Countersinking means creating a recess by using a mechanical machining operation in a hole that has been drilled. The purpose of this machining operation is to countersink a screw into the drilled material so it no longer protrudes above the surface of the material. The machining operation which achieves this is called "countersinking".
In order to countersink screws, special countersinks are used. Depending on the head of the screw to be countersunk, choose countersinks for trapezoidal headed screws, and choose counterbores or countersink milling cutters for screws such as cylinder screws or round-headed screws.