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 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and release the screws when finished without running back; (4) Machines with solid dies, in which the screws have to be withdrawn by changing the motion of the driving gearing; making in all four different types.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 machines must be stopped while fastening the rods or blanks. Clamping jaws are usually as little suited for rotation on a spindle as dies are, and generally afford more chances for obstruction and accident. To rotate the rods, if they are long, they must pass through the driving spindle, because machines cannot well be made of sufficient length to receive long rods. In machines of this class, the dies have to be opened and closed by hand instead of by the driving power, which can be employed for the purpose when the dies are mounted in a running head.

With running dies, blanks may be clamped when a machine is in motion, and as the blank does not revolve, it may, when long, be supported in any temporary manner. The dies can be opened and closed by the driving power also, and no stopping of a machine is necessary; so that several advantages of considerable importance may be gained by mounting the dies in a running head, a plan which has been generally adopted in late years by machine tool makers both in England and America.

In respect to the difference between expanding and solid dies it consists mainly in the time required to run back, and the injury to dies which this operation occasions. Uniformity of size is within certain limits insured by solid dies, but they are more liable to derangement and less easy to repair than expanding or independent dies.

Another difference between solid and expanding dies, which may be pointed out, is in the firmness with which the cutting edges are held. With a solid die, the edges or teeth being all combined in one solid piece, are firmly held in a fixed position; while with expanding dies their position has to be maintained by mechanical devices which are liable to yield under the pressure which arises in cutting. The result is, that the precision with which a screwing machine with movable dies will act, is dependent upon the strength of the 'abutment' behind the dies, which should be a hard unyielding surface with as much area as possible.

Connected with screw dies, there are various problems, such as clearance behind the cutting edge; whether an odd or even number of edges are best; how many threads require to be bevelled at the starting point; and many other matters about which there are no determined rules. The diversity of opinion that will be met with on these points, and in reference to taps, the form of screw-threads, and so on, will convince a learner of the intricacies in this apparently simple matter of cutting screw-threads.