The machining with some material easily done whereas it may be difficult in other. The machinability of material varied from material to material. It is defined as each material can be satisfactorily in machined. It can be also defined as
- The life of tool material before tool failure or Re-sharpening.
- The power consumption per unit volume of material removal.
- The quality of material surface finishing.
Generally, the good machinability associated with material removal with moderate forces.When the action of proper chips does not causes excessive tool wear and good surface finish.Sometime high hardnessof material will be give poor machin-ability because of high temperature during machining, more power consumption and high tool wear.
Variables affecting on machinability:
The different work variables affected below,
- Chemical composition of workpiece material
- The mechanical properties like that toughness, brittleness, ductility and etc.
- The microstructure composition of workpiece material
- Method of production of work material
- Physical properties of work piece material
The most important tool variables are,
- Nature of engagement of tool with work
- The geometry and tool material
- Rigidity of tool
- Machining variables
- Power and accuracy of machine tool
- Rigidity of machine
When the machine should be rigid and have sufficient power to withstand the induced cutting forces and to minimise the deflection.
The cutting speed having the greatest influence on life of tool. When the cutting speed increased, the surface finish of material will be improved. And also the dimension of cut have been influence on life of tool.
Evolution of machinability:
The following criteria suggested for evaluation. They are,
- Surface finish
- Tool life per grind.
- The rate of metal removal tool grind.
- Dimensional stability of work finish
- Each of chip disposal.
- Shape and size of chips.
- Chip hardness.
- Magnitude of cutting forces and power consumption.
- Heat generated during the cutting process.
The Advantages of high machinability:
- Less power consumption
- Less tool wear
- The high metal removal rate
- The good surface finish can be produced
- High cutting speed can be used
Machinability index (I):
It is a quantitative measure of machinability. It is used to compare the machinability of different types of materials and act as quick and reliable second method. The rated machinability of two or more material maybe vary for different process of cutting like as heavy turning, light turning, milling, forming and etc. US national standard for 100% machin-ability is SAE 1112 for hot rolled Steels. It is widely used as standard Steel for comparison.
The common material of “I” such as,
- Stainless steel = 25%
- Aluminium alloy = 390-1500 %
- Magnesium alloy = 500-2000 %
- Low Carbon Steel = 55-60%
- Red brass = 180%