Chip formation in the metal removing process

Chip formation in the metal removing process

Fig shows represents an orthogonal cutting. It is a schematic representation of a shaping operation in which the work piece remains stationary and the tool advances into the work piece towards the left. Thus the metal in front of the tool gets compressed very severely, causing shear stress. This stress is maximum along a plane called shear plane. If the material of the work piece is ductile, the material flows plastically along the shear plane forming the chip which flows upwards along the face of the tool.

Here the metal structure starts getting elongated along the line PQ below the shear plane and continuous up to line RS above the shear plane, Where its deformation is complete. The complete area represented by PQRS, within which the metal deformation occurs, is known as shear zone.

TYPES OF CHIPS :-

The chips produced during machining of various metals can be broadly classified into the following three types.

1. Discontinuous or segmental chips

2. Continuous chip

3. Continuous chip with built up edge.

1. Discontinuous or segmental chips

This type of chips are produced during machining of brittle materials like cast iron and bronze. These chips are produced in the form of small segments, as shown in fig. in machining of such materials, as the tool advances forward, the shear plane angle gradually reduces until the value of compressive stress acting on the shear plane becomes too low to prevent rupture. Such chips are also some times produced in the machining of ductile materials. When low cutting speeds are used and lubrication is not provided.

Effects :-

1. Friction between chip and tool is high

2. Chip into small segments

3. Wear of the tool is high

4. Poor surface finish

5. Smaller rake angle on the tool and too much depth of cut.

2. Continuous chip

This type of chip is produced during the machining of ductile materials like mild steel, under favorable cutting conditions, such as high cutting speed and minimum friction between the chip and the tool face. If other wise, it will break and chip tool interface can be minimized by polishing the tool face and used coolant. In this process the chip moving smoothly up the tool face.

Advantages :-

1. rake angle is high and cutting speed is high.

2. good surface finish

3. less friction between chip and tool.

3. continuous chip with built up edge :-

These chips are usually formed while machining of ductile materials, when high friction exists at the chip tool interface. The upward flowing chip exerts pressure on the tool face. The normal reaction 𝑁𝑅 of the chip on the tool face is quite high and is maximum at the cutting edge and nose of the tool. This gives rise to an high temperature and the compressed metal adjacent to the tool nose gets welded to it. The extra metal welded to the nose or point of the tool is called built up edge. This metal is highly strain hardened and brittle with the result, as the chip flow up the tool, the built up edge is broken and carried away with the chip.

Effects :-

1. rough surface finish on the work piece

2. vibrations in cutting tool

3. low cutting speeds

4. small rake angle.

Chip breakers:-

1.chip breakers break the produce chip in to small pieces.

2. these are provided to control the continuous ribbon type chips.

3. the chip breaker deflect the chip at a shape angle cause it to break the small pieces

Types of chip breakers are commonly used :-

1. groove type :-

It consists of grinding a groove on the face of the tool, behind the cutting edge, leaving a small land near the tip.

2. step type :-

It consists of grinding a step on the face of the tool, adjacent to the cutting edge.

3. secondary rake type :-

it consists of providing a secondary rake on the tool through grinding, together with a small step.

4. clamp type :-

this type of chip breaker is very common with the carbide tipped tool. The chip breaker is a thin and small plate which is either brazed to or held mechanically on the tool face.