Why Counterbore Is Used?

A counterbore is used to enlarge the opening of a hole producing a flat bottom so a socket-head screw will fit flush with the surface of the part. Lock washers may be used to ensure a secure assembly. Typically counterbores are used for a single purpose.

What is the purpose of a counterbore?

A tool referred to as a counterbore is typically used to cut the spotface, although an endmill may also be used. Only enough material is removed to make the surface flat. A counterbore is also used to create a perpendicular surface for a fastener head on a non-perpendicular surface.

Why is it called a counterbore?

The pre-drilled hole is meant to contain the fastener and to prevent the two work pieces from separating when their two flat surfaces are joined. The larger cavity matches at least the width and depth of the head of the fastener. This cavity is called the counterbore.