The lathes which are used for screwing taps and hobs are those provided with screws having either half-inch steps, or quarter-inch steps. Whether the lathe-screw has a quarter-inch step or a half-inch one, the wheels for screwing have the same numbers of teeth, although the wheels are always of a proper diameter and weight to suit their respective lathes, whether small or large. A lathe that has a quarter-inch screw will screw any size of taps or hob which is to be made with the ordinary Whitworth screw, by using only two screwing wheels for each different step. One of these is that which is put upon the lathe-spindle, and called the mandril-wheel, and the other is that which is put upon the lathe-screw and termed the screw-wheel. A screw having a half-inch step will also cut most of the screws required for taps, by using only two wheels, and when screws are made by using two screwing wheels only, the arrangement is termed simple gear. Any additional wheels that may be required are merely connecting wheels, which are put between the two screwing wheels to occupy the space and produce the particular direction of movement required for the traverse of the screw-tool. Without connecting-wheels, the motion of the mandril-wheels, which are of various sizes, could not be communicated to the lathe-screw wheels, which also are of various sizes.

Cutting a screw in a lathe is a much more important operation than selecting the proper wheels, because all the lathe-makers provide Tables which indicate the different wheels required for each stated pitch to be cut. Such Tables are hung on the walls of engineers' turneries near the screwing lathes for the use of those who require them, and to avoid calculation when a screw is to be cut. If it happens that such Tables are not provided, the proper wheels are discovered by very simple arithmetic, if necessary; but this is of no avail without a careful attention to the shape of the cutting tool, the position of the tool's point to form the thread at right angles to the work, also the proper height of the cutting edges, making the tool cut only at one side, applying the proper quantity of cut to avoid breaking the point, and all the other directions given in preceding sections, if hobs and taps are to be screwed.

In the Tables for dimensions of taps and hobs now given, a couple of wheels are indicated for each different pitch of thread. It may be noticed that several sizes of taps are screwed with threads of the same pitch for all; consequently, one couple of wheels are suitable for several sizes of taps and hobs, and the total number of wheels required for several thousands of taps is very small. In Table 5 the dimensions of hobs are given, the first column showing the name of each hob, which is the diameter of its screw-portion. The second column denotes the total length for each, including both stem and screw-part. The next three columns indicate the lengths of the screw-parts, stems, and heads, and the sixth column is useful to denote the depth to which the thread-groove is to be cut while screwing. Column seven is required to show the precise diameter to which each stem is to be turned, previous to beginning the screwing; the dimensions in this column are also the diagonals of the heads. In the eighth column is given the pitch for each size of hob, and in the ninth, two wheels are mentioned for each pitch, the first number of each couple showing the number of teeth on the mandril-wheel, and the second number showing the number of teeth on the wheel for the lathe-screw.

Table 5. Dimensions Of Hobs Having Ordinary Screws

Table 5 - Continued

When the operator desires to ascertain whether the numbers of any couple of wheels are correct, he can multiply the number of the lathe-screw wheel by the number of steps in one inch of the lathe-screw, and then divide the product by the number of the mandril-wheel; if the quotient thus obtained is the same as the number of steps in the adjoining column, the wheels indicated are the right ones, although other wheels that are not too large or too small for the lathe, may be used, if necessary.

The hobs referred to in the Table are of no use for ordinary screwing of holes, but are only available as standards of reference, or to cut a very thin shaving from a nut or other piece of work which has been screwed in a lathe, and to shape the teeth of hand-screw tools, slide-rest tools, and sometimes teeth of dies.

Table 6 contains the dimensions for long taper taps, and each diameter of taps has the same sort of thread as the same diameter of hobs in Table 5, being that which is adopted by the tool-makers ; but the length given for each tap is rather greater, to allow for wearing and also grinding the tap's point to sharpen it, so that the tool will tap a few thousands more nuts than a shorter one.

Table 6. Dimensions Of Long Taper Taps Having Ordinary Screws

Table 6 - Continued

The greater number of all taps that are made, are used to screw nuts for the ends of screw-bolts, connecting-rods, piston-rods, slide-rods, crank-pins, studs, joint-pins, spindles and keys; and because large bolts, rods, and other work of three, four, or six inches thick are now much more frequently made than when the Whitworth screws originated, it is the custom of engineers to screw the ends of such work with thin threads having short steps, that the thread-groove may not be too deep and break off the end of the rod while in use. Thin threads are termed fine threads, and are of the usual vee-shape having curved summits, the angle subtended by the two sides being fifty-five, as in the other thick threads having long steps. In Table 7 are given the various diameters of bolts and rods intended to have thin threads, and opposite are the respective numbers of steps per inch.