Taps - Plug - Metric

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.

Metric is the most common thread type in production today. Usually specified as M then a number x another number. This signifies the thread diameter and the length of the fastener.

For example, M4 x 30 means that the major thread diameter (the external distance across the outside of the thread) is 4mm and the length is 30mm. Length is usually measured from the underside of the fastener head.

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.

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