Taps - Taper - BSW

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.

BSW (British Standard Whitworth) is an imperial coarse thread first used in standard production by Joseph Whitworth during the industrial revolution. Whilst it is still used in engineering applications all across the world it is less common in the UK where metric threads have generally taken over.

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.