counterbore

A counterbore can refer to a cylindrical flat-bottomed hole, which enlarges another hole, or the tool used to create that feature. It is usually used when a bolt or cap head screw is required to sit flush with or below the level of a workpiece's surface (By comparison, a countersink makes a conical hole and is used to seat a flathead screw).

The uppermost counterbores shown in the image are the same tool. The smaller top item is an insert, the middle shows another three-fluted counterbore insert, assembled in the holder. The shank of this holder is a morse taper although there are other machine tapers that are used in the industry.

The lower image is of a plain counter bore designed to fit into drill chuck, and being smaller, is not economical to make as one piece.

• Burrs are small cutters used in die grinders, rotary tools or dentist's drills. The name may be considered appropriate when their small sized head (3 mm diameter shaft) is compared to that of a burr (fruit) seed.

  To maintain the correct surface speed and cutting conditions they are rotated at the highest speed possible, commensurate with their size and construction. The cutters shown in the image are made from tungsten carbide which allows them to run at higher speeds than similar HSS cutters, yet still maintain their cutting edges.