Chip Color and Shape
1.53
The color and shape of metal chips from
a machine tool operation are good indicators of
how well the machining is being done. The harder the tool is working the
metal, the hotter the chips become. As the chips get hotter, they gradually
change color as indicated in Table 1-7: first to straw yellow, then brown,
purple, blue, and finally gray. You can tell two important things from the
color of the chips:
•
whether the tool is cutting efficiently
•
whether the tool temperature is within limits.
1.54
The shape of metal chips tells a great deal about what kind of work
a machine is producing. The three general shapes of chips are:
discontinuous, continuous, and continuous with a built-up edge. Knowing
what causes the chips to have different shapes will help you understand
what the machine is doing.
1.55
The discontinuous chip, shown in Fig. 1-7A, forms when the
removed metal breaks into short segments as the cutting action of the tool
forces it across the tool face. Brittle materials like cast iron produce this
type of chip. A fair surface finish and a reasonable tool life usually result
F
N
F
4
0
ipr
Then F
F
speed
ipm
r
t
m
r
=
×
=
×
=
=
×
=
×
=
.
.
.
.
002
0 008
0 008
100
0 800
16
T
a
ble 1-7.
Tool
p
erform
a
nce me
a
sured b
y
color of chi
p
s
p
roduced
Temp
.
r
a
n
ge
C
olo
r
of
c
h
ips
C
o
b
alt
alloy
H
ig
h-
speed
steel
C
a
rb
ide
Type
of
tool
mate
r
ial
No color
Straw yellow
Brown
Purple
Blue
Gray
Hot
Hottest
Fair
Fair
Best
Best
Good
Best
Best
Good
Best
Best
Workpiece
Tool bit
Chip
Tool bit
Chip
Tool bit
Chip
A
. D
isco
n
ti
nu
o
u
s
(
br
o
k
e
n
pieces)
B. C
o
n
ti
nu
o
u
s
(lo
n
g
c
ur
led
st
r
a
n
ds)
C. C
o
n
ti
nu
o
u
s
w
it
h bu
ilt
-u
p
edge
Workpiece
Workpiece
Fig. 1-7.
Types of chips formed by machining operations