Machinery's Handbook, 31st Edition
MACHINING ECONOMETRICS 1207 where L is the constant in the tool life equation, Equation (4a) or (4b), and L F is the corre sponding constant in the force ratio equation, Equation (5). In Fig. 12 this function is plot ted for a = 0.5 and for L F / L = 5, 10, and 20. Accurate calculations of aforementioned relationships require elaborate laboratory tests, or better, the design of a special test and follow-up program for parts running in the ordinary production. A software machining program, such as Colding International Corp. COMP program can be used to generate the values of all three forces, torque, and power requirements both for sharp and worn tools. Surface Finish and Tool Life.— It is well known that the surface finish in turning de - creases with a bigger tool nose radius and increases with feed; usually it is assumed that the R a (or Ra, for roughness average) value increases with the square of the feed per revo- lution, and decreases inversely with increasing size of the nose radius. This formula, de- rived from simple geometry, gives rise to great errors. In reality, the relationship is more complicated because the tool geometry must be taken into account, and the work material and the cutting conditions also have a significant influence.
10
V = 475 V = 320 V = 234 V = 171 V = 168 V = 144 V = 120
1
0.1
0.001
0.01
0.1
1
ECT , mm
Fig. 13. R a versus ECT , Nose Radius r Constant Fig. 13 shows surface finish R a versus ECT at various cutting speeds for turning cast iron with carbide tools and a nose radius r = 1.2 mm. Increasing the cutting speed leads to a smaller R a value. Fig. 14 shows how the finish improves when the tool nose radius, r , increases at a constant cutting speed (168 m/min) in cutting nodular cast iron. In Fig. 15, R a is plotted versus ECT with cutting speed V for turning a 4310 steel with carbide tools, for a nose radius r = 1.2 mm, illustrating that increasing the speed also leads to a smaller R a value for steel machining. A simple rule of thumb for the effect of increasing nose radius r on decreasing surface finish R a , regardless of the ranges of ECT or speeds used, albeit within common practical values, is as follows. In finishing, (6) r 2 0.5 = c m
R R
2 1
a a
r
1
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