Bolts are usually threaded by being held firmly in a machine which runs a briskly revolving threading die along the body of the bolt as far as the thread is to extend. These dies are kept deluged with oil while they work, though the work is so severe that they wear out rapidly.

A very effective thread for the cheaper-made bolts is either cold or hot pressed on the bolt by a machine, the essential principle of which is shown in Fig. 269. The bolt B is placed between the flat dies C and D which are made to move in the direction of the arrows and are held firmly at a given distance apart by bearing against the parallel faces of the guides F and G. Each die moves forward and back once, traveling a distance in one direction equal to at least the circumference of the bolt.

Fig. 269. - Dies for Rolling Threads on Bolt Ends.

The side view of the die plate shows how it is cut to impress threads on the bolt. The notch N is cut in the face of the die to the depth of the threads.