Principles of Cutting and Shaping the Metals

 

In many cases the desired shape and dimensions of a workpiece are obtained by detaching chips from the material by means of cutting tools. Machine tools shape workpiece by cutting operations (Fig. 6.1). During the sequence of operations workpiece and tool perform certain motions relative to each other.

The process of chip removal is affected by the working motions of the machine tool (formative motions), which are transmitted either to the cutting tool, or to the workpiece, or to the both simultaneously. Working motions of the machine tool include a primary cutting motions and a feed motion (motions). Each of the working motions is specified by its speed or rate.

The primary cutting motion V provides cutting of the chip from the blank at a cutting speed V equal to the velocity with which the chip leaves the work.

The feed motions enable the cutting process to be extended to the whole surface to be machined; it may be longitudinal S₁ or transverse S_t.

In addition to the working motions auxiliary motions are needed to prepare the machine, tool and work for carrying out the cutting process. One of them is setup motion of tool S_s, which, determines the cross-sectional area of the chip, or the thickness t of removed metal.

 

 

Fig. 6.1. Schemes of the basic machining operations: a – turning; b - milling;

c – drilling; d – planning; e, f – grinding; g – broaching

A straight-line reciprocating primary cutting motion V is employed in shapers (Fig.6.1d), planers, slotters, etc. This motion can be transmitted either to the tool, as in shapers and slotters, or to the work, as in planers.

The cutting cycle consists of a working stroke V_w during which the tool cuts a chip, and the idle or return stroke V_r, when the tool or work returns to its initial position.

In turning (Fig. 6.la), drilling (Fig. 6.1c), milling (Fig. 6.1b) and grinding (Fig.6.1e, f) the primary cutting motions are rotary motions of work, or of tool. In broaching (Fig. 6.1g) longitudinal feed is absent.

There is no set-up motion in the drilling, so the width of cut is determined by drill diameter.

 

Geometry of a Cutting Tool

 

The material of the cutting tool must be of higher hardness than the material of the work. The illustrations given above show that the wedge is the basic shape of any cutting tool, whereas the specific shape of the wedge depends on the intended purpose. There are certain angles at a cutting tool, which determine the efficiency of the tool and the value of the applied cutting forces.

The principles underlying cutting-tool angles are the same whether the tool is a turning (lathe) tool, a milling cutter, or a grinding wheel. Since turning tool is the easiest to visualize, it will be discussed in details (Fig. 6.2). It consists of two parts: working (cutting) I and shank II ones. The working operations of turning are carried out by the working part; the shank is intended for fixing of the tool on the lathe.

Fig. 6.2. Elements, surfaces and cutting angles of a turning tool: I – working part; II – shank;

1 – top race; 2 – main back rake; 3 – main edge; 4 – auxiliary back rake; 5 – point;

6 – auxiliary edge; a - clearance angle; b - wedge angle; g - top rake angle; d - cutting angle

 

Due to the clearance angle a, only the cutting edge of the tool contacts with the surface of the workpiece. Thus, friction is reduced and additional rise in temperature in cutting avoided. This angle is limited by the lower face of the wedge and the surface of the workpiece.

The wedge angle b determines the resisting force of the cutting edge. The larger the wedge angle, the longer the tool life (during which a sharpened tool can be used without interruption until it becomes blunt), the higher the cutting force value. The wedge angle is limited by the top of the wedge also known as top (true) rake and the lower face, known as main back rake (flank).

The top rake angle g is formed by the top rake, also known as face, of the wedge and the (imaginary) line running perpendicular to the surface of the workpiece. The value of this angle determines the formation of chip. Large top rake angle allows the chip to peel off easily.

The cutting angle d largely determines the cutting operations. It is formed by the top face of the wedge and the surface of the workpiece. It determines, among other things, shape and size of the chips.

Shape and size of chips, moreover, are determined by plasticity of work.

Continuous chips are received, when metal has high plasticity; and discontinuous chips are formed, when plasticity is low.