Section 13. qxd

BASIC MECHANICS OF METAL CUTTING
13-51
Fig. 13.4.4
Basic types of chips produced in metal cutting: (
a
) continuous chip with narrow, straight primary shear
zone; (
b
) secondary shear zone at the tool-chip interface; (
c
) continuous chip with large primary shear zone; (
d
) contin-
uous chip with built-up edge; (
e
) segmented or nonhomogeneous chip, (
f
) discontinuous chip. 
(Source: After M. C. Shaw.)
Fig. 13.4.5
Types of
tool wear in cutting.
Section_13.qxd 10/05/06 10:32 Page 13-51

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13-52
MACHINING PROCESSES AND MACHINE TOOLS
The recommended cutting speed for a high-speed steel tool is generally
the one which produces a 60- to 120-min tool life. With carbide tools, a
30- to 60-min tool life may be satisfactory. Values of 
n
typically range
from 0.08 to 0.2 for high-speed steels, 0.1 to 0.15 for cast alloys, 0.2 to
0.5 for uncoated carbides, 0.4 to 0.6 for coated carbides, and 0.5 to 0.7
for ceramics.
When tool-life equations are used, caution should be exercised in
extrapolation of the curves beyond the operating region for which
they are derived. In a log-log plot, tool life curves may be linear over
a short cutting-speed range but are rarely linear over a wide range of
cutting speeds. In spite of the considerable data obtained to date, no
simple formulas can be given for quantitative relationships between
tool life and various process variables for a wide range of materials
and conditions.
An important aspect of machining on computer-controlled equip-
ment is 
tool-condition monitoring
while the machine is in operation with
little or no supervision by an operator. Most state-of-the-art machine
controls are now equipped with tool-condition monitoring systems.
Two common techniques involve the use of (1) transducers that are
installed on the tool holder and continually monitor torque and forces
and (2) acoustic emission through a piezoelectric transducer. In both
methods the signals are analyzed and interpreted automatically for tool
wear or chipping, and corrective actions are taken before any significant
damage is done to the workpiece.
A term commonly used in machining and comprising most of the
items discussed above is