Mechanics of Metal Cutting
Mechanics of Metal Cutting: Metal cutting operation is illustrated in Fig.1. The work piece is securely clamped in a machine tool vice or clamps or chuck or collet. A wedge shape tool is set to a certain depth of cut and is forced to move in direction as shown in figure. All traditional machining processes require a cutting tool having a basic wedge shape at the cutting edge. The tool will cut or shear off the metal, provided (i) the tool is harder than the metal, (ii) the tool is properly shaped so that its edge can be effective in cutting the metal, (iii) the tool is strong enough to resist cutting pressures but keen enough to sever the metal, and (iv) provided there is movement of tool relative to the material or vice versa, so as to make cutting action possible. Most metal cutting is done by high speed steel tools or carbide tools. In metal cutting, the tool does not slide through metal as a jack knife does through wood, not does the tool split the metal as an axe does a log. Actually, the metal is forced off the workpiece by being compressed, shearing off, and sliding along the face of the cutting tool. The way a cutting tool cuts the metal can be explained as follows. All metals in the solid state have a characteristic crystalline structure, frequently referred to as grain structure. The grain or crystals vary in size from very fine to very coarse, depending upon the type of metal and its
heat-treatment. The cutting tool advances again in the work piece. Heavy forces are exerted on the crystals in front of the tool face. These crystals, in turn exert similar pressures on crystals ahead of them, in the direction of the cut or force applied by the cutter. As the tool continues to advance, the material at sheared point is sheared by the cutting edge of the tool or it may be torn loose by the action of the bending chip which is being formed. As the tool advances, maximum stress is exerted along sheared line, which is called the shear plane. This plane is approximately perpendicular to the cutting face of the tool. There exists a shear zone on both sides of the shear plane, when the force of the tool exceeds the strength of the material at the shear plane, rupture or slippage of the crystalline grain structure occurs, thus forming the metal chip. The chip gets separated from the workpiece material and moves up along the tool face. In addition, when the metal is sheared, the crystals are elongated, the direction of elongation being different from that of shear. The circles which represent thecrystals in the uncut metal get elongated into ellipses after leaving the shearing plane.
Fig. 1 Metal cutting operation
TYPES OF CHIPS: In a metal cutting operation is carried out in machine shop. Chips are separated from the workpiece to impart the required size and shape to the workpiece. The type of chips edge formed is basically a function of the work material and cutting conditions. The chips that are formed during metal cutting operations can be classified into four types:
1. Discontinuous or segmental chips
2. Continuous chips
3. Continuous chips with built-up edge.
4. Non homogenous chips
The above three common types of chips are shown in Fig. 2 . Fig. 2 (a) shows continuous chips coming out during machining in machine shop. These types of chips are obtained while machining ductile material such as mild steel and copper. A continuous chip comes from the cutting edge of a cutting tool as a single one piece, and it will remain as one piece unless purposely broken for safety or for convenience in handling. Formation of very lengthy chip is hazardous to the machining process and the machine operators. It may wrap up on the cutting tool, work piece and interrupt in the cutting operation. Thus, it becomes necessary to deform or break long continuous chips into small pieces. It is done by using chip breakers. Chip breaker can be an integral part of the
tool design or a separate device. Fig. 2 (b) shows discontinuous chips coming out during machining in machine shop.
In this type, the chip is produced in the form of small pieces. These types of chips are obtained while machining brittle material like cast iron, brass and bronze. Fairly good surface finish is obtained and tool life is increased with this type of chips. Fig. 2 (c) shows continuous chip with built-up edge. During cutting operation, the temperature rises and as the hot chip passes over s the face of the tool, alloying and welding action may take place due to high pressure, which results in the formation of weak bonds in microstructure and weakened particles might pullout. Owing to high heat and pressure generated, these particles get welded to the cutting tip of the tool and form a false cutting edge. This is known as built-up edge
Fig. 2 Common types of chips
Non homogenous chips are developed during machining highly hard alloys like titanium which suffers a marked decrease in yield strength with increase in temperature