Machinery's Handbook, 31st Edition
1264 GRINDING FEEDS AND SPEEDS Example 3: Fig. 2b shows that maximum values of wheel-life occur along the H -curve. For the 3 speeds 1800, 2700, and 3600 m/min, maximum wheel lives are approximately 70, 14 and 4 minutes, respectively, at ECT around 17 3 10 − 5 through 20 3 10 − 5 mm along the H -curve. Left and right of the H -curve wheel-life is shorter. Fig. 2c depicts wheel speed V versus ECT with wheel-life T parameter shown as curves in log-log coordinates, similar to those for the other cutting operations, with the character istic H - and G -curves.
10000
T = 30 T = 10 T = 1
H-CURVE
1000
G-CURVE
100
0.00001
0.0001
0.001
ECT, mm
Fig. 2c. V versus ECT , T Plotted Optimum grinding data for roughing occur along the G -curve, which is determined from the V - ECT graph by drawing 45-degree lines tangent to the T -curves, as shown in Fig. 2c, and drawing a line (the G -curve) through the points of tangency on the respective T -curves, thus the location and direction of the G -curve is determined. Globally optimum data correspond to the T -curve for which wheel-life is calculated using the corresponding equivalent tooling-cost time, T V , calculated from T V = T RPL + 60 3 C E ÷ H R , minutes, where T RPL is the time required to replace wheel, C E = cost per wheel dressing = wheel cost + cost per dressing, and H R is the hourly rate. Minimum cost conditions occur along the G -curve; if optimum life T O was determined at either 10 or 30 minutes then V O = 1500 or 1100 m/min, respectively, and ECT is around 65 − 70 3 10 − 5 mm in both cases. The corresponding optimum values of SMRR are 1000 3 1500 3 67 3 10 − 5 = 1000 and 1000 3 1100 3 67 3 10 − 5 = 740 mm 3 /min/mm wheel contact width (1.5 to 1.1 in 3 /in/min). Using Fig. 1 we find optimum work speeds for depths of cut a r = 0.01 and 0.005 mm to be V w = 100 and 75 m/min, and 200 and 150 m/min (330 and 250 fpm, and 660 and 500 fpm) respectively for 10- and 30-minute wheel-life. These high work speeds are possible using proper dressing conditions, high system stiffness, good grinding fluid quality and wheel composition. Fig. 3 shows the variation of specific metal removal rate with wheel speed for several materials and a range of ECT s at 10- and 30-minutes wheel-life. ECT decreases when moving to the left and down along each curve. The two curves for unhardened 1020 steel have the largest values of SMRR , and represent the most productive grinding conditions, while the heat resistant alloy Inconel yields the least productive grinding conditions. Each
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