The turning of two or three hundred hobs is managed by using a slide-rest lathe having a long traverse, although it need not have any screwing gear. Previous to commencing the turning it is necessary to examine the pivots and put them into shape, if required, and to turn the mandril-pivot if it happens to rotate untruly. When the hob is in the lathe, the first turning consists in taking a thin cut off the entire hob, both its stem and thicker part, using for this purpose a roughing vee-tool, shown by Fig. 429 ; and as soon as the work is made to rotate truly, by turning off only a small quantity, the hob is ready for a preliminary hardening, if it were not hardened at the first centring. A number of them are therefore thus roughly turned, so that all may be taken away together to be hardened. This hardening consists in heating each hob to redness and suddenly cooling it in water, after which all of them are again heated to redness, and without straightening them after being bent with hardening, they are gradually cooled in cinders to be softened for the final turning. During this tnrning, gap-gauges made of steel plate are used, to properly adjust each hob to its thickness previous to screwing. For each hob two gauges are necessary, or one gauge having two gaps; the width of one of these gaps is equal to the thickness of the hob's stem, and the width of the other gap is equal to the thickness of the hob's screw-part. The proper mode of making such gauges, consists in providing a distinct gauge for each class or size of hobs, and forming two gaps in each gauge near one end of it, each two gaps being made for one size of hobs. A gauge of this sort is shown by Fig. 548, having the name of each gap marked, one gap having the word stem and the other having the word screw. The thickness of each hob's screw-part indicates the name of the hob, consequently, the gauge bears the same name as the hob to be turned to the gauge. The stem-gap of any such gauge should be equal in width to the finished thickness of the stem, which is always less than the thickness of the hob at the bottom of the thread-groove when finished ; but the width of the screw-gap in the gauge must be a little greater than the finished diameter of the screw, if the screw-part is roughly turned previous to screwing; but if this portion is smoothly filed or polished, the gauge-gap is of exactly the same width as the diameter of the intended screw when finished. The preferable mode is to make the gap to the finished diameter of the required screw, and to always smoothly turn the screw-parts of the hobs, instead of leaving them to be screwed while rough.

The screwing of a large number of hobs with the ordinary Whitworth screws is usually effected in a lathe entirely devoted to screwing, and if only a small number are to be screwed with the same lathe in which they were turned, the screwing is a distinct process; consequently, all the hobs are entirely turned to their respective gauges, previous to any one hob being screwed. Ordinary screw-cutting lathes are always used for screwing hobs, whether they are small ones, only a quarter of an inch thick, or large ones, three inches thick. In some cases the slide-rest tools of the lathe are made to entirely form the screw, from the first making of the thread to the last finishing polish given to the completely-shaped work. In other cases the slide-rest tools are used to effect the greater portion of the screw-cutting, and the work is finally adjusted to dimensions and polished with hand screw-tools, such as are denoted by Figs. 407 and 408.

Before commencing the screwing of hobs, the lathe is adjusted to parallelism, if necessary, and this is partly effected by shifting the poppet-head until two dots, or other marks on the head, are made to coincide with each other ; after this, a test piece of round iron is turned, and each end of the turned part is measured with callipers to discover which end is thickest; the poppet-head is then shifted a small quantity to reduce the thicker portion, until the entire turned part is cylindrical, at which time the lathe is adjusted.

In order to commence and finish a hob-screw without hand screw-tools, the slide-rest tools required are those shown by Figs. 440, 442, and 443. Tools of this sort are made of several sizes, to suit a small four-inch lathe for screwing quarter-inch or half-inch hobs, or to suit a twelve-inch lathe for screwing three-inch hobs. One-tooth vee-point tools, which are denoted by Fig. 440, are made with different angles in their cutting parts, some having ends whose angles are less than fifty-five degrees, and others having ends whose angles are greater than fifty-live degrees. This angie of fifty-five is that which is subtended by the two sides of the threads belonging to the Whitworth hobs and taps in general use throughout England; consequently, the screwing-tools are adapted to make the threads to the required angle; and because the angle of the thread belonging to a half-inch hob is the same as that of a three-inches hob, the points of all the vee-tools for finishing are of the same shape, although some are much larger than others to suit larger lathes.

To facilitate the cutting, oil is freely used during a screwing process, and the vee-point tool, which is first used to commence screwing a hob, is one whose cutting edges subtend about sixty-five or seventy degrees, and when this tool has made the width of the thread-groove nearly equal to its finished width, another vee-tool, having an angle of fifty-three, is put in, and the bottom of the thread-groove is cut out, but the mouth of the groove is not altered, because this tool cuts only at the bottom of the groove. The use of this is continued until the thread-groove is deepened to its finished depth, which is ascertained by the use of callipers having thin points; the screw-tool is then taken out, and a third tool is put into its place. This is a vee-tool having two teeth that were shaped to the thread of the particular standard hob to which the hob in the lathe is being made. This third tool is shown by Fig. 442, and will cut at the outer extremities of the thread, termed the summits, but will not cut at the bottom of the groove, because the angle of the one-point tool, previously used, is fifty-three, and the angle of the third tool is fifty-five, which is the same as that of the standard hob. This tool with two teeth is made to cut with oil and thin cuts, until the entire thread is smoothly shaped, and fits tightly in the steel gauge nut to which the hob is being screwed. After this, the final polishing of the thread should be effected with a springy tool having two teeth. One of this class is shown by Fig. 443, and when it is to be used the hob in progress requires plunging into small wood-dust to absorb the oil which is attached; it is next cleaned with a brush and dipped into soapy water, until the water will stick to the entire surface of the thread, at which time it is ready for the springy tool. In addition to cleaning the hob, it may be also necessary to clean the oil from the tool-holder and poppet-pivot; if much oil happens to be on them, and when the hob is again put on to the pivots, and the springy-tool into the tool-holder, the tool is adjusted for polishing the thread. The adjustment consists in making the two teeth of the tool fit any two steps of the thread, and is effected by carefully and slowly rotating the lathe-spindle and hob forwards, and slowly working the two upper screws of the slide-rest, until the cutting edges of the tool coincide with the sides of the thread. When adjusted, thin soap-suds are freely applied during a few light scrapings given with the tool, by which the screw is polished to its finished dimensions, and made to fit the gauge-nut. When it is intended to screw a hob so that its thread shall be nearly as possible of the same shape and dimensions with the thread of a standard hob, a steel nut must be used, which was adjusted to the standard by the hob being screwed through the nut, and cutting out a small quantity ; after which, the nut is used as a gauge-nut without hardening it.

All hob-screws should be tapered at both ends, to avoid the trouble occasioned while entering a hob into a nut, or into any other screwed hole that requires measuring. If a hob-screw is tapered, it will smoothly polish a gauge-nut instead of roughing it, and the number of steps at each end of a hob-screw to be tapered should be five or six, as indicated in Fig. 551, in which the parallel portion of the screw is that between the two short straight lines extending from the screw at right angles to the hob's length. The tapering of the screw should be done with a comb screw-tool, either before or after it is finally adjusted to the proper diameter for the parallel portion, and the two extremities are to be made a fiftieth of the hob's diameter smaller than the parallel part in the middle, being careful to make the shape of the thread in the taper parts exactly the same as that of the thread in the parallel portion.

The mode of applying oil and water during screwing consists in providing a reservoir can for oil, and another for soapy water. Each of these cans is provided with a small outlet pipe situated at a short distance above the bottom of the vessel, and at the end of this feed-pipe is a small tap for applying and arresting the oil when necessary. Whichever one of the cans is required for use is suspended with a davit, or standard, which is fixed to the lathe-carriage, so that the feed-tap's orifice is exactly over that portion of the work being screwed. By being thus attached to the carriage, the feed-cock proceeds along the work just as fast as the screw-tool; consequently, an effectual and economical supply of oil or of water is secured. In addition to the feed-pots, two dishes are necessary, one for receiving the oil as it falls from the work, and another for receiving the water; either of these dishes is fastened to the carriage when a large quantity of screwing is to be done, so that some of the oil or water may be prevented from falling about the lathe and floor.