Part 5

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 in Fig. 67. These are so arranged that whatever interval exists between _a_ and _b_, exists also between _c_ and _d_. 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.

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 edges will 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.

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. 74 represents 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 a perpendicular 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 and Fig. 72, and subsequently Fig. 71 introduced, and used in a similar way. In Fig. 76 the 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.

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 more than 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 what _ought_ to 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 a _red_ line 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 as _barely_ to 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 like Fig. 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 to turn 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.

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 tool Fig. 59, enlarge with Fig. 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, like Fig. 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 at Fig. 80. To remove the box, the thumb is placed at 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.

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.

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 sides_ of the _ball_" is used for want of a better; the _meaning_ of the author will, however, be evident.

CUTTING SCREWS.

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 in Fig. 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 is V shaped like Fig. D, and very keenly sharpened. The method used to clamp the knife in position, which is shown in Fig. 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 the _outside_ of the screw-thread to be cut, forming a continuation of that which is tapped in the lower block. A slot _b_, 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 like Fig. 82. The part _a_, 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 part _c_ must 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 in Fig. 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. This method 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 in Figs. 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.

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. The screw tool for male and female threads is represented in Figs. 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, as Fig. 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 thread is 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.

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 in Fig. 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, as Fig. 89.

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 line _e, 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 are constantly 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 to _them_ good 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 for _cutting_, but for starting the threads of a single, or double screw, or, indeed, a quadruple one. It is the invention of a gentleman whose _nom de plume_ is East Norfolk Amateur, and was by him kindly communicated to the _English Mechanic_, of June 21, 1867. The description is here given in his own words:--