Section 13. qxd

temperature
between the
tool and the chip increases with cutting speed and feed, while the influ-
ence of the depth of cut on temperature has been found to be limited.
Interface temperatures to the range of 1,500 to 2,000
F (800 to 1,100
C)
have been measured in metal cutting. Generally the use of a cutting fluid
removes heat and thus avoids temperature buildup on the cutting edge.
In cutting metal at high speeds, the chips may become very hot and
cause safety hazards because of long spirals which whirl around and
become entangled with the tooling. In such cases, 
chip breakers
are
introduced on the tool geometry, which curl the chips and cause them
to break into short sections. Chip breakers can be produced on the face
of the cutting tool or insert, or are separate pieces clamped on top of the
tool or insert.
A phenomenon of great significance in metal cutting is 
tool wear.
Many factors determine the type and rate at which wear occurs on the
tool. The major critical variables that affect wear are tool temperature,
type and hardness of tool material, grade and condition of workpiece,
abrasiveness of the microconstituents in the workpiece material, tool
geometry, feed, speed, and cutting fluid. The type of wear pattern that
develops depends on the relative role of these variables.
Tool wear can be classified as (1) flank wear (Fig. 13.4.5); (2) crater
wear on the tool face; (3) localized wear, such as the rounding of
the cutting edge; (4) chipping or thermal softening and plastic flow of
the cutting edge; (5) concentrated wear resulting in a deep groove at the
edge of a turning tool, known as 
wear notch.
In general, the wear on the flank or relief side of the tool is the most
dependable guide for 
tool life.
A wear land of 0.060 in (1.5 mm) on high-
speed steel tools and 0.015 in (0.4 mm) for car-
bide tools is usually used as the endpoint. The
cutting speed is the variable which has the
greatest influence on tool life. The relationship
between tool life and cutting speed is given by
the Taylor equation 
VT
n
C
, where 
V
is the
cutting speed; 
T
is the actual cutting time to
develop a certain wear land, min; 
C
is a con-
stant whose value depends on workpiece mate-
rial and process variables, numerically equal to
the cutting speed that gives a tool life of 1 min;
and 
n
is the exponent whose value depends on
workpiece material and other process variables.