1、13FTM02 AGMA Technical Paper Performance and Technological Potential of Gears Ground by Dressable cBN Tools By J. Reimann, F. Klocke, and M. Brumm, RWTH Aachen University, A. Mehr and K. Finkenwirth, Liebherr-Verzahntechnik GmbH2 13FTM02 Performance and Technological Potential of Gears Ground by Dre
2、ssable cBN Tools Jan Reimann, Fritz Klocke, and Markus Brumm, RWTH Aachen University, Andreas Mehr and Klaus Finkenwirth, Liebherr-Verzahntechnik GmbH The statements and opinions contained herein are those of the author and should not be construed as an official action or opinion of the American Gea
3、r Manufacturers Association. Abstract Dressable vitrified bond cBN grinding tools combine the advantages of other common tool systems in generating gear grinding. The cBN grains are a highly productive cutting material due to their high specific stock removal rate. Vitrified bonds are dressable and
4、thereby very flexible: By dressing different profile modifications can be set up and constant gear quality can be guaranteed during the tool life time. Despite those technological advantages there is only a small market distribution of these grinding tools due to high tool costs. Furthermore, only a
5、 few published scientific analysis of generating gear grinding with dressable cBN exist. Especially, the influence of the grinding tool system on manufacturing related component properties has not been analyzed yet. The research objective of this report is to determine the advantages of dressable cB
6、N tools in generating gear grinding. Copyright 2013 American Gear Manufacturers Association 1001 N. Fairfax Street, Suite 500 Alexandria, Virginia 22314 September 2013 ISBN: 978-1-61481-059-9 3 13FTM02 Performance and Technological Potential of Gears Ground by Dressable cBN Tools Jan Reimann, Fritz
7、Klocke, and Markus Brumm, RWTH Aachen University, Andreas Mehr and Klaus Finkenwirth, Liebherr-Verzahntechnik GmbH Introduction In order to improve load carrying capacity and noise behavior case hardened gears usually are hard finished. One possible process for hard finishing of gears is generating
8、gear grinding which has replaced other grinding processes in batch production of small and middle gears due to high process efficiency. Depending on grinding task and batch size different tool concepts can be used in generating gear grinding. The latest concept is dressable vitrified bond cBN grindi
9、ng tools. Dressable vitrified bond cBN grinding tools combine the advantages of other common tool systems in generating gear grinding. The cBN grains are a highly productive cutting material due to their high specific stock removal rate. Vitrified bonds are dressable and thereby very flexible. By dr
10、essing different profile modifications can be set up and constant gear quality can be guaranteed during the tool life time. Despite those technological advantages there is only a small market distribution of these grinding tools due to high tool costs. Furthermore, only a few published scientific an
11、alyses of generating gear grinding with dressable cBN exist. Especially, the influence of the grinding tool system on manufacturing related component properties has not been analyzed yet. State of the art Generating gear grinding One of the most efficient processes for the hard finishing of gears in
12、 batch production of external gears and gear shafts is the continuous generating gear grinding. Generating gear grinding is used for the hard finishing of gears with a module of mn= 0.5 mm to mn= 10 mm 1, 2, 4. By the application of new machine tools, the process can be used for grinding large modul
13、e gears (up to da= 1,000 mm) 4. The cylindrical grinding worm, whose profile equates a rack profile in a transverse section, hobs with an external gear, Figure 1 (left). The involute is generated by continuous rolling motion of the grinding worm and workpiece by the profile cuts method 5, 2. Profile
14、 cuts method in the generating processes means the profile form is generated by a finite number of profiling cuts. Due to the closed grinding worm no generating cut deviations, known from gear hobbing, occur in generating gear grinding. Figure 1. Generating gear grinding principle, machine settings
15、and contact conditions ( WZL) 4 13FTM02 In comparison with other gear grinding processes, the stock removal rate in generating gear grinding is very high. In most cases, it is only limited by the reachable gear quality 2. In generating gear grinding, always multiple points of the grinding worm are i
16、n contact. The number of contact points change continuously during the tool rotation, Error! Reference source not found. (right). The contacts on the right and on the left tool flank are equal by an even number of contact points. This leads to an consistent distribution of forces. By an uneven numbe
17、r of contact points also the distribution of forces will be uneven. This leads to an inconsistent distribution of the cutting forces. In the example in Figure 1, lower on the line of contact of the left tool flanks the forces are split in two contact points. On the right tool flank the cutting force
18、 is increased, because only one point has contact. This fact can lead to a higher stock removal at this contact point and to a higher excitation. The consequence can be the appearance of profile form deviations which reduce the reachable gear quality. Scientific publications of Meijboom 6 and Trich
19、7 describe this relation theoretical. Publications, e.g. in 8, 9, 10, and existing doctoral thesis of Meijboom 6, Trich 7 and Stimpel 11 show the influence of several parameter (e.g. axial feed, number of starts) on the process results. But several technological correlations have not been analyzed o
20、r verified in trials yet. Especially, investigations on different tool systems cannot be found in literature so far. Tool systems in generating gear grinding In gear grinding usually two types of tool system are used. On one hand for small batch production with changing gear geometries and modificat
21、ions flexible solutions are needed. Therefore, dressable vitrified bond grinding wheels made of corundum are used. A huge advantage is constant gear quality and surface roughness due to the possibility of dressing. A disadvantage can be found in short tool life times and additional dressing time. On
22、 the other hand, for mass production with only a few variants concerning profile modifications a productive grain material is needed. Therefore, electroplated cBN tools are used. In contrast to dressable vitrified bond corundum tools electroplated cBN has a higher productivity due to wear-resistant
23、grain material. A huge advantage is time saving due to reduction of dressing processes. But disadvantages can be found in changing gear quality during tool life time and the significant higher tool costs. To combine the advantages of both tool systems vitrified bond dressable cBN tools have been dev
24、eloped. Due to high tool costs and existing risk in tool handling this tool system has not been used in many industrial applications so far. Furthermore, only a few published scientific analysis of generating gear grinding with dressable cBN exist 13, 14, 15. Especially, the influence of the grindin
25、g tool system on manufacturing related component properties has not been analyzed yet. But due to new developments, since a few years the tool system gets more and more attractive for industrial applications 12. Objective and approach The research objective of this report is to determine the advanta
26、ges of dressable vitrified bond cBN tools in generating gear grinding. The manufacturing related properties of gears which are ground with dressable cBN will be analyzed and compared with conventional ground gears. The tested gear sets are manufactured identically despite the gear hard finishing pro
27、cess. In generating gear grinding different tool systems are used for the hard finishing. The properties, e.g. surface structure, residual stresses and gear quality, are analyzed and compared. After gear grinding pitting tests according to the DIN-ISO standard for low cycle fatigue area are carried
28、out 16, 17. In the end a relation between different tool systems, manufacturing related properties and fatigue strength will be deducted. Furthermore, first results showing the tool wear behavior will be shown. Performance of CBN-ground gears in fatigue tests In the following section the performance
29、, especially the flank fatigue strength, of gears ground by different grinding tools will be investigated in pitting tests. Therefore, gear geometry, initial situation of the ground gears, test rig, and test approach is described. Afterwards sample results will be shown. 5 13FTM02 Workpiece geometry
30、, test bed and approach Geometry and initial situation of test gears For the fatigue strength test, a standardized gear geometry of the Laboratory for Machine Tools and Production Engineering (WZL) was chosen. The gear set has a module of mn= 4.0 mm with 20 teeth at the pinion, 33 teeth at the wheel
31、, and a gear face width of b = 21.2 mm. The pressure angle is n= 18 and helix angle is = 20.4. This leads to a standardized center distance of a = 112.5 mm. The complete gear geometry data are shown in Figure 2. All gear sets have been machined from the same material batch (20MnCr5) and have been he
32、at treated in one batch. The gears are case hardened with a case hardening depth of CHD550HV= 1.0 mm at the flank and a surface hardness of 60+2 HRC in one batch. All soft machining operations, e.g. turning, milling and gear hobbing, have been carried out in each case with the same set of process pa
33、rameter in one batch. The only differences can be found in the gear grinding process. All gear sets were ground by continuous generating gear grinding with identical process parameters on the generating gear grinding machine LCS380 of LIEBHERR at the Laboratory for Machine Tools and Production Engin
34、eering (WZL), RWTH Aachen University. Gears and pinions have been ground by a two pass strategy. In the first pass, the gears were roughed with a cutting speed of vc= 60 m/s, an axial feed of fa= 0.50 mm and a removed flank stock of s = 0.13 mm. In the second pass the gear sets have been finished wi
35、th the same cutting speed but with an axial feed of fa= 0.37 mm and a removed flank stock of s = 0.02 mm. A grinding worm with a number of starts of z0= 1 has been used. One half of tested pinions is ground by a dressable vitrified bond cBN tool, the other half is ground by a state of the art vitrif
36、ied bond corundum tool with 30% sintered corundum. Other process parameters and the clamping situation can be seen in Figure 3. Geometrically all gear sets are identical concerning gear geometry deviations and surface topography. Figure 4 shows for example the gear quality of all gears after the gri
37、nding process. All gears are ground in gear quality 4 or better according to DIN 3962 18 Figure 2. Gear geometry test pinion and gear ( WZL) Test Gear Modulemn= 4 mm Number of teethz1= 20 Pressure anglen= 18 Helix angle1= 20.4 Flank directionleft Pitch circle diameterd1= 85.353 mm Tooth widthwK=4 =
38、42.816 mm Addendum modificationx1= 0Test Pinion Modulemn= 4 mm Number of teethz1= 33 Pressure anglen= 18 Helix angle2= 20,4 Flank directionright Pitch circle diameterd2= 140.833 mm Tooth widthwK=4 = 55.050 mm Addendum modificationx1= -0.1449120147.3 4521.25293.1 7021.252 456 13FTM02 Figure 3. Gear g
39、eometry, machine tool and tool data pitting tests ( WZL) Figure 4. Gear quality of tested pinions selected values ( WZL) In addition to the geometrical gear properties, manufacturing related component properties of the surface zone have to be considered. Figure 5 shows residual stresses in the surfa
40、ce zone in axial and tangential direction for both tool variants. In tangential direction, respectively the profile or generating direction, residual stresses tof the cBN-ground gears are significantly higher. Due to high productivity of the cBN grains, the grinding process, especially the chip form
41、ation, induces less thermal energy to the surface. Additionally, the higher hardness of cBN grains creates higher compressive stresses which also extend deeper into the surface zone. At the surface, gears ground with cBN have compressive residual stress of t= -940 MPa. The conventional ground only t
42、= -690 MPa. 4202466fhProfile DeviationsGear Datamn= 4 mmz = 20b= 21.2 mGrinding Tool DataWinterthurA120 JV6501WendtJ10A-170-150-15-M126xProcess ParameterRoughingvc= 60 m/sfa= 0.50 mms = 0.13 mmFinishingvc= 60 m/sfa= 0.37 mms = 0.02 mmGear Quality According to DIN 396235 1135Left Flank Right FlankffF
43、lank Deviations fhff7 13FTM02 Figure 5. Comparison of residual stresses in axial und tangential direction after generating gear grinding ( WZL) The surface roughness of both variants is nearly identical. Average cBN-ground gears have a surface roughness of Rz= 2.5 m and the conventional ground gears
44、 of Rz= 2.9 m. Referring to ISO 6336-1 the difference on load carrying capacity of the both variants concerning gear and surface quality is insignificant 17. Test bed and approach Gear testing has been carried out at the Laboratory for Machine Tools and Production Engineering (WZL), RWTH Aachen Univ
45、ersity. The gears were tested on a 112.5 mm center distance back-to-back contact fatigue test rig at n2= 2,450 rpm, at the pinion and with n1= 1,500 rpm, at the input shaft. As lubricant Shell Omala F220 oil at Toil= 90 C was used for splash lubrication. The used back-to-back test rig, according to
46、DIN 51354-1 16, and its principle are shown in Figure 6. Figure 6. Back-to-back test rig according to DIN 51354-1 and testing parameters ( WZL) ResidiualStressesAxial axMPaResidiualStressesTangentialtMPaGear Datamn= 4 mmz = 20b = 21.2 mmGrinding Tool DataWinterthurA120 JV6501WendtJ10A-170-150-15-M12
47、6xProcess ParameterRoughingvc= 60 m/sfa= 0.50 mms = 0.13 mmFinishingvc= 60 m/sfa= 0.37 mms = 0.02 mmSurface Distance z m01501005000-1000-1000axt-750-500-250-750-500-250corundumdressable cBN8 13FTM02 For low cycle fatigue pitting tests are carried out with constant torque at pinion following to a FVA
48、 guideline for pitting tests 18. These tests were carried out for each ground variant at two different torque levels of M2= 650 Nm and M2= 750 Nm. These torques gave peak contact stress levels of P= 1,471 MPa and P= 1,578 MPa, respectively. These values were calculated by the finite element tooth co
49、ntact analysis FE-Stirnradkette developed by the Laboratory of Machine Tools and Production Engineering. Each test involves five tests per variant (cBN or corundum) and torque level. Results As criterion for low cycle fatigue strength a damaged surface of a single tooth of four percentage (VEZ= 4%) is defined. In Figure 7 an example for typical pitting damages can be seen for differently ground variants at test end. The two pictures at the top of Figure 7 show
