A counterbore is a cylindrical flat-bottomed hole that enlarges another coaxial hole, or the tool used to create that feature. A counterbore hole is typically used when a fastener, such as a socket head cap screw or fillister head screw, is required to sit flush with or below the level of a workpiece's surface.

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.

What is the difference between countersink and counterbore?

The key difference between countersink and counterbore screws are the size and shape of the holes, counterbore holes are wider and more square to allow for the addition of washers. ... Countersinking creates a conical hole matching the angled shape on the underside of a flat-head screw.