what is counterbore
A counterbore (symbol: â´) 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, is required to sit flush with or below the level of a workpiece's surface.Whereas a counterbore is a flat-bottomed enlargement of a smaller coaxial hole, a countersink is a conical enlargement of such. 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.