Hand tapping and tap drill size

A tap is a hardened threaded tool with flutes used to cut an internal thread in a drilled hole. It is turned by a tap wrench. A die cuts an external thread on a rod, held in a die stock.

Set of taps

For through holes, taper or plug tap alone is often enough.

Procedure

1. Drill the hole to the tap drill size and chamfer its mouth.
2. Hold the work firmly in a vice with the hole axis vertical.
3. Fit the taper tap in the wrench and start it square to the work by light downward pressure.
4. Turn the wrench 1/2 to 1 turn forward, then 1/4 turn back to break and clear the chips; use cutting oil (not for cast iron).
5. Remove the tap, clear the chips and repeat with the intermediate and plug taps in order.
6. Check the thread with a bolt or a go/no-go gauge.

Tap drill size

Tap drill size = major diameter - pitch (for ISO metric threads of about 75% thread depth).

$$\begin{aligned} d_{tap} &= D - p \\ &= 12 - 1.75 = 10.25\ \text{mm} \end{aligned}$$

The nearest standard drill is 10.2 mm.

Answer: tap drill size = 10.25 mm (use 10.2 mm drill).

Die

A die cuts an external thread on a bar. The bar end is chamfered, the die is started square in the die stock and turned forward with periodic reverse to break chips, with cutting oil.