Machinery's Handbook, 31st Edition
MACHINING ECONOMETRICS 1223 5) Determine cost of tooling per batch (cutting tools, holders and tool changing) then total cost of cutting per batch: C TOOL = H R × T C × ( C E / T ) ⁄60 C TOOL + C CH = H R × T C × ( T RPL + C E / T ) ⁄60 C TOT = H R × T C (1 + ( T RPL + C E )/ T ) Example 12, Face Milling—Minimum Cost : This example demonstrates how a modern firm, using the formulas previously described, can determine optimal data. It is here applied to a face mill with 10 teeth, milling a 1045 type steel, and the radial depth versus the cutter diameter is 0.8. The V - ECT - T curves for tool-lives 5, 22, and 120 minutes for this operation are shown in Fig. 23a.
1000
G-CURVE
100
T = 5 T = 22 T = 120
10
0.1
1
10
ECT, mm
Fig. 23a. Cutting Speed versus ECT, Tool Life Constant The global cost minimum occurs along the G -curve, see Fig. 6c and Fig. 23a, where the 45-degree lines defines this curve. Optimum ECT is in the range 1.5 to 2 mm. For face and end milling operations, ECT = z 3 f z 3 ar / D 3 aa / CEL ÷ π . The ratio aa / CEL = 0.95 for lead angle LA = 0, and for ar / D = 0.8 and 10 teeth, using the formula to calculate the feed/tooth range gives for ECT = 1.5, f z = 0.62 mm and for ECT = 2, f z = 0.83 mm.
0.6
T = 5 T = 22 T = 120
0.5
0.4
t c
0.3
0.2
0.1
0
0
0.1 0.2 0.3 0.4
0.5 0.6
0.7 0.8 0.9
1
f z
Fig. 23b. Cutting Time per Part versus Feed per Tooth Using computer simulation, the minimum cost occurs approximately where Fig. 23a indicates it should be. Total cost has a global minimum at f z around 0.6 to 0.7 mm and a speed of around 110 m/min. ECT is about 1.9 mm and the optimal cutter life is T O = 22 minutes. Because it may be impossible to reach the optimum feed value due to tool
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