Machinery's Handbook, 31st Edition
GRINDING FEEDS AND SPEEDS 1265 branch attains a maximum SMRR along the G -curve (compare with the same curve in the V- ECT graph, Fig. 2c) and a maximum speed region along the H -curve. When the SMRR - values are lower than the H -curve the ECT values for each branch decrease towards the bottom of the graph, then the speed for constant wheel-life must be reduced due to the fact that the ECT values are to the left of their respective H -curves in V - ECT graphs.
10000
Vw × a , T = 30, IncX Vw × a , T = 10, IncX Vw × a , T = 30, M4 Vw × a , T = 10, M4 Vw × a , T = 30, T-15 Vw × a , T = 10, T-15 Vw × a , T = 30, 1020 Unh Vw × a , T = 10, 1020 Unh
Optimum = G-CURVE
1000
H-CURVE
100
100
1000
10000
V m/min
Fig. 3. Specific Metal Removal Rate versus Cutting Speed at T =10 and 30 Minutes Wheel Life In the figure, IncX is Inconel; M4, and T-15 are tool steels; and 1020 Unh is unhardened 1020 steel. Surface Finish, Ra.— The finish is improved by decreasing the value of ECT as shown in Fig. 4, where the Ra value (or R a , for roughness average) is plotted versus ECT at 3 differ- ent wheel lives 1, 10 and 30 minutes at constant wheel speed. Because ECT is proportional to the depth of cut, a smaller depth of cut is favorable for reducing surface roughness when the work speed is constant.
10
1
T = 1 T = 5
T = 15 T = 45 T = 100
0.1
0.00001
0.0001
0.001
ECT, mm
Fig. 4. Surface Finish, Ra versus ECT , Wheel-Life T Plotted In Fig. 5, Ra is plotted versus wheel-life at 5 different ECT ’s. Both Fig. 4 and Fig. 5 illustrate that a shorter life improves the surface finish, which means that either an increased wheel speed (wheel-life decreases) at constant ECT , or a smaller ECT at constant speed (wheel-life increases), will result in an improved finish. For a required surface finish, ECT and wheel-life have to be selected appropriately in order to also achieve an optimum grinding time or cost. In cylindrical grinding a reduction of side feed f s improves Ra as well. In terms of specific metal removal rate, reducing SMRR will improve the surface finish Ra .
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