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.

When should a counterbore be used?

A counterbore hole is usually used when the head of a fastener, such as a hex head or socket head capscrew, is required to be flush with or below the level of a workpiece's surface. For a spotface, material is removed from a surface to make it flat and smooth, usually for a fastener or a bearing.

What is 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.