[4] This doubt seems to mar the invention. It is, however, on the whole a good design.
Figs. A, B, C, D.
The above simple apparatus will, it is believed, be of great service to those who find difficulty in hand chasing of screws; it is, however, necessary to speak of other methods, and especially of that so universally used by the turners of "Tunbridge ware"—viz., the traversing mandrel. This is represented in [Fig. 91]. A is the poppet; B, the mandrel, no longer conical at the place where it traverses the collar, but cylindrical, and passing through two cylindrical collars. It is prevented from advancing towards the left in its bearings by the shoulder, K, and in the other direction by a plain cylindrical collar, or ferrule, C, which slips over the end, and is secured by a nut, D. The whole is thus ready for use, as an ordinary mandrel. To cause it to traverse from left to right, as it revolves, the nut and collar, C, D, are removed, and a ferrule, or guide, F, which has a screw of the desired pitch cut on its edge, is slipped on the mandrel over a short feather against a shoulder, where it is retained by a nut or pin. There are several such guide-ferrules supplied with the mandrel of different pitches of screws. The nut G is then removed and the piece E, which is of brass or gun metal ⅜ or ½ inch in thickness, with similar screw threads in each of the hollows is attached. This guide is slipped on at H, and secured by replacing the nut. The pin, which carries the guide, is frequently made to slide up and down the face of the poppet, by the action of a screw, M, working through a brass piece attached to that which carries the pin. It is thus readily lowered out of gear or drawn up again to touch the screw ferule. This is better than a pin screwed to the head of the poppet, and is always adopted in the best lathes. In the frontispiece is a [photograph] of this arrangement. This guidepiece acts like a half nut, and as the mandrel revolves it gears into the ferule and causes the required traverse. A single point tool, therefore, held against the work will trace a screw of the same pitch as that of the guides, and of a length equal to that of the ferule. The above is not intended for cutting long screws, which would have to be done in successive short lengths, but for screws of box lids, chucks, and similar work it is a most excellent contrivance, and peculiarly adapted for the use of the amateur. The guide threads in the commoner patterns of these lathes are cut on the mandrel itself, which is made of greater length than usual, and these several guides are cut upon the part within the poppet heads, as [Fig. 92], which represents such a lathe as is sometimes used by gasfitters and brass workers in general.[5] In the latter it will be noticed that there is added a sustaining screw at the back of the mandrel. This is a good addition, and indeed almost a necessary one if the lathe is to be used for ordinary rough work, especially drilling, as it takes off the pressure which must otherwise come against the shoulder, K, [Fig. 91], and it must be remembered that these lathes are expensive, and, therefore, ought to be taken care of. The amateur may also be warned against bad work; none but the first-class makers can turn out a reliable lathe of this description. The collars and mandrel require perfect fitting, and they must be quite hard, because there is no possibility of tightening them when worn. They must be kept well oiled, therefore, when in use, and the oil holes in the top of the poppet should be fitted with brass covers, to prevent any particles of metal, and especially emery dust, from working in between collars and mandrel. Be sure to have one guide screw of the same pitch as that on the nose of the mandrel, for the purpose of tapping chucks to fit thereon. In using the traversing mandrel, either the cord should be slackened so that the pressure of the hand on the pulley may stop the revolution of the work in a moment, or the flywheel should not be brought into use, the cord being instead grasped by the left hand, because it is generally necessary to cut to a shoulder or given point. It is, however, possible sometimes so to arrange the guides by insertion of a washer or other expedient as to cause the action to cease of itself at the required point. In a future chapter, the cutting of long metal screws will be treated in detail, but before concluding the present chapter, it may be useful to say a few words concerning the nature of the screw itself as a mechanical expedient. A screw may be defined as a continuous inclined plane—or an inclined plane wound round a cylinder—the pitch being the inclination of the plane, that is, the ratio of its height to the length of its base. From the mechanical principle of the inclined plane it follows that the greater number of threads in a given space the greater is the power of the screw when used as in a press, or to draw along its nut, as in the slide rest, in which endlong motion in the screw itself is prevented. It seems, at first sight, easy to devise a method for cutting screws of any desired pitch, but this is far from being the case, and when it becomes necessary to increase the number of threads to fifty, sixty, eighty, or even more to the inch, with such accuracy that one turn of the screw shall always produce an equal longitudinal movement of the nut, the most delicate machinery scarcely suffices for the purpose. The microscope detects and shows errors even in the best work, and it is questionable whether a perfect screw of any length can be cut by machinery, as every imperfection in the latter is communicated to the work done by it. If the amateur, therefore, requires a screw for a slide rest, eccentric chuck, microscope, or other delicate piece of machinery, or philosophical instrument, he had better get it cut by some practical mechanician, in the possession of the necessary apparatus.
[5] There should be a collar or shoulder to this mandrel, the same as at K in the other figure.
Figs. 91, 92.
In the "Manual Bergeron" is an ingenious contrivance, by an amateur, which is worth notice, although unsuited for any work where extreme accuracy of pitch is required in the screw. The following, [Fig. 93], is a description:—The mandrel is made to traverse in its bearings, as before detailed in this series, but instead of its motion being governed by guide hubs, it is dependent on the action of a pair of differential pulleys, B. A bent lever, C, is pivoted at E to the face of the poppet, having a bit of hardened steel fitted to work in a semicircular groove in the mandrel itself, and so arranged that on raising the tail or long arm of the lever the mandrel is thrust forward from left to right, while a reverse action of this lever causes a similar movement in the opposite direction. The movement of the lever is thus regulated:—At the extreme end of the long arm is a pulley and hook, as shown in the drawing; the double, or differential pulley, is fixed to the end of the mandrel, and from the smaller part depends a cord which passes thence through the pulley on the lever, and is wound round the larger one on the mandrel when its end is secured. On the hook is hung a weight. It will be evident, on an inspection of the drawing, that on putting the lathe in motion the cord on the differential pulley will coil itself round the largest part of the same, and will draw up the end of the lever with a speed proportionate to the difference of diameter between the larger and smaller parts of the double pulley. The short end of the lever will at the same time with similar proportionate motion move the mandrel and work, and cause the fixed tool to cut a spiral or screw thread on the latter—a good deal of ingenuity is displayed in the above, and it has the advantage of being easily fitted up, but it is evident that some alterations and additions would be required to adapt it to any other use but that specified. A contrivance similar to [Fig. 94], may in some cases be a sufficient makeshift, when a more perfect one is not at hand. A screw is here cut on the outside of the chuck, and a kind of double tool is used, the tracer which is in contact with the guide thread being adjustable as to its length, and the cutting tool having a sidelong adjustment as well. The rest being placed between the connecting bar of the tool and the work, the former will be held with sufficient steadiness to enable the workman to traverse the whole easily by hand. The use of this tool is of course limited, but the plan is simple and fairly effective. The only really serviceable plan is the slide rest, to be hereafter described. But one other plan is here added, which is called "Healey's chuck." The description and sketch are from Holtzapffel's work, in which it was however copied from an older treatise. The author, it must be understood, has never seen the contrivance himself, and there is a fault in its principle of construction which must militate against its use except in a very limited degree. Since, however, Holtzapffel has considered it worthy of a place in his work, it is at any rate well to introduce it to the reader, especially as its defects will not be of great importance in tracing screws of half a dozen threads or so. The apparatus is represented in [Fig. 95]. in plan. C is the chuck which carries the work to be screwed, and T is the tool which lies upon R, R, the lathe rest, that is placed at right angles to the bearers and is always free to move in its socket S, as on a centre, because the binding screw is either loosened or removed. On the outside of the chuck C is cut a coarse guide screw which we will suppose to be right handed. The nut N, N, which fits the screw of the chuck is extended into a long arm, and the latter communicates with the lathe rest by the connecting rod C, C. As the lathe revolves backwards and forwards, the arm, N (which is retained horizontally by a guide pin, G), traverses to and fro, as regards the chuck and work, and causes the lathe rest R, R, to oscillate in its socket S. The distance S, T being half S, R, a right hand screw of half the coarseness of the guide will be cut, or the tool being nearer to and on the other side of the centre, S, as in the dotted position T, a finer and left hand screw will be cut. The rod C, C, may be attached indifferently to any part of N, N, but the smallest change of the relation of S, T to S, R would mar the correspondence of screws cut at different periods, and therefore T and R should be united by a swiveljoint capable of being fixed at any part of the lathe rest R, R, which is omitted in Mr. Healey's perspective drawing of the apparatus.
Fig. 93.