Cutting Screws.

[2]Now Messrs. Mundy and Berrie.

Figs. 51, 52, 53, 54, 55, 56, 57.

Supposing a selection made of size proportionate to that of theintended box, including cover and a tolerable margin for waste and accidents, proceed as before to rough it down between two centres and thus to reduce it to a cylindrical form—there is, however, no occasion to use the chisel at present, as we only need a rough cylinder. Remove this from the lathe, and if you have no brass cup chuck into which you can fit it, proceed to make one out of a piece of beech, ash or, if you have plenty, boxwood. Do not hurry the work, but cut the chuck out neatly, screw and fit it, as previously directed, on the nose of the mandrel. We shall suppose it as yet merely a short neat cylindrical block, quite solid. Place the rest with the tee across the end of the piece of wood, the top edge a little below the centre (by the thickness of the blade of the tool). For the latter select one of the three following—either will answer well—58, 59, 60. With one or the other drill a hole in the centre, keeping the tool quite horizontal across the rest. Enlarge this hole by a left side tool, working from the centre of the piece towards the outside, not taking the whole depth at once, but a quarter or half an inch at a time. You must hollow it out about one inch, and see how nicely you can fit the hollow to the size of the piece you are going to turn. You will, of course, have squared up one or both ends of the latter, which must now be driven tightly into the hollow chuck. If you squared the ends of the cylinder correctly and left also the bottom of your chuck level and true, you will be gratified by seeing the piece run evenly at once.

Figs. 58, 59, 60.

Fig. 61shows a section of the chuck with the piece to be turned fitted inside it. Now take the gouge and chisel and reduce the piece to a plain cylinder, and take special care to square up the outer end. This may be done by the aid of a carpenter's chisel held across the rest, like the side-tool. If the end is much out of truth you had better use first the round-ended tool,Fig. 60, but if you have worked carefully from the commencement this will be unnecessary.

To ascertain the correctness of this part, apply a small steel square likeFig. 62, the blade of which slides through the brass part and is clamped by a small screw at the side. We show the method of applying this tool to gauge depth, test right angles, &c., inFigs. 63 and 64. It is a most convenient and necessary instrument, and should be at once provided. Having thus ascertained that the end of your cylinder is at right angles with the side, take the point tool,Fig. 58, or the acute corner of your chisel, and, setting the lathe in motion, mark off the intended depth of thecover, as D, C,Fig. 61. (Observe, it is the cover and not the box that first demands attention.) Now proceed to hollow out the cover as you hollowed out the chuck, but with greater care. You must allow in the thickness of the top rather more material than you will eventually require, the thickness of the sides, also, may be a trifle in excess, but take the utmost care to make the inside rectangular, that is, the linef, g, perpendicular to g, D. Upon the correctness of this the fit of the cover will depend. This being done and tested as to truth with the square, as before, you may cut off the cover with a parting tool,Fig. 65. This tool is thin, with a cutting edge at the end, and is held edgewise upon the rest. The blade is made rather thicker near the end, so that as the tool penetrates the work it may not bind, but allow the small chips made by it to escape freely. The rest must be removed from its former position and placed parallel to the side of the piece, and the tee at such a height that the latter may, when the tool is held horizontally, point to the axis of the work. The tool should be occasionally withdrawn, and the point, instead of being kept precisely in one position,may be slightly raised and lowered from time to time, describing a small arc. It will soon be ascertained in what position it cuts most easily. There are different sizes of parting tool, some very thin in the blade, for ivory and precious woods, some thicker, for box and less valuable stuff, some with a notched end, forming two points, for soft woods, the action of all being similar to a saw tooth, or, in the last, to two adjacent saw teeth set out to clear themselves in working. Care must be taken that the thin blades do not bend and twist while cutting, especially after the cut has become deep. To avoid this do not hurry the work, but take a little at a time, and be careful to keep the tool with its sides perpendicular to the rest. With these precautions the cover will soon be cut off neatly. If care is not taken to allow for the necessary thickness of the cover, the turner will be mortified by finding that instead of the latter, he has merely cut off a ring, and he will have to expiate his want of judgment by beginning a new cover and making a shallower box. We name this to put him on his guard. Supposing the above work satisfactorily accomplished—the top of the cover, however, being (as will probably be the case) either convex or concave, requiring a little touching up and finishing, it will be necessary to turn down on the solid bit of wood left in the chuck the part A, B (Fig. 66), on which the cover will eventually rest. On no account, however, must this be now turned small enough, it must be left so large as not quite to enter the cover, because if it is now nicely fitted, and the box subsequently hollowed out, the cover is sure to be too slack, the wood shrinking in the process of hollowing out. This shrinking may be accounted for by supposing the rings of woody fibre, the result of yearly growth, naturally elastic, with a tendency to contract, each one, like a series of india-rubber bands, embracingthat within it. The central ones being removed by the tool, permit the outer ones to contract, their particles approaching nearer to each other and the structure becoming more dense. This tendency causes those radial cracks so often seen in the ends of pieces of wood sawn from the trunk or limbs of the tree. The outer parts becoming drier than the inner, and prevented by the latter from shrinking, necessarily split, hence, when it can be done, the centre of such pieces is bored out, while the wood is yet full of sap, and the rest is thereby preserved. Where this cannot be done the ends may be covered with glue or resin; or paper may be glued on, to prevent access of air, and thus the drying of the outer portion may be so retarded as only to keep pace with that nearer the centre. The concentric rings thus shrink equally, and no radial splitting takes place.

Figs. 61, 62, 63, 64.

Figs. 65, 66, 68.

We will now return from this explanatory digression to the work in hand. Having cut down the flange for the cover to nearly the required size, proceed to hollow out the box. Work carefully, so that the sides shall be smooth and perpendicular to the bottom, and the latter plane and neat. Take care, as with the cover, to leave the necessary thickness of bottom, allowing for the cut of the parting tool, and, if possible, half an inch or more beyond it. Now finish that part on which the cover is to rest. Take great care, as before, to secure right angles, and cut away the wood little by little, trying on the cover from time to time, until at last it will just go smoothly and stiffly into its place. It must fit rather tightly, but take especial care not to force it on, or you will split and spoil it. We shall here introduce to the notice of the reader another form of callipers useful in such work as the above, and in many cases absolutely necessary. They are called in-and-out callipers, and are made as shown inFig. 67. These are so arranged that whatever interval exists betweenaandb, exists also betweencandd. If, therefore, the inside of a box cover (or similar article) is measured by the latter, the other end of the instrument will show the exact size to be given to the part A, B,Fig. 66. The convenience of such an arrangement for an infinity of cases will be apparent on an inspection of the figure.

Fig. 67.

The cover of the box must now be put on, the lathe set in motion, and the outside, and also the top of the cover, carefully turned and finished. If the box is to be cylindrical, care should be taken that it is truly so, and that the angle formed by the junction of the top and sides is sharp. If sand-paper is used to finish the work, the edgeswill be rounded and the workmanlike appearance spoiled. If, therefore, the article is made of box or other hard close-grained wood, this finishing-off may be done with a carpenter's chisel held so as to act as a scraper. The turning chisel will answer the same purpose, but it is a pity to spoil the edge, which should be always preserved keen and fit for use. If the box is made of soft wood, scraping will not answer; the turning chisel must then be made use of, held as previously, described. If the cylindrical form is not proposed, the sides of the box must be left thicker, and after the cover is fitted on the outside may be moulded by the gouge and chisel, and tools like 60 and 68 to 70, to any desired pattern. The only thing remaining to be done is to cut off the box with the parting tool, the same precautions being observed which we spoke of in separating the cover. If there should be any defect in the bottom after the work is detached, the box must be placed in a cup chuck turned to receive it, and the above defects removed.

Figs. 69, 70, 71, 72, 73.

In hollowing out a piece of solid ivory or similar costly material, it would be exceedingly wasteful if the central part were removed as in a common box, by being reduced to small chips. It is possible to remove the whole interior in a solid block, and with exceedingly trifling loss of material. This is effected by means of side parting tools, 71, 72, 73. A common parting tool is first used and a groove cut therewith in the face of the block to be turned.Fig. 74represents this face 75; the section after the groove is cut the depth of the box required. The shaded part in the centre represents the part to be removed. The smallest parting tool,Fig. 73, is now introduced, the back of the tool being laid across the rest, so that the crook takes aperpendicular position, A, B,Fig. 74. When at the bottom of the groove the hook is turned to the left, so that it may cut a groove underneath the block, until stopped by its shank. It is then withdrawn andFig. 72, and subsequentlyFig. 71introduced, and used in a similar way. InFig. 76the black line shows the tool in position, with the under cutting done by it. The sizes are thus increased until the last tool removes the block entire.[3]

[3]The side parting tools are sometimes inserted in the centre of the work, a hole being made for their introduction, they then cut from within outwards. In this case, however, instead of a solid piece a thick ring of the material is detached.

Figs. 74, 75, 76, 77.

We now propose to describe the method of turning a round ball or globe, and, to make the work more interesting, it shall contain a small box. The first thing necessary is to decide upon the diameter. In the present case let it be an inch and a half. Turn a cylinder of boxwood a little exceeding this, and cut off from it rather morethan an inch and a half in length. The excess is merely to allow for waste. You will thus have a cylinder whose diameter equals its length. Before removing it from the lathe, mark its centre by a groove with a point tool—subdivide the outer spaces with five lines, and from the latter remove the corners of the piece, thus reducing it to the form 77. Test the length and breadth by the callipers and take care that the ends of the cylinder are at right angles to the sides. Now place the piece in the chuck in the position shown in the figure, that is, at right angles to its original position in the lathe. It must be tested as to truth by holding a point tool on the central line E, F. If correctly placed this will only make a dot when the lathe is put in motion. If the piece does not lie evenly the point of the tool will make a small circle—it must then be corrected with a light tap or two, until it runs evenly.

If the inside of the chuck is rubbed with chalk the work will be less liable to slip. The following operation, however, must be conducted very gently and with exceeding care, or a satisfactory result will not be produced. It will be observed that the central line having been marked or cut upon the side of the cylinder is necessarily a circle, and its revolution on its axis forms a sphere.

We have therefore only to cut away the piece truly down to this line to finish whatoughtto be a perfect globe. Bergeron, however, justly remarks that although the theory is correct it is next to impossible to manage the tools with sufficient skill to complete in this way a true sphere. One great cause of this difficulty, is that as the work revolves in its new position the central line is not visible as a line, but simply becomes the boundary of the sphere. This may be in part done away by making aredline or black one (red is the best) instead of a mark with the tool. The work will then appear red as it revolves, and the gouge and chisel must be used to cut away this red part, great care being taken only just to remove what appears coloured. Thus you will in the end have cut the work away so asbarelyto remove the line. Work from the central part outwards, and always with exceeding care, and you will eventually succeed to your satisfaction. It is, nevertheless, a very difficult bit of work to finish even fairly well—mainly on account of the great obscurity of your landmark, the red line. For the more perfect finishing of the above a template of steel may be made likeFig. 78, with which to test the work—its diameter is equal to that of the sphere, and it will serve as a gauge or scraper. It should be made of saw plate if intended for the latter purpose, otherwise sheet brass will answer as well. After the semi-globe has been turned in the first chuck, it will be necessary toturn another to receive the finished part, and for the more perfect formation of the same a semicircular template of the same gauge as the concave one first made may be provided, as the more nicely the ball fits the chuck, the less chance there will be of the work shifting during the turning of the latter half of the sphere.

Fig. 78.

In order to obviate the difficulty of following the diametrical line with the cutting tool, the following contrivance has been suggested to the author by one who has followed lathe work as a profession for many years, and is an adept at the art. The lathe band is to be slightly slackened by partial untwisting (a turn or two will suffice), if of catgut, so that it will carry round the pulley, if desired, but will slip if the hand is placed on the latter. Thus, the tool may be applied, and a light cut taken, and the work instantly stopped for examination without stopping the lathe, as the flywheel continues to revolve all the time. This examination can be repeated, if necessary, every few seconds, by merely placing the hand on the pulley, and in this way the work being carried on little by little, a good result is attainable with comparatively little difficulty.

The best position for the rest during the above operations will be across the face of the work, as in hollowing out boxes, working carefully, little by little, from centre to circumference. Towards the finish a scraper should be used, the common carpenter's chisel being as good a tool as any. Now to proceed with the box. Before removing the finished ball from the chuck, bore it through with toolFig. 59, enlarge withFig. 51, and make the hole conical, unscrew the chuck, with the ball remaining in it, and put on another with a piece of boxwood large enough to make a plug to fit this hole. This plug, when fitted, is to be hollowed out, and converted into a box, likeFig. 79. The latter, when put in place, must fit so neatly that only a light circle shows its position. To conceal it still more completely, a series of circles are to be set at each of the six sides of the ball, as shown atFig. 80. To remove the box, the thumb is placedat the small end, and pressure made. This forms a neat pocket needle-case, and may be made of ivory as a present to your lady-love.

Figs. 79, 80.

There is no practical difficulty likely to be met with in the above after the round ball is itself made, unless it may arise in respect of the conical hole. Let this be turned out as directed, until at the furthest (smallest) end it will just allow a gauge, like the annexed figure, 80A, to pass through it.

Fig. 80A.

Having also gauged the large end of the hole to the desired size, take care to finish the side evenly from one to the other. The gauge may be a disc of tin on a wire, or, still better, a short cylinder of box wood, on a similar handle, as there will be a little difficulty in feeling whether the disc is placed at right angles to the axis of the hole. Unless, however, you desire to work to a pre-arranged exact measurement, the above precautions will scarcely be necessary, inasmuch as the hole is first bored, and the conical plug afterwards fitted to it. The ball may, therefore, be taken from the chuck, each end of the bore measured, and the plug gauged at each end by the callipers, and turned to an exact fit.

In the above account the unmathematical phrase of "six sidesof theball" is used for want of a better; themeaningof the author will, however, be evident.

The ambition of amateurs, especially, is very commonly centered in a desire to cut screws in the lathe, and there is good reason for this, because in the first place there is a difficulty presented which it is pleasant to overcome, and in the next place, a screw is of absolute necessity in the greater number of turned works. There is an apparatus of simple but ingenious construction called a screw box, which is commonly used by carpenters and others who have not attained the skill necessary for chasing screws in the lathe, and which is very convenient even for those who have obtained this power. A sketch of this is given inFig. 81. A, shows the tool complete, B is a view after the top plate has been removed, showing the knife or cutting tool, the latter being delineated alone at D on a large scale, C is a section. To make this tool, which is within the power of any person of average skill, a block of hard wood is first selected, and drilled with a hole corresponding to the proposed size of the screw to be cut. If no tap is at hand of the desired diameter and pitch, this block must be mounted in the lathe, and the thread chased as we shall presently describe. It is absolutely necessary that the block be nicely squared up and level on the face. A small place must then be cut to receive the knife, the edge of which is so constructed as to form part of the thread cut away to make room for it. It isVshaped likeFig. D, and very keenly sharpened. The method used to clamp the knife in position, which is shown inFig. D, permits the cutter to be advanced or withdrawn until its position is accurately determined as above. The top plate of wood is now fitted, and adjusted—the central hole,which is not tapped, but as large as theoutsideof the screw-thread to be cut, forming a continuation of that which is tapped in the lower block. A slotb,Fig. A, forms a passage from the knife, to allow of the escape of the chips. The piece to be cut into a screw should be shaped likeFig. 82. The parta, will be left plain,b, is that on which the thread is to be cut, and must be truly cylindrical, and of such size as to just enter the hole in the top plate of the screw box. The partcmust pass through the threaded part of the screw box, not loosely, but just so as not to damage the threads in the least. The lower part of the central division is sloped off as seen in the sketch. To cut the thread the screw blank is fixed in the vice by its head, which, after being turned, should be planed off at each side. The screw box being then placed upon it the lowest and smallest part of the blank should just project, as inFig. 83. This part is intended to insure the perpendicular position of the blank in respect of the screw box. The latter is then turned from left to right until the screw is cut, which ought to come from the tool clean and smooth. Box wood is especially suitable for this purpose. Thismethod is, of course, wholly inapplicable to anything but wooden screw bolts, and for practice the tyro may set to work and make three or four of the following screw clamps, which are useful to hold pieces of wood that have been glued together. The tap for screwing nuts and the jaws of these clamps, is similar to that used for metal, but, the teeth, or cutting threads, are deeper and more pointed. The jaws of the clamps shown inFigs. 84, 85, are usually made of beech, which will take a very fair thread; or of birch, which is still better; and the screws may be made of the same material, box being too costly and scarce for such purposes. In making these clamps, there is to be no thread cut on that part at which the handle of the screws project, nor is there any thread on this part of the bolts, which pass through a smooth hole in one jaw and lay hold of the other only. Other forms will suggest themselves, but the two given will be found serviceable patterns.

Fig. 81.

Figs. 82, 83, 84, 85.

The above method of cutting screws is not of anything like universal application, nor specially the work of the turner; we shall now, therefore, speak of cutting them by the chasing tool in the lathe. To effect this with certainty requires much care and long practice, and at first the attempt should never be made on a box or ornamental piece of work, otherwise finished, but on a plain cylindrical bolt, such as those of the clamps just described. For the inside, or female screw, the making of chucks will afford endless practice, and a failure in either of these will be of little importance. Thescrew tool for male and female threads is represented inFigs. 86 and 87. It is of steel, and as each tooth inclines in the direction and with the pitch of the screw, it cannot be made with a file, but is cut by being held against a revolving tap (or screw hob, which is of similar form.) There is certainly a defect in the above common form of screw chaser, and a slight modification, to be presently described, will be found easier to use, and, in many respects, easier to make. To cut a thread with the chasing tool, the top of the rest must be quite level and smooth, so that the tool may readily slip along it. Suppose an outside thread to be required on a cylinder of box or other close grained wood. The rest being firmly fixed so that the upper edge is level with the axis of the piece, and about half an inch from it, asFig. 88, the tool is advanced to touch the work, not in a line with the axis, but so as to bring the part,a, in contact with it first, and the moment the tool is felt to run along, which it will do as soon as this part of it indents the wood, the handle is raised a little so that the points of the teeth come into work. The tool in fact must describe the segment of a circle, as shown by the dotted line. If this is done cleverly the tool will not hitch, nor produce a drunken thread, but the latter will come out clean and sharp. It is, nevertheless, necessary to practise till the knack of thus chasing a threadis attained, and, considering that once acquired, the necessity of traversing mandrel or other expensive (and yet more or less defective) apparatus, no longer exists, it is evident that the young aspirant should spare neither time nor patience in becoming an adept in this useful art.

Figs. 86, 87, 88.

One great difficulty in cutting the screw-threads to the top of a box, or the inside of its cover, arises from the necessity for stopping short, and removing the tool instantly as soon as it touches the shoulder, or the top of the cover. The latter should be made rather deeper than is necessary, so that there may be a turn or two of screw to spare. This will give more room for the play and removal of the inside chasing tool.

The ordinary form of the latter is as shown inFig. 87, the part under the plane upper surface (a) being either slightly hollowed or flat, generally the former, from having been cut by a revolving cylindrical hub.

Now, although this form may be suitable for outside screw tools, which have to work on cylindrical pieces, it does not appear equally suitable for inside tools, which are to act on concave work. The writer of this article has experimented upon many patterns of chasing tool, and has found it perfectly easy to chase an inside thread with an ordinary grooved tap, which seldom makes a false cut, or crosses the threads. From this the idea naturally arose of a convex edged tool for inside chasing, and a concave one for outside work, asFig. 89.

Figs. 89, 90.

Practically, however, the convex edge,Fig. 90, will answer satisfactorily for an outside cylinder. In order to obtain an efficient cutting edge from this form, the rounding must be very slight. The out or inside tool is used with a rolling movement on the rest as it advances. If for hard wood, a notch cut across in the linee, f,Fig. 90, with a saw-file, will, by making a partially cutting edge on the convex part, cause the tool to enter more readily at starting. Tools like the above must be necessarily the work of the amateur himself. The regular makers have a great objection to make any tool or machine out of the ordinary routine. Hence the same patterns areconstantly reproduced year after year, until some one connected with the manufacture invents an improved form, or some one else of mechanical genius, and possessed of means, registers a new design. Amateurs are apt to cavil at this system, and in some cases it no doubt interferes with, and checks improvement in tools and machines, but the evil is almost a matter of necessity. Tools are made not singly, one of each pattern, but so many score or hundreds of one form are forged out, and handed over to the grinding and finishing department, and it would sadly interfere with the system and order of the manufacturer to make a single tool or two for individual purchasers of different pattern to those ordinarily used. If a design is sent in by a retail dealer who can order a hundred or so at his own risk, the above objection is obviated, and the new pattern of tool or machine is at once introduced. If, however, any new design by an amateur, being submitted to such men as Holtzapffel, Buck, Fenn, or Whitworth, appears tothemgood and saleable, they will not only not object to introduce it, but may possibly give a premium to the inventor. We have thought it necessary to make these remarks to obviate the possible disappointment of amateurs in this respect. It is but natural to suppose that some ingenious device must have long since arisen to obviate the difficulty of thus cutting screws by hand. Every turner finds the difficulty, and few perhaps have failed to try some plan or other to counteract it. There are two methods whereby this can be done: one by causing the tool to traverse at a given rate according to the proposed pitch of the screw, the other by giving similar movement to the mandrel, while the tool remains still. For such work as screwing the lids of boxes, the traversing mandrel is commonly used, but for cutting long screws in metal, the tool is fixed in a slide rest, and the latter is made to traverse, if necessary, the whole length of the lathe bed, by means of a guide screw driven by suitable gearing put in motion by the mandrel itself, the speed being adjusted by a series of cog wheels which can be interchanged to cause various rates of motion. The latter method belongs to machine lathes, and will be treated of hereafter in this series.

The author has great pleasure in here introducing a device, not exactly forcutting, but for starting the threads of a single, or double screw, or, indeed, a quadruple one. It is the invention of a gentleman whosenom de plumeis East Norfolk Amateur, and was by him kindly communicated to theEnglish Mechanic, of June 21, 1867. The description is here given in his own words:—

"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 andmade by myself, and I believe will prove as useful to others as it has to me: thedrawingwill almost explain the tool. The cutter, A, is9/16ths 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.

Figs. A, B, C, D.

The above simple apparatus will, it is believed, be of great service to those who find difficulty in hand chasing of screws; it is, however, necessary to speak of other methods, and especially of that so universally used by the turners of "Tunbridge ware"—viz., the traversing mandrel. This is represented inFig. 91. A is the poppet; B, the mandrel, no longer conical at the place where it traverses the collar, but cylindrical, and passing throughtwocylindrical collars. It is prevented from advancing towards the left in its bearings by the shoulder, K, and in the other direction by a plaincylindrical collar, or ferrule, C, which slips over the end, and is secured by a nut, D. The whole is thus ready for use, as an ordinary mandrel. To cause it to traverse from left to right, as it revolves, the nut and collar, C, D, are removed, and a ferrule, or guide, F, which has a screw of the desired pitch cut on its edge, is slipped on the mandrel over a short feather against a shoulder, where it is retained by a nut or pin. There are several such guide-ferrules supplied with the mandrel of different pitches of screws. The nut G is then removed and the piece E, which is of brass or gun metal ⅜ or ½ inch in thickness, with similar screw threads in each of the hollows is attached. This guide is slipped on at H, and secured by replacing the nut. The pin, which carries the guide, is frequently made to slide up and down the face of the poppet, by the action of a screw, M, working through a brass piece attached to that which carries the pin. It is thus readily lowered out of gear or drawn up again to touch the screw ferule. This is better than a pin screwed to the head of the poppet, and is always adopted in the best lathes. In the frontispiece is aphotographof 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, asFig. 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 requireperfectfitting, 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 anequallongitudinal 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 lengthcanbe cut by machinery, as every imperfection in the latter is communicated tothe work done by it. If the amateur, therefore, requires a screw for a slide rest, eccentric chuck, microscope, or other delicate piece of machinery, or philosophical instrument, he had better get it cut by some practical mechanician, in the possession of the necessary apparatus.

[5]There should be a collar or shoulder to this mandrel, the same as at K in the other figure.

Figs. 91, 92.

In the "Manual Bergeron" is an ingenious contrivance, by an amateur, which is worth notice, although unsuited for any work where extreme accuracy of pitch is required in the screw. The following,Fig. 93, is a description:—The mandrel is made to traverse in its bearings, as before detailed in this series, but instead of its motion being governed by guide hubs, it is dependent on the action of a pair of differential pulleys, B. A bent lever, C, is pivoted at E to the face of the poppet, having a bit of hardened steel fitted to work in a semicircular groove in the mandrel itself, and so arranged that on raising the tail or long arm of the lever the mandrel is thrust forward from left to right, while a reverse action of this lever causes a similar movement in the opposite direction. The movement of the lever is thus regulated:—At the extreme end of the long arm is a pulley and hook, as shown in the drawing; the double, or differential pulley, is fixed to the end of the mandrel, and from the smaller part depends a cord which passes thence through the pulley on the lever, and is wound round the larger one on the mandrel when its end is secured. On the hook is hung a weight. It will be evident, on an inspection of the drawing, that on putting the lathe in motion the cord on the differential pulley will coil itself round the largest part of the same, and will draw up the end of the lever with a speed proportionate to the difference of diameter between the larger and smaller parts of the double pulley. The short end of the lever will at the same time with similar proportionate motion move the mandrel and work, and cause the fixed tool to cut a spiral or screw thread on the latter—a good deal of ingenuity is displayed in the above, and it has the advantage of being easily fitted up, but it is evident that some alterations and additions would be required to adapt it to any other use but that specified. A contrivance similar toFig. 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 akind 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 inFig. 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 handscrew 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.

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