Following are major types of drilling ;
- Step drilling: More than one diameter can be ground on the drill body which saves an extra operation.
- Counter boring: Often a flat surface is needed around a hole to provide a good seating area for washer and nuts/head of a bolt. The counter boring tool has a pilot, which ensures that the counterbore is concentric with the hole.
- Counter sinking: Counter sinking provides a tapered entrance to the hole. A special counter sinking tool with a pilot is used as shown.
- Reaming: Reaming is an operation of sizing and improving the geometry and finish of a
previously drilled hole. Hand, machine and shell reamers are used for this purpose. Machine reamers are used with a drilling machine. To work efficiently, proper stock allowance is very important. Reamers cannot remove much material, but at the same time, enough material should be available all round. For holes up to 12.5 mm in diameter, about 0.38–0.4 mm of material is left as reaming allowance. A reamer follows the original hole and cannot shift its centre. - Tapping: Tapping is also done on a drilling machine with a special flexible adapter for holding machine taps. Tapping means cutting internal threads in a hole. A machine tap set consists of two tapsrough and finish. Both taps should be used in the same order. During tapping, the spindle r.p.m. is drastically reduced and a good lubricant used.
- Core drilling: Holes made in castings by use of cores, are rough and require a special kind of drill, called core drill to clean up the holes. This operation is called core drilling.
