How Does a Tap and Die Work?

Screw threads can be cut by hand using taps and dies. A tap cuts an internal thread to take a bolt and a die cuts an external thread on a piece of bar, usually to take a nut. This technique can be used for cutting threads on almost any piece of suitably shaped metal. Taps for cutting a thread from scratch, three taps are needed:

o a taper tap to start the thread. This is tapered for approximately two thirds of its length
o an intermediate (or second) tap to follow the taper tap. This is tapered for approximately the first third of its length and it continues and extends the thread started by the taper tap
o a plug (or bottoming) tap is the final tap. It is parallel along its length except for a small-angled lead and is used only for blind holes. For holes passing right through the metal, only the taper and intermediate laps are needed.

To use a tap, a hole of the correct size has to be made first - you will need tables to look this up in. Taps have square heads which should be gripped in a tap wrench.

The tap needs to be held perfectly square to the surface and the squareness checked regularly. Turn the tap a quarter of a turn at a time and then turn it back. If you try to take too large 'bites' the tap will break - getting broken taps out of blind holes is not easy. When tapping a hole in aluminum, lubricate the tap with paraffin; use a light oil lubricant when working on bronze, copper and steel. A lubricant is unnecessary for brass and cast iron.

Dies Two types of die are normally used for cutting new threads circular split dies and pipe-thread dies.

Circular split dies have a limited range of adjustments and usually come in sizes below 13mm. When using one of these you should be particularly careful to make sure that the die is perfectly square to the bar being threaded when you start.

Pipe-thread dies are circular dies filled with a guide bush to make sure that the die is square to the bar. With both types of die, chamfer the end of the bar before cutting the thread. A die nut is used with a spanner - for cleaning up old or rusty threads.