Screw

A device which has become very popular under the name of Hendey-Norton gears comprises a nest of change wheels, mounted and keyed on the end of the lead screw. A stud wheel is made to engage through an intermediate wheel with any one of the change gears, on the simple movement of a lever, giving different ratios for screw-cutting. These again are doubled or trebled by altering the ratios of other gears connected therewith, so that for each position of engagement of the stud wheel, two, or in some cases three, pitches can be cut. This avoids the waste of time involved in setting up fresh wheels on the swing-plate as often as a screw of different pitch has to be cut.

Another step in the direction of economy depends on the re moval of all screw-cutting, except those screws which are of several feet in length, from the ordinary lathe to the special chas ing and screwing machines. When there was no other method available except that of common dies operated by hand or carried in a screwing machine, there was good reason why a true cutting tool should be operated in the lathe through change wheels. But the reason no longer exists, since for the single cutting tool of the lathe the two or three cutters of the chasing and screwing ma chines (figs. 3 and 4) are substituted, and the hollow mandrel embodied in the latter permits of screws being cut and parted from the solid bars of several feet in length.

The second method of cutting screws is that by means of hobs or leaders, and either comb or single-edged tools. That is, a of the stock, or a separate tightening screw is used at right angles with the handles, or the tightening screw is set diagonally in re lation to the handle. Sir Joseph Whitworth's well known "guide" screw stock (fig. 5) is a typical example of the embodi ment of the principle just stated, the dies being cut over a hob two depths of thread larger than the screw; one, a broad die, is used for guidance only, and two narrow dies do all the cutting.

short standard screw is mounted somewhere on the lathe, at the rear, or in front, and a nut partly embracing this becomes a guide to a bar which is attached to the tool slide directly. These are termed chasing lathes. Their value lies in the cutting of screws of but a few inches in length, of which large numbers are required, a familiar example being the screwed stays for the fire-boxes of steam boilers, hundreds of which are used in a single boiler.

By Taps and Dies.

The third method embodies the use of taps and dies in their numerous designs. The simpler forms used are those operated by hand at the bench, from which all the ma chine taps and dies have been elaborated. The tap is the solid

screwed cylindrical tool which cuts an internal thread (fig. 3) ; the die is the hollow tool which cuts a thread on the outside of a cylinder (fig. 4).

In all taps and dies the problem is to cut a screw, of which the The guide-screw stock derives its name from the fact that it em bodies a guide a distinct from the cutters b,b, the guide doing very little actual cutting; it is one of the best tools for screw cutting outside the lathe, but some of the American types of dies give very accurate results, especially when they are combined with a guide placed in advance of the dies, to keep them truly parallel on the work.

Machine Work.

Hand tapping and screwing has long been confined to occasional pieces of work done by the fitter at the bench, the erecter and repairer. Screws and tapped holes required in quantities are done on machines which include numerous types, at a rate of production which would seem incredible were it not so common. For cutting common screws of no very great length the lathe has long been superseded by the various screwing ma chines. The earlier forms were provided with clutch mechanism for running the solid dies back off the thread, in imitation of the action of the hands, and the dies could not cut a complete thread at one traverse, two or three traverses being necessary in the production of a full thread. In the modern screwing machines (fig. 3) the cutters are closed and released by cam mechanism, and all threads except those of large diameter are cut at a single traverse. Common bolts and nuts are cut in machines of this kind, machine taps, which are longer than hand operated taps, being employed in the same machines.

But the smaller screws made in large quantities, and screws which have to be cut on pieces of work on which other operations, as turning, boring, facing, knurling, have to be performed, are made in the numerous capstan or turret lathes, the dies or taps being held in the turrets. In most cases, however, the dies are held in a chuck which is inserted in one of the holes in the turret and which angle of thread changes from point to root, with tools whose angle must remain constant. In taps there is no choice of angle, since they must be the exact counterparts of the tapped threads when finished. But in dies a compromise is made by cutting them with hobs, or master taps, one thread larger than the thread to be cut by the dies. Briefly, the practical effect is that the dies are only counterparts of the thread to be cut at about the middle part of their action.