The tools employed for cutting screw threads constitute a separate class among the implements of a fitting shop, and it is considered best to notice them separately.

Screw-cutting is divided into two kinds, one where the blanks or pieces to be threaded are supported on centres, the tools held and guided independently of their bearing at the cutting edges, called chasing; the other process is where the blanks have no axial support, and are guided only by dies or cutting tools, called die-cutting.

The first of these operations includes all threading processes performed on lathes, whether with a single tool, by dies carried positively by slide rests, or by milling.

The second includes what is called threading in America and screwing in England. Machines for this purpose consist essentially of mechanism to rotate either the blank to be cut or the dies, and devices for holding and presenting the blanks.

Chasing produces screws true with respect to their axis, and is the common process of threading all screws which are to have a running motion in use, either of the screw itself, or the nut.

Die-cutting produces screws which may not be true, but are still sufficiently accurate for most uses, such as clamping and joining together the parts of machinery or other work.

Chasing operations being lathe work, and involving no principles not already noticed, what is said further will be in reference to die-cutting or bolt-threading machines, which, simple as they may appear to the unskilled, involve, nevertheless [144] many intricacies which will not appear upon superficial examination.

Screw-cutting machines may be divided into modifications as follows:—(1) Machines with running dies mounted in what is called the head; (2) Machines with fixed dies, in which motion is given to the rod or blank to be threaded; (3) Machines with expanding dies which open and release the screws when finished without running back; (4) Machines with solid dies, in which the screws have to be withdrawn by changing the motion of the driving gearing; making in all four different types.

If these various plans of arranging screw-cutting machines had reference to different kinds of work, it might be assumed that all of them are correct, but they are as a rule all applied to the same kind of work; hence it is safe to conclude that there is one arrangement better than the rest, or that one plan is right and the others wrong. This matter may in some degree be determined by following through the conditions of use and application.

Between a running motion of the dies, or a running motion of the blanks, there are the following points which may be noticed.

If dies are fixed, the clamping mechanism to hold the rods has to run with the spindle; such mac............