Machinery's Handbook, 31st Edition
MACHINING ECONOMETRICS 1201 The G - and H -Curves.— The G -curve defines the longest possible tool life for any given metal removal rate, MRR , or specific metal removal rate, SMRR . It also defines the point where the total machining cost is minimum, after the economic tool life T E , or optimal tool life T O , has been calculated, see Optimization Models, Economic Tool Life when Feed is Constant starting on page 1213. The tool life relationship is depicted in the three planes: T - V , where ECT is the plotted parameter (the Taylor plane); T - ECT , where V is plotted; and, V - ECT , where T is a param eter. The latter plane is the most useful because the optimal cutting conditions are more readily understood when viewing in the V - ECT plane. Fig. 6a, Fig. 6b, and Fig. 6c show how the tool life curves look in these three planes in log-log coordinates.
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10
ECT = 0.1 ECT = 0.25 ECT = 0.5 ECT = 0.7
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V m/min
Fig. 6a. Tool Life versus Cutting Speed T - V , ECT Plotted Fig. 6a shows the Taylor lines, and Fig. 6b illustrates how tool life varies with ECT at different values of cutting speed, and shows the H -curve. Fig. 6c illustrates how cutting speed varies with ECT at different values of tool life. The H - and G -curves are also drawn in Fig. 6c.
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V = 100 V = 225 V = 250 V = 300 V = 150
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H-CURVE
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0.1
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ECT , mm
Fig. 6b. Tool Life versus ECT , T - ECT , Cutting Speed Plotted A simple and practical method to ascertain that machining is not done to the left of the H -curve is to examine the chips. When ECT is too small, about 0.03-0.05 mm, the chips tend to become irregular and show up more or less as dust.
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