Screw thread dies are used to cut external threads on engineering components. The split button die is most widely used by the fitter and a typical example, together with the die stock is shown in the diagram below.
It will be seen that the die has three adjusting screws. The centre screw spreads the die and reduces the depth of cut, whilst the outer screws close the die and increase the depth of cut.
It is difficult to control a screw threading die and any attempt to cut a full thread in one pass of the die usually results in a ‘drunken’ thread with a poor finish. A first, shallow cut should be taken with the die spread open by the centre screw. This is followed by one or more finishing cuts with the outer screws gradually closed down until the thread is a correct fit in the nut.