Counterboring

Counterboring is used to enlarge the diameter of the top of a drilled hole such that the head of a bolt or cap screw can sit flush with the top surface of the workpiece.

Counterboring may also be used to sink Woodscrews deep into a piece of timber such that the head may be covered with a plug of material (e.g. as cut using a Plug Cutter) or to avoid the use of very long screws by setting the screw head part way through the first piece to be joined