The work therefore may remain stationary a small interval of time, until from the cut of the tool the carrier overtakes the driver; which results in damage to the thread, by a partial widening of its interval, consequent as before, upon the tool commencing to cut in a false path and then gliding into the true one. To avoid accident from this cause, the position of the carrier and the driver, when either unattached or fixed together, is always observed and the two are carefully placed in actual metallic contact by the hand, immediately prior to moving the tool at the commencement of every cut.
The rod or blank for a right handed, angular threaded screw, is mounted between centers and driven by a carrier; it is first turned truly cylindrical and then reduced to very nearly the external diameter of the screw. All metal screws, long in proportion to their diameter also require the support of the back stay, figs. 142 or 143, or that of some analogous contrivance, fixed to the slide rest opposite to the tool and travelling with it; to counteract their bending, and to enable them to withstand the thrust of the tool, both for turning the blank and while cutting the thread. A portion at the right hand end of the blank, fig. 528, beyond the length, is also reduced until its diameter is slightly in excess of the diameter of the depth of the intended thread. Screws having similar shoulders at both ends of the thread, fig. 529, allow the tool to cut itself out free from the material at the termination of every trip ; but this is prevented with fig. 528 by the collar or projection. Such blanks therefore, also require a narrow circular groove turned in them, at the termination of the thread and rather less than its depth; the groove, serving to receive the point of the tool and to allow time for it to be withdrawn, to avoid its fracture by sudden contact with the solid metal beyond the end of the thread cut.
The tool or the cutter bar, with the double angle blade first fixed at the necessary inclination, is then adjusted exactly to the height of center of the lathe by packing placed beneath it; after which it is clamped in the holder of the slide rest, with the circular portion nearly or else close home to the top plate. The stem of the cutter bar or of any other tool used in screw cutting thus receives no avoidable projection, so that the tool may acquire all possible stability. With the spiral apparatus, the height of the radial arm carrying the train of wheels, is also adjusted with reference to the diameter of the work with a view to the same result. These matters being satisfactorily adjusted previously to tracing the thread, the tool is advanced by the screw of the top slide until it just touches the cylinder, and its position is noted by the micrometer or divided head upon the screw; after which the tool is withdrawn clear of the work and traversed, until it stands well to the right of the shoulder, a, where it is brought to rest. The tool is then advanced a minute distance beyond that previously noted by the micrometer, and the lathe is set in motion for cutting. The loss of time of the nut and wheels being absorbed before the tool arrives at the shoulder, a, the continued progress traces a fine screw line along the work to the required length; and as this is approached, the motion of the lathe which is slow, is gradually stopped to avoid overrunning the length of the thread, and to cause it always to entirely cease with the point of the tool in the groove turned in the work to receive it. The tool is then withdrawn by its slide, and the motion of the lathe being reversed, it is carried back to its former position on the right of the shoulder.
The faint screw line first traced, is to test that all the parts of the apparatus have been correctly fixed; if satisfactory, it is followed by a series of trips cutting the thread. The tool is slightly advanced while free of the work, prior to the commencement of the traverse from right to left and is withdrawn clear of the work, every time previously to its return to the position to recommence. The increasing depth cut by every advance, is carefully observed upon the micrometer of the slide rest screw, a wheel fixed upon it, the edge accurately divided with fine equidistant divisions and provided with a fixed index or reader, that the tool after being withdrawn, may be advanced slightly more than its last depth for the next cut, and also to prevent its being accidentally pushed too far forward, in which case it would tear the thread or be arrested and its point broken in the work. When the slide rest screw is unprovided with a micrometer, the advance of the tool, it has been said, has to be regulated by counting whole, half, quarter and less portions of turns of the screw; this is inconvenient in screw cutting, which usually requires a less advance, but a little help may be obtained by dividing the end of the tube of the winch handle into 10, 20 or more parts, read by an index fixed to, or by a line marked on the slide rest. This arrangement is improved, when the tube of the winch handle is cased with an external ring or short tube, carrying a projecting index point; which ring will slip rather tightly round upon it. While the tool is cutting, the index on the ring is shifted round slightly to the right of that upon the slide rest, to any of the divisions upon the winch handle to determine the depth for the succeeding cut; and when that is taken, the screw advancing the tool has only to be turned until the index on the winch handle is in line with that fixed to the end of the slide rest; the former being then again shifted to determine the depth of the next cut. With the micrometer, the tool can be advanced and replaced at more minute distances, and it is also found very convenient to have a copy of the micrometer at hand, in the form of a dial, which may be set every time to the same advance as the tool, and referred to for the prevention of error.