unit 1 metal cutting and chip formation - IGNOU

Theory of Metal Cutting
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Figure 1.4 : Different Types of Chip Breakers
Figure 1.5 : Development of Built Up Edge [Rao, 2000]
Discontinuous or segmented chips are produced while machining brittle materials or
ductile materials at low speeds and high friction conditions. The basic difference between
the mechanism of formation of discontinuous chip and continuous chip is that, instead of
continuous shearing of the material ahead of the cutting tool, rupture occurs
intermittently producing segments of chip (Figures 1.3 and 1.6). These chips are smaller
in length hence easy to dispose off, and give good surface finish on the workpiece.
Discontinuous chips are formed when cutting brittle materials, or cutting ductile
materials at low speed, or cutting with tools of small rake angle.
1.3.3 Types of Cutting
(a) Tear Type (b) Shear Type
Figure 1.6 : Hypothesized Discontinuous Chip Formation [Rao, 2000]
Principally, there are two types of cutting :
(i) Orthogonal cutting, and
(ii) Oblique cutting.
Orthogonal Cutting
Orthogonal cutting operation is “the simplest type of cutting operation, in which
the cutting edge is straight, parallel to the original plane surface of the workpiece
and perpendicular to the direction of cutting, and in which the length of the cutting
edge is greater than the width of the chip removed (Figures 1.7(a) and (b))”. This
orthogonal cutting is also known as Two Dimensional (2-D) Cutting. A few of the
cutting tools perform orthogonally, such as lathe cut-off tools (Figure 1.7(a)),
straight (not helical) milling cutters, broaches, etc.
In actual machining, majority of the cutting operations (turning, milling, etc.) are
three dimensional (3-D) in nature and are called as oblique cutting. In oblique
cutting, the cutting edge of the tool is inclined to the line normal to the cutting
direction, and this angle is known as angle of obliquity. This is also called the
inclination angle, i (Figure 1.7(c)). Oblique cutting can be defined as “the cutting