Machinery's Handbook, 31st Edition
MACHINING ECONOMETRICS 1221 Variation of Tooling and Total Cost with the Selection of Speeds and Feeds It is a well-known fact that tool life is reduced when either feed or cutting speed is in- creased. When a higher feed/rev is selected, the cutting speed must be decreased in order to maintain tool life. However, a higher feed rate (feed rate = feed/rev 3 rpm, mm/min) can result in a longer tool life if proper cutting data are applied. Optimized cutting data require accurate machinability databases and a computer program to analyze the options. Reason ably accurate optimized results can be obtained by selecting a large feed/rev or tooth, and then calculating the economic tool life T E . Because the cost versus feed or ECT curve is shal- low around the true minimum point, i.e., the global optimum, the error in applying a large feed is small compared with the exact solution. Once a feed has been determined, the economic cutting speed V E can be found by calculating the Taylor slope, and the time/cost calculations can be completed using the formulas described in last section. The remainder of this section contains examples useful for demonstrating the required procedures. Global optimum may or may not be reached, and tooling cost may or may not be reduced, compared to currently used data. However, the following examples prove that significant time and cost reductions are achievable in today’s industry. Note: Starting values of reasonable feeds in mm/rev can be found in the Handbook speed and feed tables, see Principal Speeds and Feeds Tables on page 1090, by using the f avg values converted to mm as follows: feed (mm/rev) = feed (inch/rev) 3 25.4 (mm/inch), thus 0.001 inch/rev = 0.001 3 25.4 = 0.0254 mm/rev. When using speed and feed Table 1 through Table 23, where feed values are given in thousandths of inch per revolution, simply multiply the given feed by 25.4⁄1000 = 0.0254, thus feed (mm/rev) = feed (0.001 inch/rev) 3 0.0254 (mm⁄0.001inch). Example 9, Converting Handbook Feed Values from Inches to Millimeters: Handbook tables give feed values f opt and f avg for 4140 steel as 17 and 8 3 (0.001 inch/rev) = 0.017 and 0.009 inch/rev, respectively. Convert the given feeds to mm/rev. Example 10, Using Handbook Tables to Find the Taylor Slope and Constant: Calculate the Taylor slope and constant, using cutting speed data for 4140 steel in Table 1 starting on page 1095, and for ASTM Class 20 gray cast iron using data from Table 4a on page 1101, as follows: For the 175–250 Brinell Hardness range, and the hard tool grade, C V T 1458 n 1 1 # = ^ h . n 027 = = = = feed = 0.017 3 25.4 = 17 3 0.0254 = 0.4318 mm/rev feed = 0.008 3 25.4 = 8 3 0.0254 = 0.2032 mm/rev
ln ( V 1 ⁄ V 2 ) ln ( T 2 ⁄ T 1 )
ln (525 ⁄ 705) ln (15 ⁄ 45)
For the 175–250 Brinell Hardness range, and the tough tool grade, C V T . n 028 = = = = For the 300–425 Brinell Hardness range, and the hard tool grade, C V T 1 ln ( V 1 ⁄ V 2 ) ln ( T 2 ⁄ T 1 ) ln ( V 1 ⁄ V 2 ) ln ( T 2 ⁄ T 1 ) ln (235 ⁄ 320) ln (15 ⁄ 45) 1 . 026 n = = = ln (330 ⁄ 440) ln (15 ⁄ 45) = For the 300–425 Brinell Hardness range, and the tough tool grade, C V T . n 031 = = = =
685 n
1 # = ^ h
894 n
1 # = ^ h
ln ( V 1 ⁄ V 2 ) ln ( T 2 ⁄ T 1 )
ln (125 ⁄ 175) ln (15 ⁄ 45)
401 n
1 # = ^ h
1
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