Tapping is the operation done for forming internal threads by means of the tool called tap. Before a tap is used for cutting internal threads, a hole is to be drilled. The diameter of the hole should be such that it should have sufficient material in the hole for the tap to cut the thread.

Tap drill size is given by:

Tap drill size = Major diameter - Pitch

Calculation:

In M10 × 1.5, 10 mm is the major diameter of the thread and 1.5 mm is the pitch diameter.

Tap drill size = 10 - 1.5 = 8.5 mm

The die nut is used for chasing or reconditioning the damaged threads.
Die nuts are not to be used for cutting new threads. The die nuts are available for different standards and sizes of threads. The die nut is turned with a spanner.