1、13FTM05 AGMA Technical Paper CubitronTMII: Precision Shaped Grains (PSG) Turn the Concept of Gear Grinding Upside Down By W. Graf, 3M2 13FTM05 CubitronTMII: Precision Shaped Grains (PSG) Turn the Concept of Gear Grinding Upside Down Walter Graf, 3M The statements and opinions contained herein are th
2、ose of the author and should not be construed as an official action or opinion of the American Gear Manufacturers Association. Abstract To date, grinding, according to the German DIN Standard 8580, is “machining with geometrically undefined cutting edges” while other machining processes such as turn
3、ing and milling are classified as processes with “geometrically defined cutting edges”. New abrasive grains, called PSG and developed by 3M, stand this definition on its head. For the first time, grinding wheels made with PSG, called CubitronTMII, can claim to be made up of “geometrically defined cu
4、tting edges” as each and every grain is exactly the same engineered shape. Hence, it might be more appropriate to talk about “micro-milling” rather than grinding. This is borne out by looking at the resulting “flowing” chips which are akin to chips seen in milling operations, just finer. These free-
5、flowing chips no longer clog up the grinding wheel and, therefore, the grinding wheel remains free-cutting and dressing becomes only necessary due to loss of from rather than loss of cutting ability. In repeated tests, this has shown to drastically reduce the risk of burning and to give consistent a
6、nd predictable results. Furthermore, tests and subsequent long term trials under production conditions have shown that grinding time can be cut in most cases by at least 50% in comparison to grinding wheels made of standard ceramic abrasives. Based on more than 100 carefully monitored and documented
7、 gear grinding trials, this paper will demonstrate how CubitronTMII grinding wheels work both in continuous generating grinding of car and truck gears, and in form grinding of large diameter gears for wind generators, for example. Furthermore, the paper will discuss chip formation, filmed with high
8、resolution slow motion; and the benefits of the free-flowing chips in terms of resulting consistent surface finish, superior form holding and extended dressing cycles. Copyright 2013 American Gear Manufacturers Association 1001 N. Fairfax Street, Suite 500 Alexandria, Virginia 22314 September 2013 I
9、SBN: 978-1-61481-062-9 3 13FTM05 CubitronTM II: Precision Shaped Grains (PSG) Turn the Concept of Gear Grinding Upside Down Walter Graf, 3M Since the introduction of ceramic abrasives some twenty years ago, the conventional vitrified bonded grinding wheel has undoubtedly undergone technical improvem
10、ents, but only in small steps. In this authors opinion, advances in gear grinding technology in the last ten years can be more attributed to improvements in machine tool technology than the grinding wheels themselves. In fact, in some instances such as spiral bevel grinding, it can be argued that gr
11、inding wheels have held back advances made in machine tool technology. Grinding wheels called CubitronTMII, made of precision shaped grains (PSG), see Figure 1, will change this situation and are challenging machine technology. In fact, some machine tool builders have already adapted to the potentia
12、l inherent in these new grains by adding spindle power and by adopting their software grinding model. To date, grinding, according to the German DIN Standard 8580 is “machining with geometrically undefined cutting edges” while other machining processes such as turning and milling are classified as p
13、rocesses with “geometrically defined cutting edges”. CubitronTMII stands this definition on its head. For the first time, a grinding wheel can claim to be made up of “geometrically defined cutting edges” as each and every grain is exactly the same engineered shape. The secret of the CubitronTMII whe
14、els excellent cutting performance lies within its aggressive grain shape. Hence, it might be more appropriate to talk about “micro-milling” rather than grinding. This is borne out by looking at the resulting chips which are fully akin to chips seen in milling operations, just finer. With any abrasiv
15、es, the grinding process is a combination of ploughing and cutting during the chip formation stage. Ploughing carries much of the responsibility of the high energy consumption inherent in conventional grinding. The chip formation of conventional grinding takes place in three stages as illustrated in
16、 Figure 2. In Stage I, elastic deformation occurs as the grain attempts to penetrate the workpiece material which flows back to some extent, this is then followed in Stage II as plastic deformation (ploughing) and finally, in Stage III, a chip is formed. As the heat generated in grinding is carried
17、away by the chips, it is of paramount importance that the chips are formed as early as possible, and that Stages I and II are as short as possible to avoid heat build-up, and as a consequence, thermal damage to the workpiece. Precision shaped grains, however, move much quicker into the chip formatio
18、n Stage III, see Figure 3. This in turn, reduces the risk of thermal damage to the workpiece. While it is difficult to witness the chip formation with direct observation, the resulting chip shapes lead to this conclusion. However, in the actual presentation, a film sequence will be shown, filmed wit
19、h 400 frames per second, of a dry grinding process in which chip formation can be witnessed. Figure 1. Precision shaped grain (PSG) 1 mm 4 13FTM05 Figure 2. Stages of material deformation during the formation of chips Figure 3. Less material deformation and lateral build-up Comparing grinding forces
20、 between PSG and conventional ceramic grains at the same cutting parameters have confirmed this. Long flowing chips have an inherent advantage in that they do not load up the grinding wheel, see Figure 4. One of the positive surprises of working with PSG has been that the grinding wheel stays very c
21、lean over its full working life. This has been observed in all gear applications under full multi-shift production conditions. Wheel loading may also be responsible for visual surface blemishes such as scratches that can be observed in processes run with conventional ceramic abrasives. Figure 4. Chi
22、ps from profile gear grinding with CubitronTMII 5 13FTM05 When first looking at the wheel structure of a CubitronTMII wheel, almost without exception, application engineers have stated that they can recognize that the cutting ability might be superior, but at the same time expressed concern that the
23、 resulting surface finish may be much coarser than with current conventional abrasives, see Figure 5. However, both in gear grinding with single rib or with threaded wheels, the surface finishes were not inferior to conventional ceramic abrasives. On the contrary, this was particularly visible when
24、looking at larger modules ( 10) that offered a large surface to observe where the resulting surface finishes were free of shading or “cloudiness”. Typically, both in generating grinding with threaded wheels and in profile grinding with single rib wheels, the surface finish was around Ra 0.3 m across
25、 a full range of modules. In generating grinding with threaded wheels, results Ra 0.3 m have been achieved, however, with lower material removal rates. How to use CubitronTM II wheels As the cutting behavior of PSG grain is very different from standard ceramic grains, machining parameters have to be
26、 adapted both in terms of dressing and feeds and speeds. First, we look at single rib wheels, generally used for the grinding of large diameter gears. The following parameters define the grinding process and shall be described in detail: Surface speed, vc, m/s Feed-rate, vw, mm/min Speed ratio (whee
27、l workpiece), qs Dressing: overlap ratio, ud Specific material removal rate, Qw, mm3/mm/sec Specific chip volume, Vw, mm3/mm Surface speed, vcIn grinding with single rib wheels, the surface speed, vc, is not an indicator of efficiency as this is determined by the depth of cut, ae, and the feed-rate,
28、 vw. A lower surface speed of around 30 m/s has proven to be the most efficient for using CubitronTMII wheels in single rib profile grinding. While this may appear low, the wheel still does not break down. On the contrary, as a low surface speed increases the chip thickness, the wheel seems to work
29、better as the pressure on the precision shaped grains increases and fosters self-sharpening. Feed-rates, vwThe feed-rate should be chosen as high as the machine tool allows. Looking at the historical development of the feed-rates of profile grinders, less than 10 years ago all seem to hover around a
30、 maximum of 4000 mm/min. Today, however, 12,000 mm/min and more have become the standard. The higher the chosen feed-rate is, the more aggressive the wheel behaves, and the lower the speed ratio, qs, between the wheel and the workpiece becomes. Gear grinding, unfortunately, hovers around a speed rat
31、io, qs, of 500, the most critical range is illustrated in Figure 6. Figure 5. Wheel structures 6 13FTM05 Figure 6. Gear grinding By increasing the feed-rate, vw, the high speed ratio inherent in gear grinding will come down towards qs120, a much safer range for avoiding thermal damage. Incidentally,
32、 the high speed ratio range of gear grinding, both for single rib profile grinding and of continuous generating grinding, predicates that only oil can be used as a grinding fluid. Dressing overlap ratios, ud The overlap ratio, ud, describes how many revolutions a grinding wheel must make before the
33、effective width, bd, of the dressing tool has move laterally by its own width. The effective width is, of course, a function of the dressing depth ad and the radius on the dressing roll. While not going into more detail here, suffice it to say that the higher the udis, the slower is the dressing fee
34、d-rate, vd, across the grinding wheel. In the past, it was common wisdom that the udmust always be greater than 1 to avoid cutting a thread into the grinding wheel. This, however, no longer holds true as recent experienced has shown that for PSG, an udof 1, in synchronous mode, is ideal for rough gr
35、inding. In finish grinding, however, the udranges between 4 and 6, with the dressing roll and the grinding working in asynchronous mode. More aggressive abrasive grains generally tend to be more aggressive on the diamond rolls, and the question is legitimate whether PSG will wear out diamond rolls q
36、uicker than standard ceramic abrasives. As PSG is made of a similar ceramic material, and only differs in shape, one could logically assume that the wear on diamond tool would be similar. This, in fact, was born out by comparative trials of an equal high number of dressing passes at equal depth. Sim
37、ply stated, the wear can be considered the same at the same number of cycles. However, as PSG requires fewer dressing cycles, the economy is in favor of using PSG. This will be shown when addressing Vw, the specific chip volume. The purpose of dressing is to sharpen worn and blunt grains, see Figure
38、 7. While often understood as a cutting process, it has to be viewed as a type of hammering process whereby the diamond will fracture the surface of abrasive grains and sharpen them in the process. Both standard ceramics and PSG have chemically integrated fracture point, or lines respectively, in th
39、eir grain structure. For standard ceramics this means that minute particles break out to generate a serrated surface. PSG grains, in turn have integrated fracture lines that result in a sharp point to maintain their aggressiveness. Figure 7. Dressed standard and PSG grains 7 13FTM05 Specific materia
40、l removal rate, QwThis value refers to a grinding wheels cutting capacity in terms of volume material removed (in mm3) per mm grinding wheel width per second, expressed as mm3/mm/sec. Q is an excellent benchmark to determine the performance level of a specific grinding wheel, and allows direct compa
41、rison with different wheel specifications. The applied formula of Qwis shown in equation 1. 60ewwavQ (1) where aeis depth of cut; vwis feed rate. However, the depth of cut, ae, is taken as the vertical infeed into the tooth gap, and the result would only be true for the root of the tooth gap. Along
42、the involute curve, the Qwvalues vary considerably. However, as everyone applies the same formula, there is practical use in it. Typical industry values of Qwhover around 10 mm3/mm/sec. CubitronTMII wheels, however, have consistently proven themselves at Qwvalues as high as 30 mm3/mm/sec. This trans
43、lates into considerable savings, allowing grinding times to be reduced by up to 50%, and often even more. Specific chip volume, VwThe specific chip volume Vwrefers to the volume removed until redressing is initiated. It is given in mm3/mm (volume removed in mm3per mm wheel width) and mostly suggeste
44、d by the machine tool software. In other words, the specific chip volume is an economic metric. The higher the value of Vw, the longer the wheel life will be. In this area, CubitronTMII wheels have outperformed standard ceramic wheels by a factor of two to three. This translates into substantial cos
45、t savings, even though the initial wheel price of CubitronTMII is higher than that of standard ceramic abrasives. Furthermore, less dressing translates into less down time and reduced diamond roll wear. Typical values of specific chip volumes concentrate around 1000 mm3/mm for standard ceramic wheel
46、s. For CubitronTMII wheels with a concentration of 100% PSG, the specific chip volume may be as high as 4000 mm3/mm, or even more. Figure 8 gives the formula to calculate specific chip volume Vw. Case histories To illustrate the performance of CubitronTMII wheels with the precision shaped grains, fi
47、ve case histories have been chosen to show the extensive range of application, see Figures 9 through 13. This encompasses the grinding of small and large module, grinding from solid or finish grinding. The first four cases show the application of single rib wheels for profile grinding. The last appl
48、ication shows a sample of continuous generating grinding with a threaded wheel. Figure 8. Grinding parameters for single rib gear grinding with CubitronTMII grinding wheels 8 13FTM05 Figure 9. Case history 1: Grinding from the solid in creep-feed mode module 14 Figure 10. Case history 2: Grinding in
49、ternal gearing module 16 Figure 11. Case history 3: Grinding a planet gear module 16 9 13FTM05 Figure 12 . Case history 4: Grinding a planet gear, module 3.5 Figure 13. Case history 5: Continuous generating grinding of a planet gear module 9 The use of CubitronTMII is of course not restricted to gear grinding with single rib wheels. Threaded wheels made of PSG grain have shown the same performance increases as witnessed with single rib wheels. Instead of measuring using Qw, the performance in often measured in Qmax, which measures the total material remov
