Machinery's Handbook, 31st Edition
1230 MACHINING ECONOMETRICS Physics Behind hm and ECT , Forces and Tool Life ( T ).— The ECT concept for all metal cutting and grinding operations says that the more energy put into the process, by increas ing feed/rev, feed/tooth, or cutting speed, the life of the edge decreases. When increasing the number of teeth (keeping everything else constant) the work and the process are sub jected to a higher energy input resulting in a higher rate of tool wear. In high-speed milling when the angle of engagement AE is small the contact time is shorter compared to slot milling ( ar / D = 1) but the chip becomes shorter as well. Maintain ing the same chip thickness as in slot milling has the effect that the energy consumption to remove the chip will be different. Hence, maintaining a constant chip thickness is a good measure when calculating cutting forces (keeping speed constant), but not when determining tool wear. Depending on cutting conditions the wear rate can either increase or decrease, this depends on whether cutting occurs on the left or right side of the H -curve. Fig. 26a shows an example of end milling of steel with coated carbide inserts, where cut ting speed V is plotted versus ECT at 5, 15, 45 and 180 minutes tool-lives. Notice that the ECT values are independent of ar / D or number of teeth or feed/tooth, or whether f z or f z 0 is used, as long as the corresponding f z / f z 0 -ratio is applied to determine ECT E . The result is one single curve per tool life. Had cutting speed been plotted versus f z 0 , ar / D , or z values (number of teeth), several curves would be required at each constant tool life, one for each of these parameters This illustrates the advantage of using the basic parameter ECT rather than f z , or hm , or ar / D on the horizontal axis.
1000
T =5 T =15 T =45 T =180
H-CURVE
G-CURVE
HS 6
SL 2
HS 2
SL 6
100
0.001
0.01
0.1
1
ECT , mm
Fig. 26a. Cutting Speed versus ECT, Tool Life Plotted, for End Milling Example: The points (HS2, HS6) and (SL2, SL6) on the 45-minute curve in Fig. 26a relate to the previous high-speed and full slot milling examples for 2 and 6 teeth, respectively. Running a slot at f z 0 = 0.17 mm/tooth ( hm = 0.108, ECT E = 0.103 mm) with 2 teeth and for a tool life 45 minutes, the cutting speed should be selected at V = 340 m/min at point SL2 and for six teeth ( hm = 0.108 mm, ECT E = 0.308) at V = 240 m/min at point SL6. When high-speed milling for ar / D = 0.03 at f z = 3.394 3 0.17 = 0.58 mm/tooth = 0.58 mm/tooth, ECT is reduced to 0.011 mm ( hm = 0.108) the cutting speed is 290 m/ min to maintain T = 45 minutes, point HS2. This point is far to the left of the H -curve in Fig. 26b, but if the number of teeth is increased to 6 ( ECT E = 3 3 0.103 = 0.3090), the cutting speed is 360 m/min at T = 45 minutes and is close to the H -curve, point HS6. Slotting data using 6 teeth are on the right of this curve at point SL6, approaching the G -curve, but at a lower slotting speed of 240 m/min.
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