MACHINING ECONOMETRICS Machinery's Handbook, 31st Edition
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t i = idle time (tool “air” motions during cycle), minutes z = cutter number of teeth The following variables are used for calculating the per batch cost of cutting: C C = cost of cutting time per batch, $ C CH = cost of tool changes per batch, $ C E = cost per edge, for replacing or regrinding, $ H R = hourly rate, $ T V = equivalent tooling-cost time, minutes T RPL = time for replacing worn edge(s), or tool for regrinding, minutes Note: In the list above, when two variables use the same name, one in capital letters and one lower case, T C and t c for example, the variable name in capital letters refers to batch processing and lowercase letters to per piece processing, such as T C = N ch 3 t c , C TOT = N ch 3 c tot , etc. FormulasValid forAll OperationTypes Including Grinding Calculation of Cutting Time and Feed Rate Feed rate: F R = f 3 rpm (mm/min), where f is the feed in mm/rev along the feeding direction, rpm is defined in terms of workpiece or cutter diameter D in mm, and cutting speed V in m/min, as follows: D V D V 1000 318 rpm = π = Cutting time per piece: Note: Constant cutting time is a straight 45-degree line in the V - ECT graph, along which tool life varies considerably, as is shown in Fig. 21. t F Dist f Dist V f Dist D 1000 rpm c R # # # π = = = where the units of distance cut Dist , diameter D , and feed f are mm, and V is in m/min. In terms of ECT , cutting time per piece, t c , is as follows: t V Dist D CEL ECT a 1000 c # # # π = where a = depth of cut, because feed 3 cross sectional chip area = f 3 a = CEL 3 ECT . Example 3, Cutting Time: Given Dist =105 mm, D =100 mm, f = 0.3 mm, V = 300 m/min, rpm = 700, F R = 210 mm/min, find the cutting time. Cutting time = t c = 105 3 3.1416 3 100 ÷ (1000 3 300 3 0.3) = 0.366 minutes = 22 seconds Scheduling of Tool Changes Number of parts before tool change: N ch = T ÷ t c Cycle time before tool change: T CYC = N ch 3 ( t c + t i ), where t cyc = t c + t i , where t c = cutting time per piece, t i = idle time per piece Tool life: T = N ch 3 t c Example 4: Given tool life T = 90 minutes, cutting time t c = 3 minutes, and idle time t i = 3 minutes, find the number of parts produced before a tool change is required and the time until a tool change is required.
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