MACHINING ECONOMETRICS MACHINING ECONOMETRICS Tool Wear and Tool Life Relationships Machinery's Handbook, 31st Edition
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Tool life is defined as the cutting time to reach a predetermined wear limit, called the tool wear criterion. The size of tool wear criterion depends on the grade used, usually a tougher grade can be used at bigger flank wear. For finishing operations, where close toler ances are required, the wear criterion is relatively small. Other alternative wear criteria are a predetermined value of the surface roughness, or a given depth of the crater which develops on the rake face of the tool. The most appropriate wear criteria depends on cut- ting geometry, grade, and materials. Tool life is determined by assessing the time—the tool life—at which a given prede termined flank wear is reached, 0.25, 0.4, 0.6, 0.8 mm, etc. Fig. 1 depicts how flank wear varies with cutting time (approximately straight lines in a semi-logarithmic graph) for three combinations of cutting speeds and feeds. Alternatively, these curves may represent how variations of machinability impact on tool life, when cutting speed and feed are con stant. All tool wear curves will sooner or later bend upwards abruptly and the cutting edge will break, i.e., catastrophic failure as indicated by the white arrows in Fig. 1.
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Low Average High
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0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 Cutting Time, minutes
Fig. 1. Flank Wear as a Function of Cutting Time The maximum deviation from the average tool life 60 minutes in Fig. 1 is assumed to range between 40 and 95 minutes, i.e., − 33% and +58% variation. The positive deviation from the average (longer than expected tool life) is not important, but the negative one (shorter life) is, as the edge may break before the scheduled tool change after 60 minutes, when the flank wear is 0.6 mm. It is therefore important to set the wear criterion at a safe level such that tool failures due to “normal” wear become negligible. This is the way machinability variations are mastered. Equivalent Chip Thickness ( ECT ).— ECT combines the four basic turning variables, depth of cut, lead angle, nose radius and feed per revolution into one basic parameter. For all other metal cutting operations such as drilling, milling and grinding, additional variables such as number of teeth, width of cut, and cutter diameter are included in the parameter ECT . In turning, milling, and drilling, according to the ECT principle, when the product of feed times depth of cut is constant the tool life is constant no matter how the depth of cut or feed is selected, provided that the cutting speed and cutting edge length are maintained constant. By replacing the geometric parameters with ECT , the number of tool life tests to evaluate cutting parameters can be reduced considerably, by a factor of 4 in turning, and in milling by a factor of 7 because radial depth of cut, cutter diameter and number of teeth are additional parameters.
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