1、14FTM02 AGMA Technical Paper Prediction of Surface Zone Changes in Generating Gear Grinding By M. Ophey and Dr. J. Reimann, WZL RWTH Aachen2 14FTM02 Prediction of Surface Zone Changes in Generating Gear Grinding Matthias Ophey and Dr. Jan Reimann, WZL RWTH Aachen The statements and opinions containe
2、d herein are those of the author and should not be construed as an official action or opinion of the American Gear Manufacturers Association. Abstract One possible process for hard finishing gears is generating gear grinding. Due to high process efficiency, generating gear grinding has replaced othe
3、r grinding processes like profile grinding in batch production of small and middle sized gears. Despite the wide industrial application of generating gear grinding, the process design is based on experience and time and cost intensive trials. The science-based analysis of generating gear grinding ne
4、eds a high amount of time and effort and only a few published scientific analyses exist. In addition, the transfer of existing knowledge from other grinding processes onto generating gear grinding is complicated due to the contact conditions be-tween tool and gear flank, which change continuously du
5、ring the grinding process. One research objective for generating gear grinding is to increase economic efficiency and productivity of the process. At the same time gear quality must be equal or higher and the external zone must not be damaged. However, especially the influence of the grinding proces
6、s on the external zone in generating gear grinding is unknown. In case of an inappropriate process design in combination with stock deviations an unwanted process result or even a thermal damage of the external zone can occur. In this report a thermo-mechanical process model, which describes influen
7、ces on the surface zone in generating gear grinding, is introduced. Copyright 2014 American Gear Manufacturers Association 1001 N. Fairfax Street, Suite 500 Alexandria, Virginia 22314 October 2014 ISBN: 978-1-61481-094-0 3 14FTM02 Prediction of Surface Zone Changes in Generating Gear Grinding Matthi
8、as Ophey and Dr. Jan Reimann, WZL RWTH Aachen Introduction and motivation In order to improve load carrying capacity and noise behavior case hardened gears usually are hard finished 1. One possible process for hard finishing of gears is generating gear grinding. Generating gear grinding has replaced
9、 other grinding processes in batch production of small and middle sized gears due to the high process efficiency. Despite the wide industrial application of this process, only a few scientific analysis exist 2, 3, 4, 5, because the science-based analysis of generating gear grinding needs a high amou
10、nt of time and effort and the continuously changing contact conditions complicate the investigation. The lack of knowledge of cause-effect relationships results in an empirical process design in industrial practice. Therefore in most cases several trials must be performed to find a stable process de
11、sign. By stock fluctuation or by an unfavorable process design an undesirable process result up to a process-related thermal damage of the external zone can occur. Therefore it is necessary to get a better understanding of the cause-effect relationships between process parameters, tool specification
12、s and process in generating gear grinding. State of the art Hard finishing technology is used to remove deviations from hardening, to machine tooth flank modifications and to meet quality requirements. The case hardening process is necessary to enable the gear to transmit high torque with smaller ge
13、ars in high power applications. In industrial applications generating gear grinding, profile gear grinding and gear honing are most commonly used as hard finishing processes for gears. Each of these high-performance processes is using geometrically undefined cutting edges. Continuous generating gear
14、 grinding has evolved to the dominant process in batch production for small and middle sized gears due to the high productivity. Generating gear grinding One of the most efficient processes for hard finishing of gears in batch production of external gears and gear shafts is generating gear grinding.
15、 Generating gear grinding is used for hard finishing of gears with a module of mn= 0.5 mm to mn= 10 mm 6, 4. By the application of new machine tools the process can be used for grinding large module gears with an outside diameter up to da= 1,000 mm 7. The cylindrical grinding worm, whose profile equ
16、ates a rack profile in a transverse section, meshes with an external gear, Figure 1 left. The involute is generated by continuous rolling motion of grinding worm and workpiece by the profile cuts method 8, 4. Profile cuts method in generating processes means that the profile form is generated by a f
17、inite number of profiling cuts. Due to the closed grinding worm no generating cut deviations, as in gear hobbing process, occur during generating gear grinding. In generating gear grinding multiple points of the grinding worm are in contact simultaneously. The number of contact points changes contin
18、uously during tool rotation, Figure 1 right. In the upper right part of Figure 1, the contacts on the right and left flanks are balanced, with an even number of contact points. This leads to a consistent distribution of forces. With an uneven number of contact points, as shown in the lower right par
19、t of Figure 1, the distribution of forces will be unbalanced. This leads to an inconsistent distribution of the cutting forces. In the example with an uneven number of contact points, the force on the line of contact of the left tool flanks is split into two contact points. On the right tool flank t
20、he cutting force is not split, because only one contact point exists. This situation can lead to higher stock removal at the single contact point potentially resulting in higher excitation. The consequence can be the appearance of profile form deviations which reduce the achievable gear quality. Sci
21、entific publications of Meijboom 2 and Trich 3 describe this relation theoretically. In comparison to other gear grinding processes the stock removal rate in generating gear grinding is very high. In most cases the stock removal rate is limited by the demanded gear quality 4. Furthermore, the appear
22、ance of a detrimental process-related surface zone inducement (grinding burn) can be the limiting factor. 4 14FTM02 Figure 1. Generating gear grinding - principle, machine settings and contact conditions 9 Publications 10, 11, 12, and the doctoral thesis of Meijboom 2, Trich 3 and Stimpel 5 show the
23、 influence of several parameters on the process results. But several technological correlations have not been analyzed or verified in experimental trials yet. Current challenges Due to limited scientific studies the technology users, grinding tool suppliers and machine tool manufactures face two mai
24、n challenges. On the one hand the process design and optimization is based on know-how of the process user. In cases, where no sufficient experience (e.g., new gear geometry, new grinding tools) exists, cost-intensive trials have to be performed to find a favored and robust process design. In this c
25、ase, several trial iterations are usually necessary to obtain high process stability. In order to reduce the number of needed iteration loops the technological cause-effect relationships must be analyzed in detail. On the other hand, the demand for increasing productivity leads to an increase of axi
26、al feed and cutting speed. With an increase of productivity the cutting force and thus the heat flow towards the workpiece are rising. This in turn leads to an increased risk of grinding burn during the grinding process. The challenge is to increase the productivity without causing grinding burn. Re
27、search objective and approach The research objective for generating gear grinding at WZL is the increase of process efficiency and process reliability in generating gear grinding by description of the technological cause-effect relationships for cutting forces as well as for occurrence of grinding b
28、urn in a model. For the analysis of the cause-effect relationships an analogy trial has been developed and will be introduced in this report. The aim of this report is to present a model to predict grinding burn for generating gear grinding. Therefor cutting forces in analogy trials are measured and
29、 the interactions between process parameters and occurred grinding burn are taken into account. The measured cutting forces will be combined in an empirical cutting force model. With the ability to calculate the cutting force the heat flow density towards the workpiece can be estimated. With a compa
30、rison of the heat flow density and the grinding burn occurrence a critical heat flow density that leads to grinding burn can be determined. Thereby a model to predict grinding burn can be derived. In conclusion the prediction model will be transferred onto generating gear grinding and will be valida
31、ted by generating gear grinding trials. Analogy trial generating gear grinding The complexity of the contact conditions between tool and workpiece during generating gear grinding complicates the analysis of generating gear grinding. On the one hand, the penetration volumes change over the tooth prof
32、ile height during grinding. On the other hand, the number of engaged tool flanks and 5 14FTM02 workpiece flanks is variable. To investigate generating gear grinding on a single fixed point on the tooth profile, a geometric-kinematic model, the analogy trial, has been developed 12, 9. The principle o
33、f the analogy trial for generating gear grinding is shown in Figure 2. For each point of the involute the local radius of curvature ycan be calculated 13. For the analogy trial the contact conditions at different positions of the involute can be approximated. The radius of the workpiece in the analo
34、gy trial rAequals the radius of curvature at the investigated point of the involute profile. Thus the diameter of the analogy workpiece depends on the number of teeth z, the module mn, the helix angle and the pressure angle nof the mapped sample gear. The rack profile of the grinding worm can be app
35、roximated in the investigated contact point by a face wheel with a conic working surface. 9 Besides the workpiece and the tool geometry the chip geometry in the analogy trial has to be comparable to the chip geometry in generating gear grinding. Therefore the cutting length lcuAand the chip thicknes
36、s hcuAhave to be comparable between analogy trail and generating gear grinding. Furthermore the kinematics of chip formation and the velocities must be fitted to generating gear grinding. During chip formation the lateral sliding speed vtA, the axial feed speed vaAand the cutting speed vcinterfere w
37、ith each other. The cutting speeds in generating gear grinding and analogy trial are the same. The lateral sliding speed vtAcan be calculated by the rotational speed nAof the workpiece and the requirement to be synchronous. The axial feed speed vaAcan be adjusted according to the generating gear gri
38、nding process as the product of rotational speed nAand axial feed fa. Rotational speed as well as axial feed in analogy trial and generating gear grinding is identical. The tool is a grinding wheel with an angled surface. The angle corresponds to the pressure angle n0of the grinding worm. The mapped
39、 grinding process and the machine tool used are shown in Figure 3. The mapped grinding process is carried out with a spur gear with a number of teeth of z = 31 and a normal module of mn= 4.5 mm. The material is 20MnCr5. The gears are case-hardened with a surface hardness of 60 HRC and a case-hardeni
40、ng-depth of CHD550HV= 1.4 mm. The material and heat treatment of the analogy workpieces match with the spur gears. All generating gear grinding and analogy trials were performed on a model LCS380 grinding machine from Liebherr-Verzahntechnik GmbH that can perform both generating and profile gear gri
41、nding. For both analogy and generating gear grinding trials, corundum tools with a grain size F120 (average grain diameter 109 m) from Winterthur Technology AG are used. Figure 2. Analogy trial for generating gear grinding principle and deduction of analogy workpiece geometry 9 6 14FTM02 Figure 3. W
42、orkpiece data and machine tool The experimental setup of the analogy trials is shown in Figure 4. The cutting force can be determined with a dynamometer which is integrated in the flow of forces. For further information a full description of the analogy trial design can be found in 12. With this exp
43、erimental setup 129 analogy trials were performed. In these analogy trials the following process parameters were taken into account. The axial feed fa, the number of starts z0, the cutting speed vc, the stock s, the pressure angle of the analogy tool n0Aand the cooling lubricant volume flowCLV. In a
44、ddition, the diameter of the analogy workpiece was varied to investigate different points on the involute profile. Prediction model for surface zone inducements Based on the empirical data of the analogy trials an empirical-physical model is built up in the following. Using the model, the occurrence
45、 of a process-related damage of the surface zone can be predicted. For this purpose, an approach to describe the surface zone inducement is presented. Subsequently the required parameters are determined and the prediction model is derived. Figure 4. Experimental setup analogy trial generating gear g
46、rinding 7 14FTM02 Analytical model approach To predict the surface zone inducement the heat flow density, which describes the energy flow in the contact zone between tool and workpiece, must be determined. To prevent grinding burn the heat flow density towards the workpiece qwmust always be lower th
47、an a critical heat flow density qw, critthat leads to a detrimental influence on the surface zone. The heat flow density towards the workpiece qwcorresponds to the current energy flow through the contact area between tool and workpiece Ac. In general for grinding the majority of the cutting energy i
48、s thermal energy, which is produced by intense friction, shear and separation processes as well as by friction of the abrasive grain and bond 14. Assuming that also for generating gear grinding almost all cutting power is converted into thermal energy, the total energy flow in the contact area can b
49、e calculated by the product of cutting force Fcand cutting speed vc. In order to estimate the heat flow density towards the workpiece qwcorrection factors must take the distribution of the heat flow from the contact zone into account. These corrections factors are KCLFwhich takes into account the cooling lubricant flow and KWwhich considers the heat flow towards the workpiece. With these factors the heat flow density towards the workpiece qw can be estimated with equation 1, 15. ccwCLFW w, critcFvqKK qA (1) Determination of model para
