Chapter 8

Figs. 116, 117.

Figs. 118, 119, 120, 121.

For light brass work, such as small model engines now so generallysold, a description of lathe may be used of which no mention has yet been made—namely, a bar latheFig. 122. Such a tool may be made for £3 or £4, and will be found sufficient for such workas specified.

Fig. 122.

In place of the double bed a triangular bar of cast iron is used, on which the poppets slide, and are clamped. D shows one of two sockets with feet which hold the ends of the bar, and by which the lathe can be mounted on any stout plank or on the window sill. The pulley A is made for a strap, because this allows of a plain flywheel, which is much cheaper than those which are bevelled and turned. The rest is shown at E and F. The part E slides upon the bar like the poppet—at the top is a dovetailed groove to receive the slide F, which carries the socket for the tee, and is fixed by a turn of the screw seen on the top of it. There is much to be said in favour of triangle bar lathes, they are very stiff, and can be fixed anywhere. The flywheel can be supported on a separate frame, which is an excellent plan, for the jerk of the treadle and crank is not communicated to the poppets and mandrel. In Maudslay's triangle bar lathe, which was made for amateurs, a slide rest was attached, and the whole work was first class—of late they have gone out of fashion, but are nevertheless good tools.

Fig. 123represents another very simple form of lathe for turning small articles of brass. It may be said to be one remove from the watchmaker's bow lathe, as it has no true mandrel or treadle; the small flywheel being attached to an arm at the back and worked by hand. The left hand is used for the latter purpose while the tool is held in the right. Through a tapped hole in the left hand poppet passes a steel pin shown at E, on a larger scale; this screws into the poppet after passing through a brass pulley B; this bolt ends in a point, and an arm fixed into the pulley becomes a dog to act against a carrier screwed on to the work as in turning iron. Thus the mandrel and point are fixtures, and the pulley only turns when motion is communicated to it by a catgut from the flywheel behind it. The back poppet and rest, which last is shown separately, slide on therectangular bar; the latter is about two inches wide, and three eighths thick, and is made with feet to screw to any convenient support. This lathe is especially adapted for work of small size which can be centred at both ends, and on which a carrier can be fixed to bear against the pin, E, of the pulley; nevertheless it is even possible to use chucks, if cast in metal with a pulley to each like G. A spindle must in this case be made like H, on which the chuck must be slipped, and fixed by the nut in the hollow of the chuck. Although however this form of lathe is sometimes met with, and may be used as a makeshift, a small triangle bar or 3in. lathe of the usual form is far preferable.

Fig. 123.

In centering a bar of iron in the lathe too much care cannot be exercised in causing it to run evenly. The ends should be drilled, first with a small, and afterwards with a larger drill, so that a countersunk hole may be obtained in orderto keep the point of the lathe-centre from touching the bottom of the hole. If this is not done the friction of the work upon its bearing will soon spoil the lathe-centre, and the work itself will speedily get out of truth, and it will not be possible to screw up the spindle of the back poppet so as to correct this. Of course in turning up very small work this drilling cannot be done. A simple hole must then be made, sufficient for the safe support of the work while being turned, but even in this case the angle of the drill point should be less than that of the conical centre of the lathe, the point of which will then run free. To mark the true centres of a round bar of metal a punch has been devised likeFig. 124. This is figured in Bergeron's work, and is veryserviceable. To insure its working truly be careful that the bar of metal is filed flat on the ends, and that the surface of the latter is at right angles to the length of the bar. It is then only necessary to place the end of the piece in the conical part of the cup (which will be best effected by fixing the bar in a vice) and by raising the spring and letting it go sudden by a sufficient mark will be made to guide the point of the drill.

Fig. 124.

The proper place at which to commence turning an iron bar is at one end, the rest being placed so as to bring the tool just upon the line of centres; if applied lower the tool would take too deep a hold, and would either be broken or lift the bar out of the lathe, damaging the centre points. The ends of the bar should be squared up before the circumference is turned, and on no account must a file be used after the turning tool has done its work. It is only at the commencement before a cutting tool has been applied, that an old file may be made use of to take off the scale and roughness left from the forging or casting.

Brass may be attacked upon or just below the line of centres, because the form of tool is such as cannot penetrate deeply. This metal is perhaps easier in some respects to turn, but the tool is apt to form undulations on its surface (is apt to chatter). This is due, partly to the impossibility of obtaining continuous shavings, and partly to the vibration of the tool, and when once this has taken place there is a tendency to deepen these channels, which makes it difficult to produce a plane and even surface. If a rectangular tool is used in the position already shown and described under the head of tools for metal turning, this chattering will be avoided. If it takes place, the undulations should be worked off by gentle usage of the angle of the tool, the rest being placed close to the work, and only a light cut taken.

With regard to the method of mounting a piece of brass in the lathe, any convenient chuck may be used, but sometimes the piece is short or irregular and requires to be bored out. In this case use solder and firmly attach the piece to a face plate of brass. The easiest way is to smear the two faces to be joined with sal-ammoniac made into a paste with water, and laying a piece of tin-foil between the surfaces, which must be quite clean and bright, apply heat. The tin will melt and a perfect union will be effected. When the piece is finished it is re-heated and detached. This may be considered a wrinkle worth knowing. The flat flanges of brass spoken of under the head of chucks are just suited for this method of working, and they are not damaged by the process, as the solder can be wiped off quite clean when the chuck is made hot. This is a good way to mount small cylinders of brass for model engines, as they can be bored and turned on the outside at one operation with great ease and certainty. If a piece is to be drilled or bored in the lathe, the following is the arrangement to be adopted.Fig. 125, A is the face plate, B, the piece to be drilled, C the drill, which is advanced by screwing up the point, E, of the back centre; D is a hand vice or similar article to prevent the drill from revolving with the work. If it is more convenient to fix the drill itself in a chuck, the point of the back centre is to be removed, and a flange of brass or iron substituted, as A,Fig. 126. If the drill is to penetrate quite through the work a piece of wood must be interposed between the latter andthe flange to receive the point of the drill and protect it from injury. The pressure should be so regulated as to be constant and equal without being excessive, or the drill will be bent or broken. Boring is simply drilling on a larger scale, and is of such general use as to require detailed notice. In the first place there are several tools used for the purpose, according to the size of the work. The first is the cylinder bit,Fig. 127. This is a most excellent tool, as it will work very truly, and can hardly get out of place if properly directed at starting. The cutting part A is half a cylinder, the centre being just left visible, the end is not quite at right angles with the length of the tool, but is sloped off a little and bevelled also slightly below.[7]This forms the cutting edge. The other end of the tool has a central hole, drilled to receive the point of the back centre by which it is kept to its work. To use this tool, let the piece to be drilled be placed in a chuck, and a recess turned in it of the same diameter as the cylinder bit, the latter is then placed in this recess B, and when screwed up it cannot possibly rise or shift its position; a hand-vice or spanner is then fixed as shown inFig. 125, and the lathe put in slow motion, oil or soap and water being freely used to lubricate the tool. Either a solid piece of metal, or a hollow casting can be thus bored.

[7]To an angle of 3°. See Appendix.

Figs. 125, 126.

Fig. 127.

These cylinder bits can be had of all sizes, from one-eighth of an inch upwards.

Pipe stems are bored with the smallest of these tools. For this purpose they are made of round steel wire, which is sometimes merely flattened with a hammer at one end to spread and enlarge it, this part being afterwards rounded on the underside with a file, and with the same tool finished on the upper flat face. These slender drills require to be delicately used, and are conveniently held in a pin vice or pair of pin pliers, the handle of which being hollow, allows the greater part of the drill shank to lie within it, a small part only being drawn out at a time for use; thus the drill will be kept from bending, and will work quickly and well.

Of recent inventions in the matter of drills, the most important is the Morse American twist drills sold, in sets, at 30s. on a neat stand with a self-centering chuck, complete. The form is that of a cylinder with spiral grooves cut round it of extended pitch. The cutting edge is as difficult to draw as it is to describe, and must be seen to be understood. It is chiefly formed by the meeting of the spiral grooves and the solid end, the latter forming a blunt angular point rendered cutting by the edge of the grooves. They should have a place in every workshop.

The next tool to be described is also much used, especially in agricultural implement manufactories, for boring out the brasses, or bearings. It is called a rose bit, or grinder, and is shown inFig. 128A and B. In this case also a recess is cut in the work as a guide, and as the tool fills up the whole interior as it proceeds, no change of position can occur. The rose bit is used as shown in125. A bit of this pattern is very useful for brass work of all kinds, such as the cylinders of small engines, bosses of wheels, bearings and collars, and one of these tools of the exact size for hollowing out the sockets of brass chucks, previously to their being tapped, will be found serviceable. The third kind of boring tool is made with movable cutters, which can be removed at pleasure, to be sharpened or replaced by more convenient ones. The simplest consists of a cutter bar, A,Fig. 129, with a slot in it to hold the tool, which is fixed by driving a wedge at the back of it. The tool here shown has two cutting edges,bandcwhich should be shaped according to the principles already enunciated respecting hand tools for iron and brass. The cutter bar is usually fixed between the lathe centres, and turned by a driver chuck and carrier, while the cylinder to be bored is clamped to the slide rest, and thereby advanced against the cutting edge. This form is chiefly used upon work that has been cast hollow, or drilled.

Fig. 128.

Figs. 129, 130.

Fig. 130is 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. Itis 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 inFig. 131A 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 itthe 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.

Fig. 131.

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 calledback 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 inFig. 132, is merely the substitution ofVshaped 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, asFig. 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.

Figs. 132, 133.

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 likeFig. 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. TheV's, underneath the frames, and the edges of the latter, must fit, so as not to be tighter in one place thananother, 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 thanto 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 theV-pieces. This is shown inFig. 136. The holes in theV-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.

Figs. 134, 135.

Fig. 136.

Two large headed screws, E, E, are tapped into the place on which theV-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 theV-pieces, driving them closer to the dovetailed slide. When thus adjusted the top screws are made use of to fix the stripsc,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 in138&139on 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 inFig. 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.

Fig. 138.

Fig. 139.

Fig. 140.

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. 141is quite the best formof 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. Therule of thumbwas 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.

Fig. 141.

Fig. 141is too much cranked; half the length of the hanging part would amply suffice.

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 lengthor 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 thework. Hence the greater ease in managing a hand-tool. The handfeelsthe 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 metalV-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, likeFig. 142, and, as itis 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.

Fig. 142.

The outside must be turned in a similar manner, and a hole drilled to receive a pointed bar or wrench, for the purpose of unscrewing it when screwed up tightly. To turn up a face plate of iron or brass proceed in a similar way, but commence from the centre with a point tool. This tool is the best for taking off the rough outside of hard wood, instead of the gouge, as well as for removing the roughness of a fresh casting. It is absolutely necessary that the faces of these flat chucks, or surface chucks, be truly at right angles with the mandrel; hence it is very difficult to turn and finish them by hand. We may here also state the necessity of knowing when the slides of the rest are at right angles to each other, without which no work can be turned correctly. It is necessary to ascertain this by help of a small steel square. Once fixed truly, it is only necessary to make a mark on the quadrant (which should be marked in true degrees), by which the same position can be found again at any time. We have spoken here of the quadrant—we mean the arc, or arcs with slot—allowing the circular movement of the compound slide rest. The chief use of these is to enable the workman to turn true cones instead of cylinders, which latter will only be produced when one slide is parallel to the lathe bed. Such is the common use of the slide rest; and it will be evident from these few remarks that there is an infinity of work, not only produced with ease by its aid, but which cannot, even with expenditure of time and labour, be produced without it; hence we advise this to be the chief ambition of the tyro after he has mastered the difficulties of ordinary hand turning (and not before). The cost of a fair one for 5-in. lathe will be £5, or thereabout. At Munro's, £7 or £8; at Holtzapffel's, £10 or £12. Both the latter are of course perfect.

We have directed to tap the chuck where it is to be screwed to the mandrel with a set of three taps, or to cut it with stock and dies if an outside thread is required. In both cases more true and satisfactory work may be produced by the chasing tool. We speak of the latter as used by hand; an account of cutting the threads by help of the slide rest we reserve for the present. Mount the work in any convenient way, either driving it into a wooden chuck, or by clamping it to the face plate if you have one. Now in this way you have advantages. In the first place you need not have a drill the exact size, though it is convenient to have such a one, and also a cylinder bit. You can drill and enlarge the hole by the slide rest tools, and you can also with the slide rest ensure the perpendicular position of the hole with respect to the end. Thus it is sure to fit up close and snug to the shoulder of the mandrel. When bored thus it will be in position for chasing. It is not difficult to chase a thread in brass, as it does not chip away like wood, but cuts clean and sharp. Follow the directions already given and you will succeed in a few turns in getting the tool to run. Then let it have its own way; hold it lightly, but steadily, and do not force it either to cut too deeply, or to advance too quickly; it will run along of itself after the faintest thread is cut or scratched, and the lathe can be worked by means of the treadle all the time as soon as you have attained the knack of dropping the chaser into its place at the commencement of its cut, and suddenly withdrawing it when it has reached the bottom of the hole. A chuck thus turned and screwed entirely in the lathe is sure to prove a good fit, and there can be no better practice than to cut screws in all your brass and boxwood chucks in this way. A very good chuck to hold flat plates of brass was invented by a Mr. Wilcox, of Bishop's Stortford, some years ago—an amateur of rare ingenuity and mechanical knowledge, and who made all his own apparatus for plain and eccentric turning. The chuck,Fig. 143, as described by him in some unpublished manuscript, is made of boxwood, or may be of metal. It is a plain disc or surface chuck with three slots A B C. and a steel centre. This last must penetrate rather deeply into the wood of the chuck, but is only kept up so as to project from the surface by a spiral spring below it. Hence, when pressure is made upon its point by the application of the object to be turned, the pin recedes into the body of the chuck, suffering the work to lie flat on its surface. Inthe three slots are three screws, with nuts at the back of the chuck the screws pass through two pieces of brass, forming a pair of jaws, one of which is shown separately.

Fig. 143.

Fig. 143shows the chuck complete. Suppose we have to face up a round disc in its central part, or to perform any surface work in which the jaws will not be in the way of the tool, the centre of the plate is marked at the back of it, and this mark laid on the central pin. The work can be clamped down by the three jaws, and the necessary work may be done. Now so far the chuck is but a simple affair, and the receding pin does not show itself to such great advantage; we will therefore suppose a plate is to be drilled at several spots; let these be marked at the back by a centre punch. It is now only necessary to bring these marks in turn upon the central point, and clamp the plate in position. Bring the point of the drill against the work and keep it up to cut by the back poppet screw. In the same way eccentric work or any operation may be done to such a plate, with the certainty that the point to be thus worked upon is precisely central with the axis of the pulley, or mandrel. Many similar applications of this chuck will present themselves to the reader upon due consideration. We present the chuck in this place because we have had occasion to speak of drilling as connected with the slide rest, and there are many pieces of work that could not otherwise be conveniently held in the desired position.[10]

Back to Index Next