GRINDING FEEDS AND SPEEDS Machinery's Handbook, 31st Edition
1267
1
T = 1 T = 5 T = 10 T = 100
0.1
0.01
0.1
1
10
100
− 1
Kst, N/mm × 10
Fig. 7. Sparkout Time versus System Stiffness, Wheel-Life T Plotted Equivalent Diameter (Work Conformity) Factor: The difference in curvature of the work and wheel in the contact region, determined by the equivalent diameter or work con formity formula, is an important factor for calculating spark-out time and forces, but has a negligible influence on wheel-life. Therefore, an equivalent diameter, D e = D /(1 ± D / D w ), with the minus sign for internal grinding and the plus sign for external grinding operations, is used to consider the effect of conformity when using internal and external grinding with varying work and wheel diameters. D e is equal to the wheel diameter in surface grinding (work flat); in internal grinding, the wheel conforms closely to the work and D e is therefore larger than in external grinding. Grinding Cutting Forces, Torque and Power.— Formulas to calculate the tangential cutting force, torque and required machining power are found in Estimating Machining Power on page 1145, but the values of K c , specific cutting force or specific energy, are approximately 30 to 40 times higher in grinding than in turning, milling and drilling. This is primarily due to the fact that the ECT values in grinding are 1000 to 10000 times smaller, and also due to the negative rake angles of the grit. Average grinding rake an- gles are around − 35 to − 45 degrees. K c for grinding unhardened steel is around 50000 to 70000 N/mm 2 and up to 150000 to 200000 N/mm 2 for hardened steels and heat resistant alloys. The grinding cutting forces are relatively small because the chip area is very small.
59000 60000 61000 62000 63000 64000 65000 66000 67000 68000
V = 6120 V = 4320 V = 2183 V = 1000
0.00001
0.0001
0.001
ECT, mm
Fig. 8. Specific Grinding Force K c versus ECT ; V Plotted As in the other metal cutting operations, the forces vary with ECT and to a smaller extent with the wheel speed V . An example is shown in Fig. 8, where K c , specific cutting force, is plotted versus ECT at wheel speeds between 1000 and 6000 m/min. The material is medium unhardened carbon steel ground by an aluminum oxide wheel. The impact of wheel speed is relatively small (2 to 5 percent lower with increasing speed).
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