Wheel Cutting in the Lathe
Among the various uses to which a lathe may be put, wheel-cutting is one of the most important, so many pieces of mechanism requiring cogged wheels of various pitches and forms of tooth. By the aid of the slide rest such an apparatus as figured may be readily arranged, and the work rapidly and accurately performed. The guide by which the cogs and spaces are determined is the division plate already alluded to, and which is not visible in the present drawing, but the index of which is shown at G. C is the cutter frame with a side pulley (one on each side) to conduct the catgut band from the revolving spindle to the overhead motion on to the flywheel. The spindle carries a pulley for the cord and a cutter wheel, I, to which an exceedingly rapid evolution is given. There are many patterns of these wheels, the edges of which, cut into teeth like a fine saw or file, are the exact form of the spaces required between the cogs; hence, some are rectangular, some have a triangular section. The thicker the wheel to be cut the larger should be the cutter, so that the bottom of the cut may be virtually level. In Fig. 181 another form of cutter is shown, which if put into sufficiently rapid motion answers as well, if not better, than the wheel-shaped cutters. It is a simple short bar of hard steel, with the edges bevelled in alternate directions, fitted into a slot in the spindle and held by a wedge or screw. The shape of the end is as before, a cross section of the space between two teeth. The cutter frame is here arranged to fit into the ordinary tool-holder of the slide rest, but the form may, of course, be varied at pleasure. It will be noticed that the slide rest, as delineated here, is different to that of which details have been given. It is made without the sole, and fits into the socket of an ordinary rest. Thus it can be turned on a centre, and becomes, to all intents and purposes, a compound slide rest. It is on this plan the small ornamental turning rest of lighter construction is made. The spindle fitting the socket projects from the centre of the lowest frame, and is cast in one piece with it. If the apparatus is compactly and strongly made, it becomes a very serviceable form, and is much used for the small lathes in sea-going steamers. The sides are made very short, so that the extent of traverse is small. We may here mention an addition to the rest socket, enabling the workman to raise this kind, or that used with drills and cutters, which is simple and convenient. Inside the iron socket a few turns of a screw are cut, and a second socket of brass with an outside thread is made to fit into it Figs. 182 A and B, the latter being a section. The edge of the inside socket is sometimes milled round, to facilitate holding it by the thumb and finger. In this way the height of the slide rest, or tee of the common rest, is adjustable to a great nicety.
When a wheel is to be cut of large size, or of substance exceeding that of clock-wheel work, the above method is not suitable. The wheel is then generally laid flat, and the cogs are shaped by a slotting machine, the chisel of which has a vertical motion. The lathe is then no longer used; as a separate machine is more convenient and economical.
A most serviceable addition to a lathe, especially an amateur’s foot lathe, is the circular saw, with guides for cutting parallel, taper, or mitred work. Great rapidity of work is thus combined with perfect accuracy. A five-inch lathe will of course take a saw nearly ten inches diameter, but it is not advisable to attach one of quite this size, for the larger the saw the greater is the leverage against which the turner has to contend, and the friction caused by a deep cut in stuff of two inches diameter is quite sufficient to make the labour considerable. When such work is necessary, it must be very gently brought to bear upon the saw, and the flywheel of the lathe should be heavy. The cord should also pass from the latter to the slowest division of the pulley. If the workman, amateur or professional, desires a lesson in practical mechanics, he has nothing to do but turn a piece of ash six inches in diameter, with the lathe-cord extending from the flywheel to the smallest part of the pulley, the diameter of which is about half that of the object to be turned. This will teach him what hard work is. Then let him try the job with the cord, from the smallest part of the flywheel rim, to the largest diameter of pulley. The change to a slower motion and greater power will not be disagreeable. It must be remembered that a circular saw of six inches diameter will not penetrate three-inch stuff, owing to the boss or nut by which it is attached to its spindle. The above size will not make good work of stuff exceeding two inches in thickness, and even less thickness would be preferable. As to the size of saw, indeed, that is most suitable to a five or six-inch foot lathe, much depends upon the proposed work, and still more upon the weight and size of the flywheel. As a general rule it is better to err upon the size of smallness. The service to which this tool is commonly put is but light; sawing narrow strips of mahogany, such as used at the angles of bird-cages, cutting strips or segments of ivory (for which let the saw be kept wet) sawing out mitred or dovetailed joints, and similar work is within the compass of a five or six inch saw, and it is better not to exceed this. The teeth should be tolerably fine for hardwood and ivory, and coarser for deal and soft woods. Smaller saws of hard steel, and made of thick plate, are used for metal.
The method of mounting saws of small size, such as are suited to be worked by the treadle of a foot-lathe, is shown in Fig. 183. E is a steel spindle, of which the diameter equals that of the central hole in the saw B. At F, about the middle of the spindle, is a fixed flange, at the base of which is a short feather or inlaid key, X 184, which fits the small slot seen in the centre of the saw and reaches also within a similar slot in the movable flange, G, but it must not be so long as to come through to the back of the latter. This flange and nut H having been removed, the saw is slid upon the spindle till it rests against the fixed flange; the movable one is now to be brought against it and clamped by the nut. The spindle is sometimes made with a square end to fit the square hole chuck, and centred at the other, or it is drilled at both ends, so as to be driven by the carrier or driver chuck, C, D. It must be so placed as to run towards the operator. The above arrangement must now be made complete by the addition of a platform, B, on which to lay the work that is to be sawn, and on which some contrivance can be adjusted to guide the passage of the saw through the same, so as to cut the work in parallel pieces, or at any desired angle, such as would be necessary for mitred joints. For the general uses of the amateur a mahogany or hardwood platform is as good as any, and such as is delineated in vol. ii. of Holtzapffel’s valuable work is perhaps the best arrangement. The saw table rests on the opposite ends of a kind of open box, which is represented without the two sides, although they may be added if desired, and the whole when removed from the lathe would then form a case for the saws, or serve other similar purpose. The platform is hinged, so as to overlap, as seen in the Fig. 184A, and there is in the middle of it a slit cut by the saw itself, which, when it is mounted on its centres will be in the position shown. If sides are added a notch must be made in the upper edge of both for the passage of the spindle. The fillet B fits between the bearers of the lathe, securing the parallelism of the whole. When made with four sides the box must first be placed on the lathe bed and loosely held by the bolt beneath. The saw is then mounted, and the box adjusted to its place and fixed. The cover is then (if for the first time) brought carefully down upon the saw, and the lathe being put in motion the slit is made, and its position will be truly at right angles with the spindle and the lathe bed. Of course, in future operations the platform is lowered over the saw before the holding down bolt is permanently screwed up. The sides of the saw-kerf may be edged with brass if preferred, but on the whole the plan is not to be recommended, for if, as will occasionally happen, the saw should get slightly out of truth, or vibrate a little when in use, the teeth will come into contact with the metal and be blunted or broken. If the saw-kerf by constant wear should widen too much, the whole platform is renewable at little cost, or a new piece can be let in, and a fresh saw-kerf made. There are several guides for parallel work. The one shown in Fig. 185 is precisely such as is to be seen in the ordinary parallel ruler—A is the back bar screwed to the platform at the right-hand edge; B the guide or fence which, when the connecting links C, C are perpendicular to A, should touch the saw; D, D are arcs of circles of which E, E are the centres. They may be arcs of brass pivoted to the links and passing through a slot in the bar, A, A, or may themselves be cut as mortises in the platform, in which fit a pair of bolts with thumb-nuts passing through the links, by which to clamp the fence in any desired position. This form of parallel guide is not very substantial, and is not correct in practice unless the pins are very nicely fitted, and the links precisely of a length. The second figure shows the sectional form of the fence G, the links being represented at H, and the fixed bar at K. The following is a more solid and unyielding guide, and much to be preferred. Holtzapffel attributes it to Professor Willis. It is merely a modification of the T-square as used with the ordinary drawing board with an arrangement for fixing it in the required position. The present arrangement differs from that in Holtzapffel’s work in the manner of fixing it and the addition of a second T-piece on the side next the workman. In Fig. 186 A is the upright part of the fence, B the bottom or sole, to which is attached at each end the T or cross pieces, E, E, which slide along the straight edges of the platform and secure the constant parallelism of the fence to the surface of the saw; C is the groove or slot, in which a screw, D, traverses, and the fence is thus fixed by a turn of the thumb-nut, D. This fence can by no possibility get out of truth; it is easily removed by taking out the single screw, and it is far more simple and more easily made than the one previously described. The nearest edge of the platform may be marked in inches and eighths, and the fence can then be instantaneously adjusted for sawing pieces of any desired width. It is not always, however, that straight, rectangular, or parallel strips are required, and an additional arrangement is needed to form a guide for sawing angular pieces. Now it is not sufficient to lay the guide fence at a given angle, for if the latter were arranged for that adjustment by taking off the tees and causing it to turn upon the screw which secures it in place, and a piece of board were placed against it to be sawn, the latter would press against the saw sideways as seen in Fig. 187. The guide fence for angles must itself therefore travel in a line parallel to the saw and carry with it the piece to be cut. The simplest and usual arrangement is that given in Holtzapffel’s work. A dovetailed groove in the platform running in a direction parallel with the saw carries a sector attached to a bar which fits the groove, and this bar is free to move forward or backward without lateral movement. The piece to be sawn is thus rested against the fence forming the straight face of the sector, and the whole is moved forward together against the edge of the saw. Fig. 188 explains this. It will be seen that several grooves are made side by side, all of which fit the slide alike, and by moving the latter into either of these a lateral adjustment is effected to suit pieces of different widths. The sliding strip should be made of hard wood and nicely fitted, and may be lubricated with soap, or polished with black lead, either of which will cause it to slide with diminished friction. Fig. 189 shows a somewhat different arrangement, by which more lateral adjustment may be given. The sector is replaced by a T-square, the blade of which has a slot through which a screw passes into the sliding bar. A second jointed rod is added, passing through a staple in the slide, and by a screw in the latter the T is fixed as required. The staple must turn on a centre to accommodate itself to all positions of the T-square, and a second eye may be placed on the opposite arm to allow the guide rod to be removed to that side, which is sometimes more convenient. The sector also may be made adjustable as in Fig. 190, and clamped by a simple screw in the slot. Either of the above methods will allow sufficient range for small work, such as is likely to be the object of amateurs or those who add small saws to the foot lathe. The guide for angular pieces may indeed be in many cases dispensed with by making use of patterns of wood chiefly in the form of triangles; these sliding against the parallel guide, and carrying the work with them, will answer well in a number of cases where other provision for such work has not been made in arranging the saw table. It must be remembered, however, that in this case the length of the piece (supposing it to be the side of a picture frame to be mitred) is limited to the space between the fence and the saw, which in the more perfect arrangement is an evil plainly avoided. It may sometimes be necessary to cut pieces with various angles not in the same plane. By using the hinged platform, and adding a screw attached to the front of the box, and standing perpendicularly, the front of the table can be raised to a given angle, but those who are likely to enter extensively into the cutting mathematical figures are referred to vol. ii. of Holtzapffel’s work, where the subject is fully explained and illustrated. The following remarks upon the proper speed of saws and sizes of teeth are copied from that work, and may therefore be relied on:—
“The harder the wood the smaller and more upright should be the teeth, and the less the velocity of the saw.
“In cutting with the grain the teeth should be coarse and inclined, so as rather to remove shreds than sawdust.
“In cutting across the grain the teeth should be finer and more upright, and the velocity greater, so that each fibre may be cut by the passage of some few of the consecutive teeth rather than be torn asunder by the passage of one tooth only.
“For gummy or resinous woods and ivory, the saw teeth must be keen, and the speed comparatively slow, to avoid the dust becoming adhesive (by reason of the heat engendered by friction), and thus sticking to, and impeding the action of the saw.”
By raising the platform so as only to expose a small portion of the saw, it is easy to cut rebates, grooves for tongueing, and other work of a similar kind. The above arrangement of hinged table facilitates this application of the saw.