Machinery's Handbook, 31st Edition
MACHINING ECONOMETRICS 1197 The introduction of the ECT concept constitutes a major simplification when predicting tool life and calculating cutting forces, torque, and power. ECT was first presented in 1931 by Professor R. Woxen, who both theoretically and experimentally proved that ECT is a basic metal cutting parameter for high-speed cutting tools. Dr. Colding later proved that the concept also holds for carbide tools, and extended the calculation of ECT to be valid for cutting conditions when the depth of cut is smaller than the tool nose radius, or for round inserts. Colding later extended the concept to all other metal cutting operations, including the grinding process. The definition of ECT is: ( ) ECT CEL Area mm or inch = where A = cross sectional area of cut (approximately = feed 3 depth of cut), (mm 2 or inch 2 ) CEL = cutting edge length (tool contact rubbing length), (mm or inch), see Fig. 1 on page 1092. An exact value of A is obtained by the product of ECT and CEL . In turning, milling, and drilling, ECT varies between 0.05 and 1 mm, and is always less than the feed/rev or feed/ tooth; its value is usually about 0.7 to 0.9 times the feed. Example 1: For a feed of 0.8 mm/rev, depth of cut a = 3 mm, and cutting edge length CEL = 4 mm, the value of ECT is approximately ECT = 0.8 3 3 ÷ 4 = 0.6 mm. The product of ECT , CEL , and cutting speed V (m/min or ft/min) equals the metal removal rate MRR , measured in terms of the volume of chips removed per minute: / / / min min min MRR V Area V ECT CEL V Area 1000 1000 mm cm or inch 3 3 3 # # # # = = = The specific metal removal rate SMRR is the metal removal rate per mm cutting edge length CEL , thus: / / / / / / min min min SMRR V ECT V ECT 1000 mm mm cm mm or inch inch 3 3 3 # # = = and MRR Example 2: Using the Example 1 data and a cutting speed of V = 250 m/min, the specific metal removal rate becomes SMRR = 0.6 3 250 = 150 (cm 3 /min/mm). ECT in Grinding: In grinding ECT is defined as in the other metal cutting processes, and is approximately equal to ECT = Vw 3 ar ÷ V , where Vw is the work speed, ar is the depth of cut, and A = Vw 3 ar . Wheel life is constant no matter how depth ar , or work speed Vw, is selected at V = constant (usually the influence of grinding contact width can be neglected). This translates into the same wheel life as long as the specific metal removal rate is constant, thus: / min/ SMRR Vw ar 1000 mm mm 3 # = In grinding, ECT is much smaller than in the other cutting processes, ranging from about 0.0001 to 0.001 mm (0.000004 to 0.00004 inch). The grinding process is described in a separate chapter GRINDING FEEDS AND SPEEDS starting on page 1261. Tool Life Relationships.— Plotting the cutting times to reach predetermined values of wear typically results in curves similar to those shown in Fig. 2 (cutting time versus cut- ting speed at constant feed per tooth) and Fig. 3 (cutting time versus feed per tooth at constant cutting speed). These tests were run in 1993 with mixed ceramics turn-milling 82 RC (Rockwell C scale) hard steel at the Technische Hochschule Darmstadt.
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