Twist Drill Angles - Rake/Helix, Lips Clearance, Point/Cutting, Chisel Edge

 

Read: Twist Drill Nomenclature/ Parts of a Twist Drill

The geometry and sharpening of the tool are crucial in the performance of twist drill. The special tool grinders are available for sharpening and reshaping of tools cutting edge.
The various angles of twist drill are given below

Rake angle/helix angle

It is the angle of the flute in relation to the work surface. The value of rake angle varies between 18° to 45°; its typical value is 30°.

The rake angle governs the tightness with which chip curls and hence the amount of space occupied by the chip. The higher rake angle cause tightly rolled chip while the small rake angle tends to chip curl into more loosely rolled helix. Large rake angle causes excessive vibration and chatter of drill.

Lips clearance angle

It is the angle formed by the flank and a plane perpendicular to the axis. Lip clearance is the relief given to the cutting edges that allow drill to enter into the workpiece without any hindrance.   In general purpose drills, the clearance angle behind the cutting edge is 12°. Too much clearance angle cause the breaking of the lip of drill due to insufficient support. Clearance angle should be kept minimum in order to provide enough strength and rigidity to cutting edge. Too little clearance results drills having little or no cutting edge.

Cutting angle/ Point angle

The cutting angle is the angle between two lips when it is projected on a plane parallel to the axis. Simply it is the angle between the lips in the side view of a twist drill. Normal of value of Point angle is 118°.  Both cutting edges are sharpened at the same angle, so each makes angle of 59° to the center axis.

Chisel edge angle

It is the angle included between the chisel edge and lip of the twist drill when measuring on a plane normal to the axis.