MACHINING ECONOMETRICS Machinery's Handbook, 31st Edition
1208
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
10
1
V = 260 V = 215 V = 175
V = 170, r = 0.8 V = 170, r = 1.2 V = 170, r = 1.6
0.1
0
0.05
0.1
0.15
0.2
0.25
0.01
0.1
1
ECT
ECT, mm
Fig. 14. R a versus ECT, Cutting Speed Constant, Nose Radius r Varies Fig. 15. R a versus ECT , Cutting Speed Varies, Nose Radius r Constant In roughing, multiply the finishing values found using Equation (6) by 1.5, thus, R a (Rough) = 1.5 3 R a (Finish) for each ECT and speed. Example 1: Find the decrease in surface roughness resulting from a tool nose radius change from r = 0.8 mm to r = 1.6 mm in finishing. Also, find the comparable effect in roughing. For finishing, using r 2 =1.6 and r 1 = 0.8, R a 1 / R a 2 = (1.6⁄0.8) 0.5 = 1.414, thus, the surface roughness using the larger tool radius is R a 2 = R a 1 ÷ 1.414 = 0.7 R a 1 In roughing, at the same ECT and speed, R a = 1.5 3 R a 2 = 1.5 3 0.7 R a 1 = 1.05 R a 1 . Example 2: Find the decrease in surface roughness resulting from a tool nose radius change from r = 0.8 mm to r =1.2 mm. For finishing, using r 2 =1.2 and r 1 = 0.8, R a 1 / R a 2 = (1.2⁄0.8) 0.5 = 1.224, thus, the surface roughness using the larger tool radius is R a 2 = R a 1 ÷ 1.224 = 0.82 R a 1 . In roughing, at the same ECT and speed, R a = 1.5 3 R a 2 = 1.5 3 0.82 R a 1 = 1.23 R a 1 . It is interesting to note that, at a given ECT , the R a curves have a minimum, see Fig. 13 and Fig. 15, while tool life shows a maximum, see Fig. 6b and Fig. 6c. As illustrated in Fig. 16, R a increases with tool life T when ECT is constant, in principle in the same way as does the force ratio.
10
1
ECT = 0.03 ECT = 0.08 ECT = 0.12 ECT = 0.18 ECT = 0.30
0.1
1
10
100
1000
T, min.
Fig. 16. R a versus T , Holding ECT Constant The Surface Finish Relationship: R a is determined using the same type of mathematical relationship as tool life and force calculations: ( )ln( ) y K M x H N L R 4 Ra Ra Ra Ra Ra a 2 0 = − − − − where K Ra , H Ra , M Ra , N 0 Ra , and L Ra are the five surface finish constants.
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