For practical purposes, the hardwood or ivory external screw of large or small diameter, is first turned cylindrical, somewhat larger than the size to which it is required to be finished, and its shoulder to a surface, figs. 510. 511; the front corner is turned to a small bevil, or if the screw be short, just sufficiently rounded to destroy the arris. A shallow groove is then turned in the cylinder with a narrow round tool, at its junction with the surface, or at the point where the length of the screw is to terminate; in order that the screw tool may cease to cut, and to allow time for it to be withdrawn, before the traverse carries the side of the tool into contact with the shoulder of the work; an accident which by suddenly checking the traverse of the tool, damages the thread. The screw, when struck and chased in the manner described, is then reduced to size and the tops of the thread to one level, with the flat tool; the screw tool is then re-applied with light pressure to finish the thread smooth and clean, but the action of the tool is not allowed to entirely obliterate the effect produced by the flat tool, the tops of the threads being much stronger when left slightly truncated, in about the proportion of fig. 512. The length of the screw is reduced last, the end being turned as a surface, and then slightly conical with the flat tool, applied at a little greater angle than that of the thread, to permit the end of the external, more readily to enter the internal screw.
For practice in cutting the internal screw, the aperture is hollowed cylindrical and rounded in front, fig. 513. The inside screw tool is held upon the arm rest, lying flat upon it, its surface radial; but first attempts may be sometimes easier if made without the armrest, the tee of the hand rest being fixed across the mouth of the aperture. Except as regards the difference in the form of the tool and the use of the armrest, the method of striking the thread is virtually the same as that for the external screw, and is often more easily acquired. The tool and armrest move together both in striking the thread and in chasing the screw, being advanced by the endlong thrust of the right hand, the left giving way, the body slightly swayed with the tool. The first stroke takes effect principally about the center of the curve turned at the mouth of the aperture; the line of points travelling in the curved path a b, and having also a slight upward tendency, from the advance of the tool being accompanied by a slight twist upon its handle to increase the penetration as described in the action of side cutting tools for hardwood. The second stroke on the line c d, drops into and continues the effect of the first, prolonging the screw line towards the cylinder, into and along which, it is then led by subsequent parallel strokes. The advance of the tool within the cylinder is guided by the screw line already obtained, cutting action being given by pressure with the right hand, accompanied by a gentle pull upon the armrest. The internal screw tool is always replaced in that portion of the thread, close to the mouth of the aperture, that can be observed; and so soon as this portion is cut with a thread of sufficient depth, the tool is replaced in it with only sufficient contact to carry it forward; the cutting pressure being added, when it is judged or felt that the tool has arrived opposite the unfinished portion hidden within the hole. The least forward motion of the hand or body, at the completion of every traverse, disengages the tool from the thread.
Fig. 512. Fig. 513. Fig. 514. Fig. 515.
Internal screws for practical use, figs. 514. 515, corresponding with the external lately considered, are hollowed as parallel holes, the external face surfaced, the edge of the aperture slightly rounded or bevilled, and a groove, - rather exaggerated in the figures, - to allow time for the withdrawal of the tool, is cut with the inside round tool, fig. 400, at the junction of the internal cylinder and surface, or at the end of the thread, as required. When the screw has been cut it is regulated to diameter and parallelism, with a right side tool upon the armrest, followed by the screw tool for clean finish; and then by the right side tool or flat tool, applied at an angle to the edge of the mouth, leaving that smooth and slightly conical.
The lengths of the external and internal screws for uniting two portions of the work, range from about one eighth of an inch, to about one inch, and often bear but a small proportion to their diameters. The two pieces are turned nearly to their respective diameters, and then the one reduced or enlarged to exactly fit the other in the process of cutting the thread. It is not very material which of the two screws be made first, but the internal thread must be always slightly the longer, to allow the shoulders of the two screws to arrive in contact. The external screw being more observable and accessible to measurement by callipers, is the more easily finished parallel; this is therefore more usually turned first, and the internal screw is gradually enlarged to fit it; but convenience in chucking, or the form of the work, may make it preferable to cut the internal screw first. External screws longer than their diameter, are nearly invariably cut first and their nuts then fitted to them. Screws cut in ivory, more especially upon portions of the work intended for frequent separation, suffer less wear and obtain increased smoothness of action, when polished with whiting and water, applied on the end of a slip of deal; which process is invariably followed with the best works.
It is occasionally necessary to cut the external screw at the chuck end of a piece of work, or the opposite position to the screw, fig. 511. In striking the thread in this position, the shaft of the screw tool is held at about right angles to the mandrel axis, nearly parallel with the shoulder of the screw, which is cut from the groove towards the chuck; the action is rather cramped by the confined space between the shoulder and the chuck, but otherwise presents no difficulty.
Metal screws for fixing and adjusting purposes, when possible, are cut with the diestocks or other methods, detailed in the second volume and in this chapter; but a large number of screws in brass, gun-metal, iron and steel cannot be so pro-duced, from unsuitability in their diameters or other proportions. Many of these are struck by hand in the manner described for wood, but with somewhat less facility according to the increasing hardness of the material, which renders the first traverse of the tool less certain. The penetration is considerably less and is also proportioned to the hardness of the metal, to avoid retarding the tool in tracing the first faint but accurate screw line, to be afterwards gradually deepened to form the thread.
Brass, gun-metal, and sometimes copper screws, are chased without lubrication, but oil or soap and water are generally used for the last; for cast iron the tool is dipped in water, for wrought iron and steel, it is plentifully supplied with oil to relieve the friction. The angles of the screw tools for metal range from 50° to 90°, varying with the material and somewhat, with the dimensions of the screw. Tools of the latter obtuse angle are used for the tubes of telescopes and analogous thin works, which do not admit of a groove at the termination of the thread. A narrow ring is sometimes soldered around the tube and the screw is cut upon it, when it stands above the line of the former; otherwise both screws are cut slightly taper, at a small angle to their respective tubes, when the screw tool cuts out free of the surface at the end of the thread. Cast iron requires coarse and shallow threads, produced either with a tool of considerable angle, or one with the points rounded, to give the thread the form of fig. 630 Vol. II.; the small cohesion of cast iron is not very suitable for external screws, they are little used and seldom cut by hand.
Very many iron and steel fixing screws, may be first marked out with the diestocks, and then have the bulk of the material removed with the screw tool, the work mounted between centers. Equal pressure is maintained upon the tool that it may cut uniformly, and when desirable, the thread may be subsequently regulated and the screw made parallel, with the dies, carefully passed once or twice along its length. The sharp tops of the threads of iron and steel screws, may be slightly reduced with a flat file while in revolution, and the screws still revolving in the lathe, are then polished with fine emery and oil applied on the end of a piece of deal.
The hand screw tools may be made to cut double, triple, or quadruple threads, by employing a relatively increased speed in their traverse; thus, if the tool be moved twice as rapidly, instead of the series of points advancing as usual, the second to the position of the first, the third to that of the second and so on, in one exact revolution of the work; the third point will have arrived at the position of the first, the fourth at that of the second, one point passing over; when the thread resulting will be double, two coils winding round the shaft, their angle or rake twice that of the single thread. If traversed three times as fast as for striking the single thread, the fourth and fifth points, arrive at the previous positions of the first and second, two teeth passing over, when the thread will be triple. The double and the half thread frequently occur by accident, but striking them intentionally requires practice, while the triple and quadruple threads present considerable difficulties; the tool requires to be tilted at a great vertical angle to agree with the increased rake, and this, not easily maintained in striking the external thread, is almost insuperable with the internal. Multiplex threaded screws are more accurately and conveniently made by other means, and when intentionally struck by hand, it is usually as examples of dexterity.