Part 6

"I think the plan to be described will produce to a certainty any required number of screws and turns to the inch. The screws are entirely cut with a common comb tool, but started by a revolving cutter set to the required angle, and applied firmly to the work, on the T-rest. I call it 'the universal screw guide tool,' contrived and made by myself, and I believe will prove as useful to others as it has to me: the drawing will almost explain the tool. The cutter, A, is 9/16ths of an inch in diameter, turned to a cutting edge, and finely tempered. The stem, B, in which it revolves, is round, and fits into the shoe, C, having a graduated collar, D, in front of C, to set the cutter to the required pitch or angle, the set screw, E, makes it fast; having turned a piece of rod, of brass, iron, or steel, a little above the size necessary, and supposing a quadruple screw is to be cut having ten turns to the inch, there would, of course, be forty threads when complete; if one of these four can be truly traced, the comb tool will easily follow by inserting the outside tooth, either right or left hand, as found convenient, in the line traced, when the other three will soon appear with perfect accuracy, provided the first one exactly corresponded to five points of the comb, which is easily accomplished after a few trials, and if not successful at first,[4] can be removed by a dead flat file several times, without reducing the rod too much. When found to exactly fit the five points, the cutter may be applied with more force to leave a good chase for the comb. The T-rest requires a smooth surface for the shoe, C, to slide freely on, and to be set parallel with the work, and the tool held at a right angle as it proceeds along the rest, or the lines formed would be of unequal distance. After a little experience it will be found to work with beautiful accuracy, and for those who have not screw-guide mandrels, and are not practised hands at flying common screws, it will be found a great assistance, as it sets to anything. I described only the quadruple, but the same rule applies to all quick screws; for a double the chase must correspond to three points, and so on for any number, that is, one more point of the comb than the number of screws to be cut, and for a common one the chase must fit the comb altogether."

[4] This doubt seems to mar the invention. It is, however, on the whole a good design.

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 3/8 or 1/2 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.

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.

This is one of the least perfect modes of originating screws, it should, therefore, be only applied to such as are very short, as, owing to the variation in the angular relation of the parts the motion given to the tool is not strictly constant nor equable. When in the midway position the several parts should lie exactly at right angles to each other in order as far as possible to avoid the error. The inequality of the threads is imperceptible in a short screw. A little modification of the screw-chuck of Healey would result in a more correct and serviceable arrangement.

The disadvantage, for instance, of being obliged to set the T of the rest across the face of the work is apparent at once, and it is difficult to understand how such an arrangement could be made to answer when the work might be of a length to require the support of the back poppet. The following plan, Fig. 96, would obviate this and the other disadvantages, and make a more efficient apparatus. B is the chuck with screw chased on the outside, A the nut travelling upon the same. To this is attached a bar H, which passes through the bar K, to which it is clamped by the binding screw visible at H. Two short pillars, F, F, are screwed into the bed of the lathe, in which there might be more than one hole for each to permit the pillars to be fixed at different distances from the line of centre. Through slots in these passes the square bar of polished iron or steel E, supporting the traversing rest socket, D, of which two other views are given at Figs. 97 and 98. It will be evident on inspecting the drawing that as the nut A travels to and fro, it carries with it in a line parallel to the lathe bed the rest socket and T. To enable the workman to steady the tool, the latter should rest against two short pins fixed in the top surface of the rest in holes made for the purpose; with the aid of these the tool will be made to traverse the work with great ease and regularity. In chasing a right-handed screw, the tool would have to lie on the left side of the pins, and the latter would insure its traversing with the rest. A longer T (or half T) being turned in its socket to stand across the face of the work will enable an inside thread to be cut with the ordinary tool either of one or more points. Of course, in this modification of Healey's chuck, the screw cut will be of the same pitch as that of the chuck itself; but as the latter may be of boxwood, there would be no difficulty in having from three to six with the most generally useful pitches of screws, as the arm may be screwed into the near side of the nut, and, therefore, it, and all the other parts of the apparatus, would answer for the whole set of chucks. If the work to be screwed is merely a box cover, or some such work where great length of screw is not required, it is evident that to rig up this kind of contrivance, or Healey's, or indeed anything of similar elaboration, would appear like summoning a gang of navvies to remove a mole-hill. Hence, if a traversing mandrel cannot be obtained, by far the simplest plan is the chasing tool used by hand; it is therefore well worth while to get into the knack of using this tool. To give confidence (which is essential to success, the least nervousness generally proving fatal to such work) the part on which the screw is to be cut may be left larger than will finally be necessary. The screw is then commenced, and if a failure takes place it is again levelled, but if, as is more probable, the attempt is successful, the chisel and chase are alternately used, the cuts of the latter not being obliterated by the former but always left sufficiently deep to form a guide until the desired object is satisfactorily accomplished.

Having treated of screw cutting so far as we are able without trespassing on that section of the present series which is to be devoted to machine work with slide rest and change wheels, we shall enter on the matter of spirals, or Elizabethan twists, the method of making which was long kept a secret by the trade. These twists are essentially screws of very extended pitch, and generally rounded threads. The latter sometimes embrace the central cylinder or core, and sometimes are detached so as to assume a more open form like a corkscrew, and in the latter case two, three, or more threads can be cut, so that the spirals appear to intertwine. These spirals, too, are frequently ornamented with the aid of the eccentric chuck, and thus the work becomes fit for the adornment of the drawing-rooms of the highest in the land. No turned work can in short exceed in beauty these delicate and elaborate specimens of the turner's art. To commence with a single twist of one thread. A cylinder of the requisite length must first be turned. The number of turns the thread or cord of the spiral is to make in a given space must next be determined. We will suppose the cylinder one foot long, exclusive of any mouldings or tenons at the ends, and that the spiral is to make three turns round it, that is, one turn in four inches of its length. Divide the cylinder into three equal parts by the lines B, C, Fig. 99. Next rule equidistant lines D, E, F, along the cylinder in the direction of its length; in the present case let there be _four_ such lines. The three divisions first made must be subdivided each into four (always the number of the longitudinal lines). The angles of the parallelograms thus formed must then be connected diagonally as shown in the figure, which diagonals being continued, will be found to describe a spiral line. A second similarly constructed spiral determines the thickness of the thread or cord of the twist. These spirals may now be cut with a tenon saw, and all the material outside the cord carefully removed with the gouge, so as to form a semicircular hollow between the threads. This cannot be done by putting the treadle and flywheel in motion, but the work itself must be grasped with the left hand, while the right holds the gouge upon the rest and guides its edge. After the work has thus been roughed out, it must be finished by rasps and files, or by a kind of plane with a semicircular cutting edge. While the latter is being used, the flywheel must be brought into action, so that the work may be made to revolve with sufficient rapidity to ensure a clean and smooth cut. A hollow plane may be used to round the cord of the twist, and the whole finished with glass paper and polished.

The "kind of plane" named was designed by a regular workman for his own use, and was made thus and grasped in the fist, or rather hollow of the hand. The iron was like _c_, sharp at one edge, the block of wood being slightly bevelled off in front of the cutting edge. The workman, who made scores of these twisted works for the trade, of all dimensions, would run this along the hollows, while the lathe was in motion, with great speed and accuracy. He marked the spirals as described, and then grasping the work in the left hand, and the gouge in the right, turning the work round with the former (the cord thrown off the wheel), he cut out the wood boldly in large pieces with little apparent care, and perfect ease. Then came the plane described above.

Fig. 100 shows a simple spiral thus made. If the piece to be turned is dark it is not easy thus to mark out the divisions. In that case the following method will answer equally well. It is the plan used and contrived by the writer,[6] and specially handy when a number of similar twists are to be cut, as in ornamented pieces of furniture. A, Fig. 101, is a straight edge of hard wood, through which, at any given angle (regulated by the number of turns the cord is desired to make in a given length of cylinder), a knife edge is fixed. If this is held as in the figure, and the blade is pressed down upon the cylinder to be cut, the lathe being put in motion, a very correct spiral will be traced, which can be at once deepened by a tenon saw as before. The thickness of the cord being determined it only remains to place this tool again upon the work so that the second line shall be traced at the required distance from the first. A second, or any number of cords, may be thus traced in succession parallel to each other by this simple method. By a slight modification of this instrument, which allows the knife to be clamped at any desired angle with the straight edge, the inclination of the cords, and, consequently, the pitch of the spiral can be varied at pleasure, and a second blade can be added to trace the second line, determining the thickness of the cord with one movement of the tool along the cylinder. The edge of the knife may be across the rest, the piece of wood just overlapping the T on the side next the workman, if the blade is long enough to reach across the work when thus held. The tool will be steadier and perhaps more easy of management in this position. One hand should then lightly press the back of the knife, while the other retains the wood against the rest as a straight edge. East Norfolk Amateur's design of a screw guide is on the same principle, the only difference being the substitution of a revolving instead of a fixed knife edge. In using either it requires some care to allow the tool free traverse, as the least check would spoil the thread.

[6] A similar plan is noticed in Holtzapffel's mechanical manipulation, which the writer had not seen. It is satisfactory to him to find the method thus authorised.