Dies are held in stocks. In the common die stocks one adjustable die is moved forward with a screw, which forms one of the handles is better for the cutting of the finer screws. More valuable than any other single improvement is the automatic opening of many dies used in turret lathes, by which the running back of the die over the work is avoided. They are so beautifully contrived that contact with a stop, the position of which can be regulated, arrests the cutting action and causes the dies to fly open away from the screw, so that the turret can be slid away instantly, while the dies close in readiness for the next screw.
Sizing Taps are used for the finishing of threads which are re quired to be finished so uniformly as to be interchangeable one with the other. These are ordinary plug or second taps, generally short in length, and as they remove but a mere trifle of material they retain their size for a very long time. The case of sizing taps is more difficult than that of dies, because a die can be readily compressed to an extent sufficient to compensate for wear, but a tap has to be expanded.
latest development in screw-cutting, involves the use of a special machine, something like the lathe in outline, the piece of work to be threaded being rotated in the axis of the machine. The cut ter is carried in a head, with swivelling arrangements, to provide for variations in screw angles, and is rotated at speeds suitable for the metal or alloy being cut. The necessary traverse is im parted either to the work or to the cutter, according to the design of machine, by lead screw and change gears. This method is em ployed to a considerable extent, chiefly for cutting coarsely threaded screws and worms. The great advantage which the re volving cutter possesses over the single-edged tool is its rapidity of action, by which threads may be produced more quickly than in the lathe.
Much of the precision finishing of screws is now performed with the aid of grinding wheels, which will operate on hardened screws as well as on unhardened. Some of this work is done in small precision lathes, while there are worm-thread grinders for accurately finishing the flanks of the threads of large screws or worms.