GRINDING FEEDS AND SPEEDS Machinery's Handbook, 31st Edition
1269
Table 1. Grindability Groups
Group
Examples
Group 1 Unhardened Steels Group 2 Stainless Steels
SAE 30201-30347, 51409-51501
Group 3 Cast Iron Group 4 Tool Steels Group 5 Tool Steels Group 6 Tool Steels Group 7 Tool Steels
M1, M8, T1, H, O, L, F, 52100 M2, T2, T5, T6, D2, H41, H42, H43, M50
M3, M4, T3, D7
T15, M15 Group 8 Heat Resistant Steels Inconel, Rene etc. Group 9 Carbide Materials P30 Diamond Wheel Group 10 Ceramic Materials
For each grindability group there is one table and 2 graphs (one with Taylor lines and the other with SMRR versus wheel speed V ) that are used to get a first estimate of standardized machine settings, assuming a good system stiffness (K ST > 30 N/ μ m). These data are then calibrated with the users own data in order to refine the estimate and optimize the grinding process, as discussed in User Calibration of Recommendations . The recommendations are valid for all grinding processes such as plunge grinding, cylindrical, and surface grinding with periphery or side of wheel, as well as for creep feed grinding. The grinding data machinability system is based on the basic parameters equivalent chip thickness ECT , and wheel speed V , and is used to determine specific metal removal rates SMRR and wheel-life T, including the work speed V w after the grinding depths for roughing and finishing are specified. For each material group, the grinding data machinability system consists of T - V Taylor lines in log-log coordinates for 3 wheel speeds at wheel lives of 1, 10 and 100 minutes wheel-life with 4 different values of equivalent chip thickness ECT . The wheel speeds are designated V 1 , V 10 , and V 100 respectively. In each table the corresponding specific metal removal rates SMRR are also tabulated and designated as SMRR 1 , SMRR 10 and SMRR 100 respectively. The user can select any value of ECT and interpolate between the Taylor lines. These curves look the same in grinding as in the other metal cutting processes and the slope is set at n = 0.26, so each Taylor line is formulated by V 3 T 0.26 = C, where C is a constant tabulated at four ECT values, ECT = 17, 33, 50 and 75 3 10 − 5 mm, for each material group. Hence, for each value of ECT , V 1 3 1 0.26 = V 10 3 10 0.26 = V 100 3 100 0.26 = C. Side Feed, Roughing and Finishing.— In cylindrical grinding, the side feed, f s = C 3 Width, does not impact on the values in the tables, but on the feed rate F R , where the frac tion of the wheel width C is usually selected for roughing and in finishing operations, as shown in the following table. Work Material Roughing, C Finishing, C Unhardened Steel 2 ⁄3-3⁄4 1⁄3-3⁄8 Stainless Steel 1⁄2 1⁄4 Cast Iron 3⁄4 3⁄8 Hardened Steel 1⁄2 1⁄4 Finishing: The depth of cut in rough grinding is determined by the allowance and usually set at a r = 0.01 to 0.025 mm. The depth of cut for finishing is usually set at a r = 0.0025 mm and accompanied by higher wheel speeds in order to improve surface finish. However, the most important criterion for critical parts is to increase the work speed in order to avoid thermal damage and surface cracks. In cylindrical grinding, a reduction of side feed f s improves R a as well. Small grit sizes are very important when very small finishes are required. See Fig. 4, Fig. 5, and Fig. 6 for reference.
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online