Machinery's Handbook, 31st Edition
1216 MACHINING ECONOMETRICS Determination of Machine Settings and Calculation of Costs Based on the rules and knowledge presented in Chapters 1 and 2, this chapter demon strates, with examples, how machining times and costs are calculated. Additional formulas are given, and the speed and feed tables given in SPEEDS AND FEEDS TABLES starting on page 1090 should be used. Finally the selection of feeds, speeds and tool-lives for optimized conditions are described with examples related to turning, end milling, and face milling. There are an infinite number of machine settings available in the machine tool power train producing widely different results. In practice only a limited number of available settings are utilized. Often, feed is generally selected independently of the material being cut, however, the influence of material is critical in the choice of cutting speed. The tool life is normally not known or directly determined, but the number of pieces produced before the change of worn tools is better known, and tool life can be calculated using the formula for piece cutting time t c given in this chapter. It is well known that increasing feeds or speeds reduces the number of pieces cut between tool changes, but not how big are the changes in the basic parameter tool life. Therefore, there is a tendency to select “safe” data in order to get a long tool life. Another common practice is to search for a tool grade yielding a longer life using the current speeds and feeds, or a 10 to 20 percent increase in cutting speed while maintaining the current tool life. The reason for this old-fashioned approach is the lack of knowledge about the opportunities the metal cutting process offers for increased productivity. For example, when somebody wants to calculate the cutting time, he/she can select a value of the feed rate (product of feed and rpm), and easily find the cutting time by dividing cutting distance by the feed rate. The number of pieces obtained out of a tool is a guess work, however. This problem is very common and usually the engineers find desired tool- lives after a number of trial and error runs using a variety of speeds and feeds. If the user is not well familiar with the material cut, the tool life obtained could be any number of seconds or minutes, or the cutting edge might break. There are an infinite number of speeds and feeds, giving the same feed rate, producing equal cutting time. The same cutting time per piece t c is obtained independent of the selection of feed/rev f and cutting speed V (or rpm), as long as the feed rate F R remains the same: F R = f 1 3 rpm 1 = f 2 3 rpm 2 = f 3 3 rpm 3 , …, etc. However, the number of parts before tool change N ch will vary considerably, including the tooling cost c tool and the total cutting cost c tot . The dilemma confronting the machining-tool engineer or the process planner is how to set speeds and feeds for either desired cycle time, or number of parts between tool changes, while balancing the process versus other operations or balancing the total times in one cell with another. These problems are addressed in this section. Nomenclature f = feed/rev or tooth, mm f E = economic feed f O = optimum feed T = tool life, minutes T E = economic tool life T O = optimum tool life V = cutting speed, m/min V E = economic cutting speed V O = optimum cutting speed, m/min Similarly, economic and optimum values of: c tool = piece cost of tooling, $ C TOOL = cost of tooling per batch, $ c tot = piece total cost of cutting, $ C TOT = total cost of cutting per batch, $ F R = feed rate measured in the feeding direction, mm/rev N = batch size N ch = number of parts before tool change t c = piece cutting time, minutes T C = cutting time per batch, minutes t cyc = piece cycle time, minutes T CYC = cycle time before tool change, minutes
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