Taps and Dies - these are used for cutting threads into metal. The tap cuts the female thread and the die cuts the male thread. The female thread is the one in the nut and the male thread is the one round the bolt.
Taps and dies for workshop use come in sizes ranging from 3mm to 13mm, but other sizes are available. The thread inside the hole is cut in three stages. The hole that is drilled to take the thread is called the tapping hole and it is smaller than the thread size. The actual size is usually looked up in a chart. For each thread size there are three taps, the taper tap, the intermediate tap and the plug tap. The plug tap will not fit into the small hole so you start with one that is thinner at the end, this enlarges the hole slightly cutting part of the thread but not to its full depth,  this is followed by the intermediate tap which enlarges the hole a little  more then the plug tap cuts the thread to its full depth.
There is only one die per thread size but this can be adjusted slightly to enlarge or decrease the thread diameter. Both the taps and dies have to be held in a holder to make the cutting process easier.

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Die holder

Tap holder

Three types of tap

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