A bolt threader is a hand or powered tool used to cut threads onto new steel bolts or to re-cut the threads on bolts with damaged threads. Both hand-operated and electrically-driven bolt threaders use a standard head or die that rotates around the steel stock or bolt. These heads are equipped either with an integral cutting surface or sets of interchangeable cutting inserts. The powered bolt threader is generally a fairly large, static tool typically mounted in a workshop, while hand-operated varieties are portable and usually fit inside the average tool box. Both types are used with a specially formulated cutting paste that acts as a lubricant and aids in the removal of cuttings from the head.
The characteristic spiral threads that supply the motive forces that allow a bolt or corresponding nut to be tightened are cut into blank steel round stock with a thread cutter. These tools may be hand-operated or driven by electric motors. In both cases, a cutting head equipped with sets of cutting ridges is rotated around the steel stock cutting the thread while advancing along its length. The cutting ridges are either integral parts of the head or are cut into the ends of removable inserts that slot into channels milled into the head. Integral heads, or dies, can obviously only cut one particular thread pitch and profile, while interchangeable insert heads may be used to cut any number of different threads.
Electrically-driven bolt threader types are fairly large, heavy machine tools that feature a stock clamp and a motor-driven cutting head attachment both mounted on a sturdy table. Hand-operated bolt threaders generally consist of a double-sided handle into which the die or cutting head locks. In both, the cutting head is rotated around a blank steel round bar to cut new threads or around a stripped bolt to re-furbish existing threads. As the threads are cut, they serve as a guide to advance the cutting head along the length of the bar or bolt, cutting new threads as it advances.
Both types of bolt threader are always used with a cutting paste. This is a special compound which lubricates the cut point, easing the amount of stress placed on the cutting edges. It also aids in removing swarf, or cuttings, from the cut area and away from the cutting head. No bolt threader should ever be used without cutting paste, as it can lead to accelerated die wear or even total failure of the cutting surfaces.