MACHINING ECONOMETRICS Machinery's Handbook, 31st Edition
1202
1000
H-CURVE
G-CURVE
100
T = 1 T = 5 T = 15 T = 30 T = 60 T = 100 T = 300
10
0.01
0.1
1
ECT , mm
Fig. 6c. Cutting Speed versus ECT , V - ECT , Tool Life Plotted The V - ECT - T Graph and the Tool Life Envelope.— The tool life envelope, in Fig. 7, is an area laid over the V - ECT - T graph, bounded by the points A, B, C, D, and E, within which successful cutting can be realized. The H- and G -curves represent two borders, lines AE and BC. The border curve, line AB, shows a lower limit of tool life, T MIN = 5 minutes, and border curve, line DE, represents a maximum tool life, T MAX = 300 minutes. T MIN is usually 5 minutes due to the fact that tool life versus cutting speed does not follow a straight line for short tool-lives; it decreases sharply towards one minute tool life. T MAX varies with tool grade, material, speed and ECT from 300 minutes for some carbide tools to 10,000 minutes for diamond tools or diamond grinding wheels, although systematic studies of maximum tool-lives have not been conducted. Sometimes the metal cutting system cannot utilize the maximum values of the V - ECT - T envelope, that is, cutting at optimum V - ECT values along the G -curve, due to machine power or fixture constraints, or vibrations. Maximum ECT values, ECT MAX , are related to the strength of the tool material and the tool geometry, and depend on the tool grade and material selection, and require a relatively large nose radius.
1000
T = 1 T = 5 T = 15 T = 30 T = 60 T = 100 T = 300
H-curve
Big Radius To Avoid Breakage
A
A'
G-curve
O F
Tool Breaks
B
E'
E
O R
Tmax
100
D
C
0.01
0.1
1
ECT , mm
Fig. 7. Cutting Speed versus ECT , V - ECT , Tool Life Plotted Minimum ECT values, ECT MIN , are defined by the conditions at which surface finish suddenly deteriorates and the cutting edge begins rubbing rather than cutting. These conditions begin left of the H -curve, and are often accompanied by vibrations and built-up edges on the tool. If feed or ECT is reduced still further, excessive tool wear with sparks and tool breakage, or melting of the edge occurs. For this reason, values of ECT lower
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