Cutting speed distribution
Drills have two cutting edges that are rotated and fed into the workpiece and this is the basis of how a drill works.
The cutting speed of a drills’ cutting edge varies along the cutting edge. A higher cutting speed
is experienced at the peripheral cutting edge and gradually lowers towards the centre. The chisel edge has zero cutting speed.
Edge cutting conditions
The rake angle becomes smaller towards the centre of the drill. At the chisel edge the rake angle is negative. This can be seen in cross-section C in the image and tends to pushes the workpiece rather than cut it.
Thrust generation during cutting
50％～70％ of all thrust experienced during drilling is created by chisel edge. To reduce the amount of thrust web thinning can be used.