can be ensured. During assembly of the diamond roller, great care must be taken while observing extreme cleanliness. Both the arbour and the diamond roller bore hole should be washed down, after careful cleaning, with a clean high pressure oil, not only to remove any residual dirt and fibre residues, but also to prevent any corrosion which may possibly be caused during prolonged use of the diamond roller due to temperature fluctuations (condensation water) or penetrating cooling water, and which would cause difficulties for disassembly. During disassembly it must be avoided to subject the roller bearings to hard lateral punches or pressure because this would lead to defects of the sensitive antifriction bearing races. It is advisable to use a suitable pulling-off device or a press-off part. For major diamond roller widths and sets, it is useful to design the device in such a way that the diamond roller can be removed from the device together with the arbour. This will enable a perfect assembly and disassembly outside of the machine, possibly also by the manufacturer of the roller dresser. An extremely sophisticated, well designed sealing is very important for the life of the devices because particularly the roller bearings of the diamond roller will very intensively come into contact with the cooling water enriched with removed grit and particles of the material. Predominantly in the case of surface grinding machines, the table sets are subjected to extreme strains, therefore suitable counter measures must be taken, also as regards the protection of the electrical terminals. It has been mentioned before that an efficient cooling water feed-in at the dressing device is of paramount importance. The cooling effect must be initiated even prior to the start of the dressing operation. Coolant quantity and pressure during dressing should be as large as during grinding. When grinding complicated profiles, and those with high shoulders, the delivery orifice of the spray nozzle should be adapted to the profile as far as possible, in particular during dressing operations when grinding wheel and roller dresser are rotating in the opposite direction. The coolant must be applied at the side where the higher circumferential speed of the grinding wheel draws the coolant into the dressing zone. As has been mentioned furthermore, the means of setting an optimal speed ratio is another important prerequisite regarding the diamond roller dressing device. For this reason, the speed of the diamond roller dressing device should be continuous adjustable. In order to accommodate different roller diameters and to make the necessary adjustments due to the grinding wheel diameter becoming smaller in the course of use, a range of adjustability of 5 : 1 is advisable. For those devices which are arranged on the gearless head of surface, cylindrical or similar grinding machines, an adjustable high-precision mechanism equipped for preselection is required, with the assistance of which infeed and retractive movements with high speed as well as inching infeed movements of the roller dresser in steps from 0.1 to 12

13 1.5 micron per grinding wheel rotation can be carried out in the course of the dressing operation. As has been mentioned before, a maximum dwell time of 0 to 5 seconds is required subsequent to the final infeed impulse when dressing with grinding wheel and roller dresser rotating in the same direction, which fact should also be taken into consideration in connection with the automatic process control. In conclusion, however, the following fact should again be emphasized: The most important requirement made regarding a diamond roller dressing device concerns the degree of rigidity and stability of its design as well as the whole system. All other prerequisites cannot warrant a satisfactory result of the dressing operation, unless these basic requirements are met. 9. Compensation of the quantity of dressing The compensation of the quantity of dressing can be effected in different ways. It is easiest to be achieved in the case of table sets for surface and cylindrical grinding machines. In the case of surface grinding machines, the superior tangential contact line of the diamond roller is adjusted to the finished dimension of the workpiece. Consequently, the lower edge of the grinding wheel is automatically adjusted to the finished dimension line after repolishing. Prior to that, the diamond roller must be adjusted exactly in the transverse direction to the workpiece. Depending in the characteristic features and the type of the lower final point at the gearless head or column of the grinding machine, the lower final point setting device is moved downwards prior to the dressing operation by the quantity of dressing required. Subsequently, either the grinding wheel is moved vertically with the required infeed speed and dwell time against the rotating diamond roller, or the roller dresser is moved along below the grinding wheel. These two dressing processes must not by necessity achieve the same dressing results, that means the same final maximum peak to valley height of the grinding wheel, since the setting parameter can be different. In the case of the table sets, a high precision adjustment in terms of height amounting to a few millimeters only is advisable it offers the possibility to carry out dimensional corrections at the workpieces themselves. In the case of cylindrical grinding machines, a compensation of the quantity of dressing is possible to carry out in a similar way. Here, the front edge of the diamond roller is adjusted to the point which is predetermined by the dimension of the workpiece. Quite in general, the machine table must be exactly positioned for dressing and grinding, when these dressing processes are carried out. This is, admittedly, often very time consuming and causes some trouble. These problems are eliminated if the diamond roller dressing device is mounted on the gearless head of the machine. Then the compensation of the quantity of dressing must be exactly adjusted either by means of the machine infeed with a possibility of a readjustment, or the machine must be equipped with an automatic measuring control device. In the latter case, all that has to be done is the setting of the infeed quantity. The dimension at the workpiece and the requisite control of the machine, such as rough 13

14 grinding, finish grinding, dwell time and retractive movement of the grinding wheel, is then controlled and checked by the measuring control device. 10. Various design of diamond roller dressing devises Depending on the individual manufacturer and type of machine, purpose and use intended as well as type of grinding machine, the designs of the diamond roller dressing devices will vary. The most simple design are table sets for surface grinding machines, which are equipped only with a roller drive and roller bearings. The design which is most frequently used on surface grinding machines is a type with a high precision adjustment in height. In this way it is possible to carry out a dimensional correction at the workpiece if the diamond roller and the finish dimensions of the workpiece are supposed to be at the same level. It is only very seldom that this design is in addition equipped with a transverse adjustment device, rather is such a transverse adjustment effect achieved by means of hardened and ground spacers of different width, which are inserted at the sides between the collar of the roller arbour and the diamond roller, or between the nut and the diamond roller. Of similar simplicity in the design are table sets for circular grinding machines and other production plants of this type. These devices must be equipped with a high precision adjustment mechanism working with highest perfection, since any correction in relation to the workpiece diameter will have double the effect in terms of magnitude. It is advisable, in addition, to equip the device also for coarse adjustment for the various workpiece diameters. The most favourable arrangement of the diamond roller dressing device for cylindrical grinding machines, thread grinding machines etc. is either on top of the gearless head so that the roller dresser can be moved towards the grinding wheel from above, or at the side of the gearless head. In this way, the axis of the roller dresser, grinding wheel and workpiece are arranged in parallel, and the infeed movement is effected vertical to the grinding wheel axis, in which connection the device continues moving together with the gearless head. Infeed and compensation are simpler and surer to be mastered, both as regards the overall design and the manipulation. 14

15 There is, though, one drawback in this arrangement, namely that in the case of high plane surfaces a maximum peak to valley height at the grinding wheel shoulders is achieved that is too small. It is, however, also possible to arrange the device in a stationary position on the machine column, and to move the diamond roller from the rear side towards the grinding wheel. If this type of arrangement is chosen, the grinding spindle head must be precisely arranged also in its rear positioning. With angular infeed grinding machines, the diamond roller dressing device can be used in a similar manner, with the only exception that in this case the grinding wheel is arranged on the opposite side, that means the diamond roller dressing device must be arranged in the opposite direction. In addition, the diamond roller axis must have the same angle in relation to the grinding spindle axis as the grinding spindle axis does in relation to the workpiece axis. The infeed direction runs then vertical to the grinding spindle axis. Only the angular infeed grinding technique and the requisite dressing processes have made it possible to also grind high plane surfaces at workpieces, and to generate a favourable maximum peak to valley height at the shoulders of the corresponding grinding wheel cutting surfaces. 15

16 STEINMETZ Schleiftechnik Präzisions Abrichttechnik Nutzen Sie unser Know-how! Precision Dressing Technology Make use off our Know-how! Technische Änderungen vorbehalten Subject to technical alterations An den Hirtenäckern 2 D Karlstein / M Phone / Fax / info@steinmetz-schleiftechnik.de Internet: 16