Machinability

MACHINABILITY

Machining may be easier in some materials whereas it may be difficult in other. This difference may be attributed to the machinability of various materials. Machinability is defined as the ease with which a material can be satisfactorily machined. It can also be measured by the following factors.

• The life of tool before tool failure or resharpening.

•  The quality of the machined surface.

• The power consumption per unit volume of material removed.

In general, a good machinability is associated with the removal of material with moderate forces. The action of proper chips does not cause excessive tool wear and it produces a good surface finish. Generally, high hardness gives poor machinability because of high temperature, more power consumption and high tool wear.

1. Variables Affecting Machinability

1. Work variables:

The various work variables affecting machinability are:

• chemical composition of workpiece material

• micro-structure composition of workpiece material

• mechanical properties such as ductility, toughness, brittleness etc.

• physical properties of workpiece material

• method of production of the work material.

2. Tool variables:

The various tool variables affecting machinability are:

• the geometry and tool material

• nature of engagement of tool with the work

• rigidity of tool.

3. Machine variables:

The machine variables are:

• rigidity of the machine

• power and accuracy of the machine tool.

The machine should be rigid and it has sufficient power to withstand the induced cutting forces and to minimize deflections.

4. Cutting conditions:

Cutting speed has the greatest influence on tool life. The surface finish is improved by increasing the cutting speed. Dimensions of cut also have an influence on tool life.

2. Evaluation of Machinability

The following are the criteria suggested for evaluating machinability:

• tool life per grind

• rate of metal removal per tool grind

• magnitude of cutting forces and power consumption

• surface finish

• dimensional stability of the finished work

• heat generated during cutting

• ease of chip disposal

• chip hardness

• shape and size of chip.

3. Advantages of High Machinability

The following are the advantages of having high machinability.

(i) Good surface finish can be produced.

(ii) Higher cutting speed can be used.

(iii) It needs less power consumption.

(iv) Metal removal rate is high.

(v) Less tool wear occurs.

4. Machinability Index

It is a quantitative measure of machinability. It is used to compare the machinability of different metals and acts as a quick and reliable checking method. The rated machinability of two or more metals may vary for different processes of cutting such as heavy turning, light turning, forming, milling etc. US National Standard for 100% machinability is SAE 1112 hot rolled steel. This steel is widely used as standard steel for comparison.

Machinability index,

The machinability index for some common materials is given below.

Low carbon steel - 55 - 60%

Stainless steel - 25%

Red brass - 180%

Aluminium alloy - 390 - 1500%

Magnesium alloy - 500-2000%.