Cutting Process of Metals | Industries | Metallurgy

In this article we will discuss about the elements and observation of cutting process of metals in the industries.

Elements of Cutting Process (Mechanics of Metal Cutting):

Any cutting process involves work-piece, tool (including holding devices), chips and cutting fluid. For removing the metal, wedge shaped tool is constrained to move relative to the work-piece so that it removes the metal in the form of chips.

Referring in Fig. 22.1 (which shows the position of cutting tool in relation to work in order to cut metal with less effort), it will be seen that there are three basic angles of importance, viz., rake angle, clearance angle and setting angle.

First we shall consider the geometry involved in the cutting process assuming it to be a two dimensional process, i.e. we shall concentrate on one representative plane. This is with a view to simplify the treatment. This is also known as orthogonal cutting. In it the cutting edge of the tool is at right angles to the direction of relative motion between the tool and the workpiece.

Examples of orthogonal cutting such are- turning at the open end of a tube or planing of a rib with the tool wider than the rib width. Actual cutting processes involve tools and processes which are three dimensional. (Oblique cutting).

In orthogonal cutting, the cutting edge of tool is located at 90° to the motion of work-piece. In oblique cutting, the cutting edge of the tool is inclined and not perpendicular to motion of work-piece. It may be mentioned that principles developed for orthogonal cutting generally also apply to oblique cutting.

 

Observations of Cutting Process:

In any cutting process, following observations can be made:

(a) Metal is cut by removal of chips which may be in the form of continuous ribbon or discontinuous chips composed of individual segments, which depends on the work material and cutting conditions. The chip is thicker than the actual depth of cut and it is correspondingly shortened. The hardness of the chip is usually much greater than the hardness of the parent material.

(b) There is no flow of metal at right angles to the direction of chip flow.

(c) Flow lines are evident on the side and back of a chip, which suggests that cutting involves a shearing mechanism. On observing the flow lines on the surface of a chip it will be obvious that chips are formed by blockwise slip of metal.

However, the front surface is usually smooth due to a burnishing action. It is important to note that chips are formed due to process of deformation or plastic flow of material, which takes place by means of phenomenon called slip.

(d) A lot of heat is generated in the process of cutting due to friction between the chip and tool. The friction can be reduced by having sharp cutting edge and better tool finish, increased sliding speed, improved tool geometry, use of low friction work or tool materials, and use of a cutting fluid. The temperature of the cutting tool reaches a high value when taking a heavy cut at high speed.

(e) In Fig. 22.1, line AB is the dividing line between the work and the chip. The material above this line is deformed by an internal shearing process and comes out in the form of chip. The rate at which metal is deformed is high. The material below this line is under formed. Shear plane is the plane along line AB and perpendicular to the plane of paper.

(f) In front of the cutting tool point, generally no crack is observed. Due to strain hardening, the hardness of metal in chip, the built-up edge and near the finished surface is usually greater than that for the metal.

(g) Sometimes a built-up edge is formed at the tip of the tool and it significantly alters the cutting process. It deteriorates the surface finish and rate of tool wear is increased.

(h) The inclination of plane AB with respect to surface of work is known as shear angle. This angle increases when the tool friction is decreased; and increase of shear angle means that shear plane will be of smaller length and the thickness of chip will also be less.

If the shear stress on the shear plane be assumed to remain constant, then the force along the shear plane will also vary as the area of the shear plane. Further the amount of plastic deformation to which the chip is subjected also decreases as the shear angle increases.

(i) In the cutting process, three areas of interest requiring due consideration are shown in Fig. 22.1 by circles I, II, and III. The first one is along the shear plane, second is the interface between the chip and the tool face, and the third is the finished or machined surface and the material of tool adjacent to that surface.

(j) The steady state conditions on which the cutting theories are based often get changed by the formation of discontinuous chips. The tool geometry is also altered by the presence of built-up edge.