Section 13. qxd

Continuous chips
are formed by continuous deformation of the work-
piece material ahead of the tool, followed by smooth flow of the chip
along the tool face. These chips ordinarily are obtained in cutting duc-
tile materials at high speeds.
Fig. 13.4.2
Basic mechanics of metal cutting process.
Chip
Rake 
face
Clearance 
face
α
φ
α
β
β−α
Tool
Workpiece
R
F
F
s
F
c
A
o
F
n
F
t
N
12
t
1
Fig. 13.4.3
Force system in metal cutting process.
Section_13.qxd 10/05/06 10:32 Page 13-50

---

Discontinuous chips
consist of segments which are produced by frac-
ture of the metal ahead of the tool. The segments may be either loosely
connected to each other or unconnected. Such chips are most often
found in the machining of brittle materials or in cutting ductile materi-
als at very low speeds or low or negative rake angles.
Inhomogeneous (serrated) chips
consist of regions of large and small
strain. Such chips are characteristic of metals with low thermal con-
ductivity or metals whose yield strength decreases sharply with tem-
perature. Chips from titanium alloys frequently are of this type.
Built-up edge chips
consist of a mass of metal which adheres to the
tool tip while the chip itself flows continuously along the rake face. This
type of chip is often encountered in machining operations at low speeds
and is associated with high adhesion between chip and tool and causes
poor surface finish.
The 
forces
acting on the cutting tool are shown in Fig. 13.4.3. The
resultant force 
R
has two components, 
F
c
and 
F
t
.
The cutting force 
F
c
in
the direction of tool travel determines the amount of work done in cutting.
The thrust force 
F
t
does no work but, together with 
F
c
, produces deflec-
tions of the tool. The resultant force also has two components on the shear
plane: 
F
s
is the force required to shear the metal along the shear plane,
and 
F
n
is the normal force on this plane. Two other force components also
exist on the face of the tool: the friction force 
F
and the normal force 
N.
Whereas the cutting force 
F
c
is always in the direction shown in Fig.
13.4.3, the thrust force 
F
t
may be in the opposite direction to that shown
in the figure. This occurs when both the rake angle and the depth of cut
are large, and friction is low.
From the geometry of Fig. 13.4.3, the following relationships can be
derived: The