1/4 X 3/8 Shank High Speed Steel 2 Flute Solid Cap Screw Counterbore hole

1/4 X 3/8 Shank High Speed Steel 2 Flute Solid Cap Screw Counterbore hole

1/4 X 3/8 SHANK HIGH SPEED STEEL 2 FLUTE SOLID CAP SCREW COUNTERBORE(2007-0105)

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. Source Wikipedia

What is the difference between countersink and counterbore?

The difference between them is the angle at which the larger hole is drilled. A countersink’s larger hole is tapered at an angle; whereas a counterbore is drilled straight into the material and leaves a flat bottom between the counterbore and the narrower inner shaft.

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.