6.2 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;  - clearance angle;  - wedge angle;  - top rake angle;  - cutting angle

Due to the clearance angle , 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  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  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  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.

6.3. Cutting Speed and Chip Formation

In machining metals the cutting speed is of great importance. It is defined as the speed of the relative motion between the cutting tool and work. In planning and shaping, for example, the tool moves through a straight path over the work, detaching chip. The cutting speed V can be calculated by the following formula:

(6.1)

Cutting speed is normally expressed in meters per minute; for high-speed tools, e.g. grinding wheel, the cutting speed is expressed in meters per second.

In the case of rotating tools or workpieses the formula of the cutting speed is as follows:

, (6.2)

where D is diameter of workpieses (turning) or tools (milling, grinding) in mm;

n is revolution speed of workpiece or tool, revolution per minute, or (r.p.m).

When the cutting tool is pressed into the material, chip is produced. The production of chips and their shape are determined by the properties of the material to be tooled, the shape of the tool edge, the cutting edge geometry and the cutting speed. The different types of the chips produced are known as flowing (Fig. 6.3a) shearing (Fig. 6.3b) and tearing chips (Fig. 6.3c).

In the case of flowing chip, which is cut from tough materials (steel 10, 15, 25, 30, Cu, Al, Mg) at high cutting speed, a very smooth surface of the workpiece is produced. When tough materials are tooled at low cutting speeds, or when materials with low toughness are tooled, the shearing chip is produced (steel 50X, 65Г, 60XTC).

Fig. 6.3. The basic types of chip: a – flowing; b – shearing; c – tearing

In the case of brittle materials (grey cast-iron, hard steels, hard brass) the advancing crack is relatively large. The brittle materials fracture and discontinuous chip is formed. This type is called tearing chip. At low cutting speeds the work surface will be very rough.