Section 13. qxd

coefficient of friction
at the tool-chip interface is given by
m
(
F
t
F
c
tan 
a
)/(
F
c
F
t
tan 
a
). The 
friction force
along the tool is
F
F
t
cos 
a
F
c
sin 
a
.
The 
shear stress
in the shear plane is 
t
(
F
c
sin 
f
cos 
f
F
t
sin
2
f
)
/ A
0
, where 
A
0
is the cross-sectional area
that is being cut from the workpiece.
The coefficient of friction on the tool face is a complex but important
factor in cutting performance; it can be reduced by such means as the use
of an effective cutting fluid, higher cutting speed, improved tool material
and condition, or chemical additives in the workpiece material.
The net 
power
consumed at the tool is 
P 
F
c
V
. Since
F
c
is a func-
tion of tool geometry, workpiece material, and process variables, it is
difficult reliably to calculate its value in a particular machining opera-
tion. Depending on workpiece material and the condition of the tool,
unit power
requirements in machining range between 0.2 hp
min/in
3
(0.55 W
s/mm
3
) of metal removal for aluminum and magnesium
alloys, to 3.5 for high-strength alloys. The power consumed is the prod-
uct of unit power and rate of metal removal: 
P
(unit power)(vol/min).
The power consumed in cutting is transformed mostly to 
heat.
Most 
of the heat is carried away by the chip, and the remainder is divided
between the tool and the workpiece. An increase in cutting speed or feed
will increase the proportion of the heat transferred to the chip. It has been
observed that, in turning, the average interface