| Screw threads can be cut by hand using taps and | | | | Turn the tap a quarter of a turn at a time and |
| dies. A tap cuts an internal thread to take a bolt | | | | then turn it back. If you try to take too large |
| and a die cuts an external thread on a piece of | | | | 'bites' the tap will break - getting broken taps out |
| bar, usually to take a nut. This technique can be | | | | of blind holes is not easy. When tapping a hole in |
| used for cutting threads on almost any piece of | | | | aluminum, lubricate the tap with paraffin; use a |
| suitably shaped metal. Taps for cutting a thread | | | | light oil lubricant when working on bronze, copper |
| from scratch, three taps are needed:o a taper | | | | and steel. A lubricant is unnecessary for brass and |
| tap to start the thread. This is tapered for | | | | cast iron. |
| approximately two thirds of its lengtho an | | | | Dies Two types of die are normally used for |
| intermediate (or second) tap to follow the taper | | | | cutting new threads circular split dies and |
| tap. This is tapered for approximately the first | | | | pipe-thread dies. |
| third of its length and it continues and extends the | | | | Circular split dies have a limited range of |
| thread started by the taper tapo a plug (or | | | | adjustments and usually come in sizes below |
| bottoming) tap is the final tap. It is parallel along its | | | | 13mm. When using one of these you should be |
| length except for a small-angled lead and is used | | | | particularly careful to make sure that the die is |
| only for blind holes. For holes passing right through | | | | perfectly square to the bar being threaded when |
| the metal, only the taper and intermediate laps | | | | you start. |
| are needed. | | | | Pipe-thread dies are circular dies filled with a guide |
| To use a tap, a hole of the correct size has to be | | | | bush to make sure that the die is square to the |
| made first - you will need tables to look this up in. | | | | bar. With both types of die, chamfer the end of |
| Taps have square heads which should be gripped | | | | the bar before cutting the thread. A die nut is |
| in a tap wrench. | | | | used with a spanner - for cleaning up old or rusty |
| The tap needs to be held perfectly square to the | | | | threads. |
| surface and the squareness checked regularly. | | | | |