Taps - Plug - UNC

Taps are tools used by engineers for creating a female thread inside a hole. The tap is inserted into a special wrench and turned whilst cutting oil is used to lubricate the tap.

When forming a brand new thread in a blind hole, it is usually a three-stage process.

UNC stands for Unified National Coarse. It is a common imperial thread type with less threads per axial distance than its UNF equivalent.

Unified thread types (UNF and UNC) were introduced at the beginning of World War 2 to enable the interchangeability of machinery manufacturing between the USA, Canada and Great Britain. Still commonly found on a vast array of old machines and vehicles in the UK, UNF and UNC threads continue to be the standard thread in the USA and Canada.

Using taps

Firstly a taper tap is used. This tap has a pronounced taper to the cutting edges, ensuring that the first cut is less aggressive.
After the taper tap comes a second tap. This also has a tapered edge, but not as pronounced as the taper tap. This slight taper helps align the tap when it is first introduced to the hole.
The third and final stage is carried out with a bottoming tap, often referred to as plug tap. Bottoming taps have hardly any taper and a continuous cutting edge.

When cutting shallow female threads, it may be possible to skip the third stage and just use the taper or second taps.

If tapping into a soft material it is possible to cut the thread with only a second tap.