To soften the steel

To add cutting ability

To increase wear resistance

To refine the grain structure of the steel

Protect work surface

Raise the head width

Provide leak proof joint

Provide access for tools

Check the hole with perfect

Check the inside threaded dia

Check tapered hole with perfect fit

Check the taper accuracy of outside dia

For lubrication

Permit clearance

For expansion

Permit feeding of lapping compound

Diamond

Boron carbide

Silicon carbide

Aluminium oxide