Part 9

Fig. 130 is another form of boring tool for large and heavy work. A boss, A, is fixed to the cutter bar, having a series of dovetailed grooves, or slots, on its surface, in which cutters are fixed by wedges. In this and every similar form, it is expedient always to complete the circle, or, at any rate, two-thirds or three-quarters of it, by driving in blocks of wood in the slots not occupied by the cutters. This preserves the concentricity of the tool. One edge of these movable cutters should be radial to the centre of the bar, or boss, the other rather less than a right angle, which will ensure a good cutting edge. The best lubricant is oil for the first cut, and soap and water, or pure water, for the finishing cut. The surface will thus be left bright. It is not well to finish with emery any collar in which an axle is to work (as the collar in which the mandrel of the lathe runs). This substance imbeds itself in the pores of the metal, and by forming a grinding surface, considerably increases the friction and wear and tear of the parts.[8] Although boring and drilling are capable of being done in the lathe, a far superior plan is to employ an upright boring apparatus, as is now generally used in making steam cylinders. The work is not then suspended between two points, or carried on the slide rest, but takes up a firm bearing on a fixed support, and the boring tool descends by a pressure screw, or self-adjusting contrivance, as the work proceeds.

[8] Oilstone powder may be substituted, especially for the best brass work.

We have spoken of the slow motion as necessary for turning metal work. This is represented in Fig. 131 A B C D. The first is a plan seen from above. The poppet is cast double like F, so as to afford a bearing for the mandrel, and a second for the back spindle seen at A. This back spindle, it will be observed, passes through its two collars or bearings, and can slide freely in them from side to side. This can, however, be prevented by dropping a pin through a hole in the top of the poppet, which falls into a semicircular groove in the spindle. The pulley is securely attached to a small cog wheel, and can be firmly united to a larger one, as seen at A2, and separately at C and D. This pulley and small cog wheel run loosely on the mandrel, and do not revolve with it until clamped to the wheel, C, which is itself keyed to the mandrel. Suppose them to be thus free to revolve, and the wheels in position shown in the plan, A. On putting the fly wheel in motion, the pulley will revolve on the mandrel, carrying with it the small cog wheel, which in turn will act on the large wheel on the back spindle. The small cog wheel on the latter will thus put in motion the large one geared with it, the which being keyed to the mandrel, will put the latter in motion. There are many ways of clamping the pulley to the large cog wheel, perhaps the following is as good as any. It must be so clamped for wood turning when the back spindle is to be slipped on one side out of gear.

In the face of the pulley, which is concave, is a piece of brass flush with the rim, and which forms a dovetailed groove, into which the head of a clamping screw, E, fits. This screw projects through a slot in the wheel D. When it is required to fix the pulley, this screw is slid up towards the rim till the head rests in the dovetailed projection, and it is clamped in that position by a nut. When it is desired to put the back action into gear, this nut is loosened, the screw-bolt dropped towards the axle (thus freeing the head from the dovetail), and again fixed by the nut. The wheel and pulley are thus independent of each other, the back spindle is slipped sideways into gear, and held by the pin, and the slow motion will be obtained.

There is one fault in the arrangement of the back geared lathe that with amateurs in a private house is especially disagreeable, and it is questionable whether in large machinery establishments it might not with great advantage be corrected. In the action of toothed wheels, nuts and screws, and similar gearing, there occurs what is called _back lash_. If, for instance, the tool holder of a slide rest is advanced, and then the action is to be reversed, the movement of the nut and tool holder does not commence simultaneously with the movement of the screw. This is due to the play, or necessary looseness of the working parts, the pressure coming on one side of the thread when the screw is turned in one direction, and on the contrary side when the motion is reversed. In toothed wheels a similar defect exists, and gives rise to that disagreeable and ceaseless noise which assails the ear on entering a building where machinery is in motion. This may be avoided by the use of frictional gearing, a simple but excellent mechanical contrivance which deserves far more extensive notice than it has yet received. It is the invention of a Mr. Robertson, and is patented. A lathe fitted with it would be almost noiseless, and would work with a delicious smoothness, very conducive to the comfort of the workman. This gearing, represented in Fig. 132, is merely the substitution of V shaped or semicircular grooves for cogs, the former running round the periphery of the wheel like the grooves in an ordinary lathe pulley. In this method of gearing, it would be necessary to move the back spindle to-and-fro, the usual horizontal movement not being possible. This is easily effected by a screw or a cam, either of which might be made to act on the frame which carries the back spindle, and which may then work on a centre, as Fig. 133, where A is the poppet, B, the support of the spindle, D, a cam; when the handle of the latter is raised, the standard, B, is allowed to fall back out of gear into the position shown by the dotted line, C. A screw movement would have the advantage of enabling the workman to regulate with greater precision the pressure of the friction pulleys against each other. The drawing shows the grooves of these pulleys larger and deeper than usually made. They are generally rather shallow and numerous, and it is astonishing with what firm hold they grip each other without that violent pressure which it might be imagined would be necessary to prevent slipping when in use.

The ordinary slide rest for hand lathes is made as follows:--That for ornamental turning will have a separate notice. Fig. 134, shows the slide rest viewed from above, and it is evident if the tool is clamped to the holder F on the top plate, it can be advanced from end to end of the top slide B, and also (with the upper frame itself) along the lower frame A, A, these movements being at right angles the one to the other. For parallel work this is sufficient. In this compound rest a third motion is arranged for turning cones or taper plugs like those of stop cocks, taps for screw plates and such like articles. For this purpose the upper frame is cast like Fig. 135, with a flat surface, but with two ribs underneath, uniting the frame to a circular plate with two concentric slots in it. This plate revolves on the plate G G, turning on a central pin, and it can be clamped by the two screws which pass through the slots into the plate in any desired position; once clamped at the required angle a piece of metal can be bored with a conical hole and a plug turned to fit it without possibility of failure. The details of construction allow of considerable variety, and different makers keep to their respective patterns; the main desideratum is strength and solidity, combined with accurate adjustment of the moving parts. The =V='s, underneath the frames, and the edges of the latter, must fit, so as not to be tighter in one place than another, and the upper and lower frames must cross each other accurately at right angles. It is likewise essential that the tool traverse the work in a perfectly horizontal line. Every part must, therefore, be accurately made by means of the lathe and planing machine, and the whole carefully put together. Notwithstanding the above desiderata, a slide rest is not necessarily beyond the skill of the amateur. We have, indeed, seen one thus made quite equal to the work of a professed mechanic, though the file and scraper had to take the place of the planing machine. The rough castings can be bought for about half-a-crown, suitable for a five-inch centre lathe, and it would be much better to try and fit up a set of these castings than to attempt such a substitute as a wooden slide rest. The latter has nevertheless been made, and we remember seeing one of mahogany edged with brass, the work of a cabinet maker, which did good service in turning and ornamenting wood.[9] This, however, was upwards of twenty years ago, since which time the facilities for obtaining slide rests of metal, properly constructed, have materially increased. As the dovetailed edges of the slides wear away by use, it is necessary to provide means for tightening up the =V=-pieces. This is shown in Fig. 136. The holes in the =V=-pieces through which the top screws pass are not round, but oval, so as to admit of lateral movement.

[9] In the Paris Exhibition of this year (1867) are some slide-rests made of hardwood and metal.

Two large headed screws, E E, are tapped into the place on which the =V=-pieces rest, and when these are screwed up, their heads (which are sunk for the purpose in two recesses in the lower plate) press against the =V=-pieces, driving them closer to the dovetailed slide. When thus adjusted the top screws are made use of to fix the strips _c_, _c_. By this method the slides can be adjusted to work with the utmost ease and accuracy, without shake or side play. The edge of the circular plate and the heads of the leading screws are very frequently marked in graduated divisions, so that the advance of the tool or the angle to be made with the work by the tool can be accurately measured and preserved. There should at any rate be a mark on the circular plate to show when the rest is set for parallel work. There are several patterns of tool holder, of which the forms shewn are convenient for light work. The one shown in 138 & 139 on the rest is somewhat different. The plate F, Fig. 134, is cast with a boss and socket, like that of an ordinary rest. In this socket the tool holder fits, and can be not only turned round so as to set the tool at any angle, but also slightly adjusted in height, which is a great advantage. The tool is clamped by a single screw as shown in the sketch. The drawback to this form, and that on the rest, is this single screw, which will indeed hold the tool when the work is easy, but will not always retain it with sufficient firmness when the work is rough, or of tolerable size. In large workshops one usually meets with the holder represented in Fig. 140. A plate, A, with central block B, and slide C, are in one casting. Through A pass eight screws. The tool lies on either side of the central square block and is clamped with three screws, it has thus a fair bearing on two sides, and the screws form a third above, so that accidental shifting of the tool during the progress of the work is hardly possible. The tool holder of Professor Willis, which is described in the Appendix, is perhaps the best of all at present in use. It holds the tool firmly at any desired angle.

It is quite possible that the novice who has seen mere boys working with slide rests at manufactories will be disappointed at his own first attempts to use this piece of machinery. All the difficulty lies in the shape and set of the tool. When turning metal with hand-tools it is easy to feel one's way. If the cut is not satisfactory, the hand at once modifies the angle of the tool, and regulates its direction to a nicety, but the slide rest cannot thus adapt itself to its work. It must be set with its slides in position, and the tool once fixed must pursue its own course. Hence it requires a very accurate knowledge of the nature of cutting tools, such as we have given in the Appendix to this work. If the tool is well placed upon the axis of the work for iron, a little below it for brass--it will cut cleanly and easily, without rubbing or jarring, both of which are proofs of either a wrong angle of edge, or a wrong form of tool. The work should proceed with as much apparent ease as if the metal were an apple, and the shaving should curl off like its peel. Moreover, this case is not merely apparent, it is perfectly easy to cut iron, and the strain on the tool, whether held by the hand or by slide rest, is comparatively slight, if the tool is properly made and held. Fig. 141 is quite the best form of tool for surfacing cylinders with the slide rest. It is to be so placed that both edges are made to cut near the point; hence the crank should slightly curve away to the left. It is not possible to cut metal quickly, be content with fine clean shavings curling off freely. You will soon see whether you can take a deeper cut with safety. The tool here sketched is not at all likely to dig in and hitch in the work; if it is not properly placed it will spring and jump, or its side will rub against it, and no cut will be made. To describe the exact position is very difficult, but the principle once grasped, little difficulty should be experienced in making and setting to work any tool, whether for inside or outside work. The _rule of thumb_ was all well enough in olden days and in the infancy of the art of metal working, but it is time to discard it; to master and man it is equally advantageous to do so. Indeed, in some of our leading firms, the old system of follow-my-leader, when the leader was as ignorant of his work as the follower, is waning, and the "how" is now, as it ought to be, coupled with the "reason why." One of the best papers ever written on this subject is to be found in Weale's series, "Mechanism and Construction of Machines," by T. Baker, and "Tools and Machines," by J. Nasmyth, 2s. 6d. The latter part is that specially referred to, and is well worth the whole price of the work. The remarks, however, of that eminent mechanic are embodied in the paper in the Appendix, and therefore, after the reader has studied the latter, he should make trial for himself of the principles laid down. Expend a quarter or a half an hour experimenting thus, with keen and obtuse tools, held at divers angles, and you will see and understand what is meant by setting a slide rest, or hand tool to cut metal as it ought to do.

Supposing the tool fixed in the tool holder in the position indicated, and just overlapping the circumference at one end (the right). Take hold of the handles, one in each hand, and with that which advances the tool from end to end of the bar try very carefully whether the tool will cut cleanly by making a turn or two while the lathe is in slow motion. If the tool bites too deeply, turn the other handle and ease it. If you still find the tool sticking into or scraping the work, instead of bringing off a fine shaving, look well to its position, and observe whether the edge is well placed on the axis of the piece. If it has been hitching in the work it is probably too low, if rubbing it, too high, and touches at some point below its edge. It is presupposed, of course, that the tool is made correctly as to angle of cutting edge. Do not lower the point by packing the end of the shank; _pack the whole length_ or none. It is astonishing what a great difference is made to the cutting power of a tool by slight adjustments of this kind, and how smoothly a tool will work with proper attention to these details, which would otherwise be probably cast aside as unfit for the work. Hence the greater ease in managing a hand-tool. The hand _feels_ the error, and at once, if experienced, corrects it by an almost imperceptible movement--slightly raising or depressing the handle or gently varying the angle sideways on the rest. When once the tool is found to cut as it ought to do, nothing remains but to turn the handle in the right hand, and thus cause the tool to progress steadily along the work from end to end. Then free it by a half turn of the other handle, reverse the movement until the tool has arrived at its old place, and having slightly advanced it to take a fresh bite, repeat the process until the whole bar is reduced to the required size. If the piece is slender and bends away from the tool, add to the slide rest a support; let it be fixed opposite to the tool, and it will keep the work steadily up against the cutting edge. It can be fixed (if a hole is made for the purpose) anywhere about the slide which traverses in the direction of the length of the work. It is well to drill and tap a few holes about the slide rest, and some along the side of the bed of the lathe. These will be found very useful at various times for fixing apparatus. For, be it observed, (and we shall recur to this with some practical hints by and by) the lathe may and ought to do many kinds of work beyond ordinary turning. It may become a machine for planing, slotting, drilling, wheel cutting, &c., and is to be pressed into the service of the jack-of-all-trades, without compunction.

When ordering a slide rest let steel screws and nuts be specified, and gun metal V-pieces, and let the parts be strongly made (too strong for the supposed work); for the latter may unexpectedly turn out to be sometimes rather heavy. We have found the top plate of a 3in. slide-rest so weak that when the tool was clamped on the top of it, by the screw of the tool holder, the slide itself became jammed; a defect quite beyond remedy, except by the substitution of a new and stiffer plate. The same advice may be given respecting the tools. Let these also be strong; neat and pretty tools are all very well, but you seldom see them in a workshop; you don't require pretty tools, but good and serviceable ones. Nevertheless, let the material be of the best quality possible; and that you may not be ever at a loss, you should learn to make tools yourself. Procure some small square and round steel bars; save up as directed your old files, and you need but heat them red hot (not on any account white hot), and with hammer and file shape them to your mind. Then temper to a deep straw colour, and after being accurately ground and finished on the stone, they will be fit to use upon any metal. The form of tool given as the best for slide rest work may be exchanged, when the bar is nearly turned, to the required size, for a fresh one, keen, sharp, and of an almost flat edge instead of point. A tool, of which the edge is a segment of a very large circle, will serve the best as a finishing tool, just to take off the lines left by the pointed tool. With regard to lubricating the work, we may observe that the chief object is to prevent the point or edge of the tool from heating and losing temper; oil, water, or soap and water will therefore answer, but it is a curious fact that oil will not produce so polished a surface as water will. We should advise in all cases soap and water. Soft soap is best, boiled in water, and allowed to cool. The drills with which the huge armour plates of ships six inches thick are drilled are thus lubricated, and instead of throwing out dust in a wet state as usual, these large drills fairly turn out curled shavings similar to those produced by the planing tool. It is by no means a bad plan to lay the shank of the tool which falls upon the top plate of the rest, upon a piece of leather, wood, or sheet lead. The surface of the iron, when planed and finished, is often too smooth, and the tool will sometimes slip from this cause, unless screwed unduly tight, to the detriment of the rest. By the above plan this annoyance will cease at once.

We will now say a word about hollowed work. Finishing a chuck will serve our purpose, and here be it advised not to go to much expense about chucks--get those which must be of brass in the rough, and practice metal turning by finishing them yourself. If no slide rest is available do it by hand. However, we are supposing the slide rest to have been procured, and may therefore proceed to use it. First you must drill the back part of your chuck as directed in a previous chapter. The drill is to be the size of the diameter of the hollows in the mandrel screw, that is, smaller by the depth of a thread, than the full size of the nose. Having drilled it, proceed with the most tapering of your taps, which we suppose to be provided to form the internal thread (external if your mandrel has female screw, in which case, instead of a drilled hole, the chuck will have a projection to be turned truly cylindrical, and a screw cut outside with stock and dies or chasing tool).

Follow up with the intermediate and finish with the plug tap. If you were careful to square up the shoulder, the drill having been likewise placed perpendicularly to the face of the chuck, the latter will fit truly up to the collar or shoulder on the mandrel. If not, you must go to work again, and square up the back of the chuck till a good fit is produced.

Now, if you have a compound slide rest--that is, one in which the slides turn on a centre pin--you can loosen the screws and turn it a quarter of a circle. If not, you simply put the tool into the holder, at right angles to its former position, so that the movement of that slide which is parallel to the lathe bed will become that requisite to advance the tool into the hollow of the chuck. Whichever way you set to work put in a side tool, like Fig. 142, and, as it is a brass chuck, remember that the bevel underneath is to be very slight. Introduce the tool so as to take a light cut at first, until the roughness is taken off, after which you may cause it to bite more freely. Repeat this until the chuck is sufficiently hollowed out, when you may substitute a similar shaped tool, but with a flat or slightly rounded edge, to take off the marks left by the point tool. To finish the bottom of the inside you will require a tool which cuts on the end, but it should not have a perfectly flat end--at any rate, not until, by means of a pointed or small round-ended tool, you have cut away the roughness left from the process of casting. This has always in its interstices a number of grains of sand fused, and very hard and detrimental to cutting edges of all kinds. The point tools dig these out very effectually, and should always precede those of other forms.