Turning Spheres.

Fig. 193.

Akin to the circular saw are the various revolving cutters used either for the purpose of ornamentation or for grinding, such as circular files for flat surfaces, in which the teeth are cut upon the face or tool-cutters of particular sections for cutting the teeth of wheels, in the manner already described, to which may perhaps be added milling and embossing tools, although the action of the latter is rather that of a revolving die by which the work is stamped or indented with the pattern formed upon the edge of the cutter. The small grindstones and emery laps belong also to this section, as their action results from the abrasion of the material by means of the combined cutting power of innumerable small points or miniature teeth formed by the particles of emery or other material attached to the surface of the laps.

Most of the steel cutters may be made by the amateur, the metal being turned to the required shape and the teeth cut by small files or punches while the material is in a soft state. The little discs are then hardened and mounted, by a central hole previously made, on suitable spindles, the latter being either attached at one end to the mandrel as arbor chucks, or centred at both ends and driven like miniature circular saws. The ornamental cutters for embossing,Fig. 193B, A and B, are turned to the form of short cylinders, and the patterns cut by punches. These and the milling tools are mounted alike,Fig. 194. The rest is placed a short distance from the work, and the tool revolves against it. Some pressure is necessary to imprint the design, and this is easily obtained if the cutterwheel is placed so as to attack the work below the axis; the rest then becomes a fulcrum, and the shank and handle of the tool acting as the long arm of a lever supply the required force with little exertion on the part of the operator. In this way the milling is done on the edges of screw heads, and the embossed patterns on soft wood boxes. It is not easy to understand how the patterns in these cases are produced clearly without one part cutting into and effacing another, unless the size of the work is exactly a multiple of that of the tool. The error is plain if the work is stopped exactly at the end of the first turn, but in successive revolutions this error becomes gradually obliterated, and the pattern is eventually impressed clear and well defined. The same shank is arranged for different patterns of wheel-cutters, as the pin which forms the central axis is readily withdrawn and is made to suit the holes in several sets of discs.

Fig. 193B, 194.

Fig. 195.

By the aid of the above simple tool a neat finish is readily given to many small works in wood and metal. A modification of the beading tools is here shown very similar to the screwing guide already given, but made with figured instead of sharp-cutting edge. This was communicated to theEnglish Mechanic, Nov. 2, 1866.E, 195, is the guide which is placed on the handle A, B, C, D, and fixed by screw F. The marki, on the guide, is placed against the rim, A, B, which is graduated and numbered. Each figure, as it is brought up and placed oppositei, will cut a different pattern when the guide is fixed. The tool must be held very firmly on the rest (the bottom of the guide G, H, being flat, is carried on the rest), the tool is advanced to the wood. The tool must be worked very steadily; but with a little practice, any amateur will soon use it perfectly, and produce many very pretty patterns. It is evident that provision is here made for placing the cutter at different angles to the work, by which means the circles of patterns may be traced spirally and in other positions varying from the ordinary one at right angles to the axis of the work.

The laps alluded to, which are to be mounted on spindles like the circular saw, are composed of wood and metal of determinate forms. First, there is the thin sheet-iron slitting wheel used by lapidaries, and which, when charged with emery, or sand and water, forms the nearest approach to the circular saw. Itis, in fact, the circular saw of the stone worker, the ordinary saw used in their trade being a flat blade without teeth, stretched in a wooden frame and similarlyfed with sharp sand and water in lieu of being made with teeth, the latter being replaced by the grittiness of the material. The circular plate of iron, brass, or lead alloyed with antimony, mounted on a spindle vertically, is used in a similar manner for grinding flat surfaces with the aid of emery, crocus, oilstone powder, and other substances, and the same is used edgewise for other work, as grinding and polishing tools, the section of the lap being such as will form the article required by reproduction of the revised plan of its own edge. Thus a lap running horizontally with a convex edge will produce the concave form required in beading tools, the latter, however, are more conveniently ground on brass cones mounted like the arbor chucks used for turning washers, rings, &c. The face of the tool or flat side is held towards the small end of the cone, and the latter is armed with flour emery. SeeFig. 196.

Fig. 196.

Before describing the eccentric and other compound chucks, a few examples will be given of the method of turning some of those forms in which the circle does not appear, where, in short, the boundaries of the figures are straight lines. It seems at first sight impossible to produce in the lathe by any simple means such solids as are formed from squares or triangles, of which the cube or die is the most common and most generally understood. This can, however, be accomplished, and a number of mathematical problems may be clearly demonstrated to the eye by such works skilfully made in the lathe. In this kind of work it is absolutely necessary to strive after perfection. In short, as Bergeron rightly says, the work imperfectly done is simply worth nothing at all; but when accomplished with exactness, nothing can be more worthy of a place in a cabinet of curiosities. The usual method of turning a cube is by shaping it out of a perfect sphere, but as the latter can hardly be made without a special slide rest made for the purpose, a method of turning the cube by hand alone will be given, the foundation of it being that which the lathe so readily produces, namely, a cylinder. By far the most proper material for the work in question is sound boxwood, and special care is to be taken to keep all angles as sharp as possible, and therefore to cut clean with sharp tools, and to avoid as far as possible the use of sand or glass paper.Fig. 197shows a square described in a circle. The non-mathematical reader may be told to draw through the centre two diameters at right angles to each other, and to complete the square, as in the figure, by joining the extremities. The largest square that is possible to be drawn in the circle is thus described. This square will form one face of the cube, and the diameter of thepiece of wood must be regulated according to the proposed size of the finished work. It is evident that this diameter is equal to the line drawn from corner to corner called the diagonal of the square. Now, in turning a ball or sphere, a cylinder would be turned of the exact length of A, C, because every measurement taken on a diameter of the sphere would be of that length. In the present case, however, the cylinder must be of the same length as the line A, B. Turn, therefore, with great care a cylinder of any desired size, gauging it carefully with the callipers and squaring off the ends truly. On one end, in which the centre point just remains visible, draw diameters and construct the figure197. Turn out a chuck to fit it exactly, and let the bottom of this chuck be truly square. Take the precise length of the line A, B, with finely pointed compasses that can be fixed with a screw, and measure off the same, and mark it on the side of the cylinder. When the figure is placed on the chuck, a mark must be traced round it at this point, A, B,Fig. 198,and this must remain to the end. It may be made with a hard pencil or steel scriber, and, though distinct, must be very fine. It is at this line the wood will have to be cut off, but in this operation keep beyond it so as not to erase it. While the piece is in the chuck, rule lines, as E, G, from the points C, D, E, F, 198. Bisect also the length of the cylinder by the line, H, K, also finely drawn. Draw two more diameters, as shown by the dotted lines bisecting the sides of the cylinder, which is now divided round its circumference into eight equal parts. The lines, E, G, &c., can be ruled along the edge of the rest, or, if the latter is untrue, the cylinder may be laid on a plane surface, and a scribing block, or gauge,Fig. 199, may be drawn across it, or, lastly, a small steel square may beused; one part being carefully placed on the lines at the end of the piece in succession, the other part will lie evenly along the side, and will form a ruler by which to work. A division plate on the lathe pulley will facilitate the above measurements, but they can be readily made without it, and once carefully marked, all the guides required for cutting out the cube will be complete, and the work can be proceeded with, with confidence and decision. Proceed, therefore, to cut off the piece at A, B, with a parting tool, but with the precaution already named of not erasing the line by so doing. Now prepare a chuck which will take the piece lengthwise,Fig. 1992, and insert it in that position to the depth of the diameter so as to hold it securely, and the central line will show whether it lies evenly (which is, in fact, the use of this line). If even, a point-tool held on this line will form a mere dot upon it; if uneven, it will make a circle as it revolves. Place the rest across the face of the work, and, beginning at the centre, cut carefully towards the outside until you have cut away the wood as far as the line, A, B. You will thus complete one face of the cube, and an inspection of the end of the piece, shown black in the sketch to show the quantity removed, will prove that you thus produce a right line, C, D. Take out the work, and reverse it, and operate similarly on the opposite face; but in every case do not quite obliterate the lines first marked as guides. You have now a piece shaped likeFig. 200, A and B, and must make a smaller chuck to hold it; you have then to cut away in a similar manner the remaining parts of the cylindrical portion, and the cube will be complete. To finish the sides more neatly, lay upon the table the finest sand paper, and tack it at the corners, and with gentle movements work down precisely to the guide lines. This requires extreme care, for if once the piece is but in the slightest degree tilted up, an angle will lose its sharpness, and the beauty of the work will be marred. Hence we recommend to cut with sharp tools, instead of trusting to the finishing process. To cut, however,exactlyto the line is very delicate work, and to the less practised hand the useof the sand paper on the faces in succession may be thenecessaryexpedient. The main secret of sharp edges in works of wood and metal is to finish with hard substances, as emery stick or glass paper glued to a piece of wood, or the same nailed on the bench, and to try always to work on the centre, leaving the edges or angles to take care of themselves. When the reader has made a cube, as above, that will bear delicate measurement, he will be more than usually gratified, and will be qualified for still more difficult work. A chuck figured by Bergeron,Fig. 201, is very convenient, as it holds the work truly central; the jaws work simultaneously by a right and left-handed screw as in the die chuck. It is, however, perfectly easy to do good work without it, but the chucks should be carefully made, turned very flat at the bottom, with side truly perpendicular. A little extra care bestowed upon chucks will save many disappointments, and conduce to good work. The formation of a four-sided solid, consisting of triangles solely, in the lathe is a work of difficulty, owing to the impossibility of fixing the work satisfactorily in an ordinary chuck. The natural way to form such a solid is to turn a cone A, B,Fig. 202, and on its base to mark a triangle of the required size. It is then necessary to place the cone in a chuck, so that the ends of one of the lines thus marked and the apex of the cone are precisely level with the surface of the chuck, as shown inFig. 203. But it is evident that adequate support is not thus obtainable. The apex of the cone cannot, in point of fact, be inside the chuck at all, as it is necessary to cut clean to its extremity, and even the base of the cone is imperfectly held at two points. Hence it becomes necessary to make use of "turner's cement," and to imbed the work fairly in it, while both are warm, to such a depth as will hold it securely and still allow the guide lines to be seen. The latter should be carried from the three angles of the triangle marked on the base to the apex. On the whole, this is the easiest method of fixing such work in the lathe; and if the piece is itself warmed before being placed, there will be time to adjust it precisely before the cement is cold. To do this, place the rest parallel to the lathe bed, hold a pointed tool steadily upon it, and note whether, as the work slowly revolves, the three points in question, viz., the two angles of the triangle and thetop of the cone, are in one plane. When they are so placed, the rest is turned to face the work, and the material is carefully cut away till the gauge lines are just reached.[14]A pyramidical solid with a square base may be similarly turned. The following is the method of preparing the above turner's cement:—Burgundy pitch, 2 lb.; yellow wax, 2 oz. Melt together in a pipkin, and stir in 2 lb. of Spanish white. When the whole is well mixed, pour it out on a marble slab and roll it into sticks. Fine brickdust, whiteing, or any similar substance finely pulverised, will answer equally well to add to the pitch and beeswax. This cement is very useful, as it will hold the work firm enough to turn carefully, and nevertheless a slight blow will loosen it. To clean it off, warm or dip the work in hot water and wipe quickly with a piece of cloth. The above is from the "Handbook of Turning," the author of which has copied from Bergeron both the recipes given in his work; the one here described is stated to be specially serviceable in cold weather. It is perhaps rather less brittle than the first of Bergeron's, and for this cause is the best for general use. Holtzapffel sells this at one shilling the stick, which is of convenient size and generally of excellent quality. Bergeron gives a method of turning pilasters or balustrades, which is of great ingenuity, and applicable to work of various sections. The rectangular and triangular sections illustrated are not, indeed, strictly rectilineal figures, as the sides of the balustrades thus formed are not flat but rounded surfaces; they are, however, sufficiently curious, and when well turned are interesting specimens of lathe work. Bergeron's description relates to the pole lathe, or to work mounted between two centres with a pulley cut on the work itself to receive the lathe cord. The ordinary method of mounting on a foot-lathe will, of course, be much better and the whole operation of more easy performance. Let it be required to turn a moulded pillar or balustrade of the sectionFig. 204, viz., a triangle with slightly curved sides. A piece of wood of the requisite length is planed up accurately to a triangular form, or, as it generally happens that a set of such pilasters are required, a number of such pieces are prepared which must be accurately planed to the same size, for which purpose a gauge or template,Fig. 205, should be made use of. Six or eight of these, if not too large, may be operated upon at the same time; but as six pieces of two inches across each face require a cylinder of about eight inches diameter, the number must depend on height of centres. The larger the cylinder, however (which, in fact, forms a chuck), the more nearly will the sides of the finished work approach to plane surfaces, as they will form arcs of a larger circle. Let a cylinder, then, be turned of sound wood with a reduced part at one end so as to form a shoulder. Divide the circumference into twice as manyequal parts as there are pieces to be turned, the divisions being equal to one of the sides of these pieces as measured in a cross section. These divisions are to mark the positions of the grooves or channels in which the triangular strips are destined to lie. Half the circumference will be so cut out, the alternate divisions being left. Thus, to turn four pieces, each two inches wide, a cylinder of sixteen inches will be required, affording four grooves two inches wide, and four intermediate pieces forming the partitions between the grooves. The latter must, by means of the saw and chisel or other tools, be made to receive the strips exactly, and the ends of the latter being carefully squared off, are to be made to rest against the plate C,Fig. 206, which is cut out and screwed against the shoulder, after the above-mentioned grooves have been cut. The whole are then secured by two rings, with screws. Probably the stoutest india-rubber rings now made would answer as well as the iron clamps in Bergeron's description. Lay the strips in their places, but as they are flat they will not form part of the cylindrical surface, but will lie lower, asFig. 207, or higher,Fig. 208; the latter is the best, and the pieces may, for this cause, be cut out a trifle deeper than ultimately required and they may be planed down a little to remove the angles and assimilate them to the cylindrical surface. The whole may then be turned together so as to form a plain cylinder, the clamping rings,Fig. 209, being shifted when requisite. The whole is now to be formed into a balustrade, but as the proportions of the mouldings of such a large cylinder would not suit the small pieces, the hollows must be less deep, and the raised parts less prominent than they would ordinarily be made. The clamps are now to be loosened, and the pieces reinserted with another face upwards, the flange or plate against which their ends rest forming a gauge or stop to ensure their position, without which precaution the mouldings would not eventually meet at the angles. In cutting a fresh side the utmost care is requisite, for if the work on the original cylinder is cut by the tool, it will be impossible to restore it, and the work will be spoiled. In replacing the strips, let the finished part lie below, so as to come first in contact with the tool, by which the angle will be clean. Extra care is for this purpose required in cutting the last side. It appears to the writer that another precaution should be taken which Bergeron omits, namely, to arrange the mouldings so that certain parts are left of the original size of the wood, in order to retain a certain number of points of contact with the sides of the grooves, so that the strips shall not fall deeper into them than at first. The extremities of the strips should certainly not be left smaller than the central portion, or the pieces will rock on the latter while in process of being turned. The two ends, therefore, should be allowed to retain their original triangular form, forming base and capital of the pillar, or the pattern may be so planned that, after the extremities have been left as supports they may be cut off when the work is complete. Care must be taken, in working as above, to have the cylinders so large above the estimated size that the inner apices of the triangles do not nearly meet at the centre, else the whole chuck would be very weak and split into triangular strips. The lathe called a spoke-turning lathe would accomplish this kind of work in a far more easy and speedy manner. The balustrade would have to be made by hand as a pattern, and cast in metal, and any number could be produced precisely similar. The lathe in question is on the following principle:—The frame carrying the tool (a set of revolving gauges) is made to oscillate backwards and forwards to and from the piece to be operated on. This is accomplished by its having attached to it a roller or rubber working against the cast-iron pattern placed parallel to the work, and below or one side of it. The rubber and frame are kept against the pattern by a strong steel spring. The cutters also travel in a direction parallel to the axis of the piece. Hence any elevated part of the pattern causes the tool to recede from the work in a corresponding degree, and a hollow allows a nearer approach of the tool. Thus, as the tool is carried by a screw slowly from end to end of the work, it is made to advance and recede in exact correspondence with the form of the pattern. An immense deal of work is done in this way, such as balustrades, spokes of wheels, the long handles of the American felling axes, and similar irregular forms.

[14]With the universal cutting frame this kind of work is much more readily accomplished.

Figs. 197, 198.

Figs. 1992, 199.

Figs. 200, 201.

Figs. 202, 203, 205.

Figs. 206, 207, 208, 209.

Theprinciple, indeed, is not new, as the rose engine is a similar tool, and, so long back as Bergeron's time, pattern plates were used giving any desired motion, endlong or otherwise, to the tool, or, which in effect is the same, to the work to be operated on.

We must now recur to the sphere of which we have already spoken. The method previously given for producing it is not sufficiently accurate, although a very close approximation can thus be made to the perfect figure. It is probably impossible without special apparatus, rendering the tool independent of the hand, to turn out an absolutely correct sphere—indeed, it is a sufficiently delicate operation, even with the following or similar apparatus. For ordinary purposes, indeed, where the object is simply to produce a croquet ball, a spherical box, or a globe to be afterwards covered with paper, or any such work, the plan already given will generally suffice, and, indeed, is very extensively used. Some practised workmen, too, will, without even the aid of ruled lines, turn out spheres of average excellence by the eye alone, aided by a template. When, however, it is proposed to hollow out a sphere so as to leave a mere shell of ⅛ in. or less, and perhaps include a number of such shells one within the other, and a star in the centre of all, it evidently becomes necessary to work with greater accuracy, and still more so with respect to billiard balls, in which even the slight variation caused by increased temperature will seriously affect the result of the most skilful play, and cause the very best players to fail. The principle of the spherical rest is displayed by the diagram,Fig. 210.

Fig. 210.

Fig. 211.

A is the chuck carrying the ball to be turned, of which C is the centre. In a right line with the latter, and below it, is a pin fixed to a block between the bearers of the lathe, and on this the arm, D, turns. The latter carries a tool-holder in which a pointed tool, E, is fixed. The point of this tool will evidently move in a circle, when the arm is moved by means of the handle, D; and, as the centre of the circle is exactly under that of the proposed sphere, the latter will be correctly shaped when the lathe is put in motion.Fig. 211gives another view of the tool-holder. It is essential that the point of the tool should be in a line with the centre of the lathe mandrel, so that it shall act on a diametrical plane as it is carried round the work. Such is the principle upon which a practically useful tool for turning spheres has to be arranged.

The faults in the above simple machine are many. In the first place no provision is here made for the advance of the tool towards the work. In the second place the requisite firmness and stability cannot be obtained by merely causing the bar to revolve upon a centre-pin; and thirdly, as the tool post is fixed to the horizontal bar, the diameter of the ball must be limited. In point of fact, therefore, the above arrangement would not answer, and it is only described in order to illustrate the principle of all inventions for the production of spheres in the lathe. To give steadiness of action the pin forming the centre of motion is connected with a circular metal plate truly turned, upon which a second similar plate works, and to the latter is attached the tool-holding apparatus. It is difficult to make choice of a circular or spherical rest so as to give it precedence, since most of the patterns ordinarily made are good. To obtain the requisite movement is, indeed, by no means a matter of difficulty; and one or two adjustments in respect of the height and radius of the tool beingprovided, a very simple apparatus will answer the purpose. To commence with Bergeron's, which, though venerable, is by no means inefficient. This is represented inFigs. 212 and 213. A, B, is the base, the tenon, B, accurately fitted to slide between the bearers of the lathe, the whole being held down as usual by the bolt and nut,c. The top part of the base is surmounted by the accurately faced plate214, on which a side sectional view is given inFig. 215. This is fastened to the base plate (which, in Bergeron's description, is of wood) by four countersunk screws. It is turned with a recess, so that the outer part stands up in the form of a rim, and from its centre rises a conical pin,b, the upper part of which is first octagonal, and then rounded and tapped. It is this strong pin which forms the centre of motion, and it must stand with its axial line precisely in the centre of the lathe bed, so that if the plate were slipped close to the poppet head this line would bisect the nose of the mandrel. This is essential in all patterns of spherical slide rest. Upon the lower circular plate rests that represented inFig. 216, A and B, the latter being the sectional representation. This plate is drilled in the lathe with a central hole, the lower part conical to fit the pin in the base plate, the upper part countersunk as in the figure, to receive the octagonal part of the pin and the nut. The projectionsa, andb, in B, represent the projecting rim,a, in theFig. 216, A, and this is made to fit very nicely within the rim of the lower plate, while the adjacent part,c, rests upon the rim itself. The accuracy of these bearing surfaces is of the utmost importance, It is evident thatthis arrangement is calculated to give great stability during the revolution of the upper part of the rest, which is fixed securely to the plate last named. This plate has a hollowed edge cut with a set of fine teeth to be acted on by the tangent, screw D,Figs. 212, 213,and shown inFig. 217on a larger scale. The bearings of this screw are attached to the base plate, and the screw is prevented from moving endwise by collars as usual.

Figs. 212, 213.

Figs. 214, 215, 216, 217.

Figs. 218, 219, 220.

On the upper surface of the top plate are fixed parallel bars chamfered beneath like those of a slide rest, and between these the tool holder slides, the advance of the latter being effected by a screw precisely as in the slide rest already described. In the plan of this instrument, as seen from above,Fig. 218, A, B, are the bars. The tool is seen in position at C, the tangent at D. A scale is attached to the sliding part of the tool holder for determining the size of the sphere. The tool is again seen in position in219, and detached in220. In Bergeron's description of the above spherical slide rest the method of using the apparatus is thus described:—"Commence by placing in a chuck a cylinder of some sound wood, and reduce it to a convenient diameter, which should be a little greater than that of the proposed ball. With the gouge work down this, and give it roughly the form of a ball attached to a cylindrical base." This base serves to sustain the ball during the operation, and the form of an inclined plane is to be given to it where it is attached to the ball, as seen in the drawing, to facilitate the passage of the tool. After this preliminary work, place the instrument on the lathe bed, and cause the tool holder to advance by means of a screw (the one attached to the lower slide, not the tangent screw) until, reckoning from 0in., the starting point, the index attached to the sliding part has travelled over the graduated divisions of the scale, so as todenote the size of the ball in its present rough state. Then slide the instrument along the lathe bed, until the tool, accurately adjusted as to height, just touches the ball at the quarter circle. This will be better understood by the diagramFig. 221, in which A, B, are the lathe bearers; C, the tool in position; D, the ball; E, the chuck or the base of the cylinder. Having previously determined the size of the finished ball, the work may now be carefully begun by clamping the rest underneath the bed, and making use of the tangent screw. Little by little the work is to be reduced, taking care not to cut too near the base on which the ball is yet carried. This base is to be cut away little by little as the tool comes round, and at the last cut the ball will drop off finished, and it is not to be further touched with sand paper or other material. Hence, as it approaches the finish, the tool must be delicately and steadily made to traverse, so as to leave a finished surface as it advances. Bergeron adds certain precautions as follows:—"If the material is very rough from the gouge, so that at any point the tool is likely to meet with such resistance as would endanger the work, such part may be pared down again by using the machine as a common rest for gouge or chisel; for the apparatus once arranged should not be altered nor the fixed tool shifted until the work is done. The tool throughout is to be advanced very gently forward at each turn by means of the screw, which causes the parallel movement, and the tool is to be accurately adjusted so as to be exactly in a line with the centre of the mandrel; it is also to be very keenly sharpened, and even polished." The first impression given by an inspection of the above figures is that the ball would be liable to drop off before it could be fairly severed by the tool. The writer determined to test this objection personally. He selected a piece of sycamore, which is very fit for the purpose, and useful as a kind of medium between hard and soft woods. A ball was turned by hand from this material having a diameter of about two inches. The neck by which it was held was retained of the size of a cedar pencil during the final shaping of the rest of the ball. It was of course not thus reduced until the ball was nearly spherical. Gradually the neck was cut away until it was perhaps as small as the lead of a pencil, yet still the ball retained its position, and the final stroke cut it off truly and tolerably cleanly,E, 212 and 213. It is really astonishing how small a portion of sound wood will retain a ball so turned, but the lathe should not be allowed to stop, else the tendency to hangdown or sag would overcome the sustaining power of the fibres. Bergeron states, indeed, plainly, that the process answers satisfactorily, and as a man of large experience his opinion is certainly reliable. Nevertheless, since sand papering or after process is to be eschewed, it does appear to the writer that the final cut would leave a minute portion untouched requiring to be afterwards removed. Not having one of the rests in question the writer's opinion is to be takenquantum valeat. The spherical rest more commonly used is that represented in the "Handbook of Turning," since it is necessary not only that the machine should be efficient for turning a sphere, but likewise applicable to the ornamentation of the same by the revolving cutter and other apparatus used in such processes. This and some other forms will presently be described, and also an ingenious adaptation of the ordinary slide rest by means of guide pieces or templets to work of this character. A very good form of chuck for holding spheres during the operation of hollowing them out or forming stars or cubes within them will, however, be first introduced here as described by Bergeron. It will be at once seen how simply and efficiently a ball can be thus held during such processes.

Fig. 221.

Fig. 222represents a chuck for holding balls, A being a sectional view, B an elevation. In the first,b, b, represents the body of the chuck, made as usual to screw on the mandrel. Ata, the chuck is formed with a shoulder like an ordinary box. This part of the chuck is to be hollowed out to fit the ball on which it is intended to operate. On the side of the chuck atb, is to be cut a screw of medium pitch. Mounting another piece of wood in the lathe a kind of cover,c, c, is now to be made to fit over the body of the chuck like the cover of a box, but hollowed out to the curvature of the ball. A ring of brass or wood of section D being screwed on the inside to the pitch of the male screw on the outside of the chuck will hold the three separate parts of the apparatus firmly together. Let them be thus arranged and finished as one piece on the mandrel. Afterwards drill a holein the centre of the part which forms the cover, and enlarge it so that its diameter shall slightly exceed that of the openings necessary to be made in the ball for the purpose of hollowing out and forming within it stars, or lesser spheres, box, cube, or other design, at pleasure. In this chuck the ball will be held not only centrally but securely. It is of course necessary to have a chuck specially made for each different sized ball, but when it is considered that such things as these are merely turned as curiosities and to prove the capabilities of the workman, it is not probable that more than two or three sizes of chuck will be needed, and the difficulty of making them is not great nor the necessary expenditure of time, either. They should be made entirely of sound boxwood, and so arranged as to the size of the respective parts that when the ball is inserted and the cover placed on, the latter shall not quite reach the shoulder on the base of the chuck. In the spherical rest of more modern times the principle of that already described is almost necessarily retained. It is figured in223. The sole A is formed like that of an ordinary hand rest, so that it can be advanced across the lathe bed, and secured by the nut and screw underneath as usual. From this rises a central circular plate, which need not be more than a quarter of an inch thick, but turned truly flat, that it may be parallel with the surface of the lathe bed. From this rises the conical central pin upon which works the plate B, the edge of which is racked to be moved by the tangent screw C. Across this circular plate is securely fixed the chamfered frame D surmounted by the part E which carries the socket and tool receptacle, the details of which will be entered into when describing the rest for ornamental work.[15]

[15]The drawing shows a band at X encompassing the screw. This is an error, as the whole upper part, including this screw, is made to revolve by means of the tangent screw.

Fig. 222.

Fig. 223.

The method of turning from a pattern acting on the tool has been alluded to. In some cases a similar method is pursued, in which the hand supplies the place of automatic machinery, an instance of this is the application of pattern plates or templets to the small slide rest now to be described, by which not only parallel or spherical work may be done, but the elevations and hollows in moulded work may be followed without difficulty. The pattern plates can be made by the amateur or workman so that not only is no extra cost incurred, but any desired form can be given to the work and as many duplicates made as may be requisite. The simple slide rest itself is represented inFig. 224. The lower iron or steel frame is rectangular, chamfered underneath, and is cast with a projecting part B underneath to the socket of the hand rest. There is, therefore, no sole or saddle required, and the height is adjustable at pleasure to suit 3, 4 or 5-inch centres. C is a plate of brass, cast with oneV-piece similar to D, the second E being removable at pleasure, or bothV-pieces or guide bars may be attached by screws. They are attached by two or three screws passing through oval holes in the plate and tapped into the bar, so that a little play to or fro is allowed, by which they can be adjusted to grasp with more or less friction the iron bevelled frame. They are kept up to their places by a pair of large headed screws tapped into the edge of the brass plate and markede, e, in the drawing of the rest. On the top of the brass plate is fixed in a likemanner another pair of chamfered bars similar to those of a slide rest for metal, but in the ornamental turning slide rests, where lightness and accuracy are more needed than strength, the parts are proportionally smaller. The frame, for example, may be six inches or six and a half inches in length by two in width, the brass plate two inches square, or perhaps two and a half by two, the longest measurement in the direction of the upper guide bars, between which the tool receptacle will slide. This will be quite large enough for a five-inch lathe, although the measurement may be more or less if desired. If taken as above, the iron frame may be half an inch deep and the face of each side of similar size, leaving one inch between, in which the screw will lie.224 Bshows the under side of the frame with a cross piece to which the part, B 224, is attached. The guide bars for the tool receptacle may be similarly small and light. If the plate is ¼ in. thick and two in width the bars may be ⅜ in. stuff before being bevelled, the attaching screws can then be ⅓ in. or3/16in. in the shank, the heads being large and flat, and not countersunk. It is probable that many amateurs would like to attempt such a rest themselves, hence we have given the above details. We must however, warn them that as the above is for ornamental turning, where accuracy is of the utmost importance, great care must be exercised in the work, and the various parts must be fitted to the utmost perfection. The upper face of the lower frame must be quite level, and the sides quite parallel, and the upper or cross slide must be fitted precisely at right angles to it. Above all, the screw by which the upper part is advanced in either direction requires great precision. It should be made with a fine thread deeply cut, and must fit its nut under the brass plate, D, without shake. The nut itself should be long, and must be carefully bored and tapped; it should be also sawn through its length underneath, to give it a spring, so that it may grasp the screw tightly yet easily, and this will also compensate for wear. This will be understood when it is stated that the milled head by which the screw can be turned is graduated round its circumference, as is also the face of the bed or lower frame of the rest. Hence to give the head a turn, or half, or a quarter turn, must draw the sliding plate exactly the same distance to or fro, at whatever part of the frame it may be at the time. This necessitates all the threads being precisely alike. [We say precisely with a certain mental reservation, for, strictly speaking, perfection is hardly possible, and the skill and science brought to bear upon screw cutting when perfect work has been necessary, as for astronomical instruments, would hardly be credited, so extremely difficult is this part of the mechanical art.] It would be better for the amateur to get the screw and nut cut by Holtzapffel, Munro, or other first-class maker, who has the requisite means and can command the highestskill. Between the upper guide bars are fitted various tool receptacles. The only one which need as yet be spoken of is that which holds the little inch-long tools for ordinary work or for use with the eccentric chuck. This consists of a brass plate bevelled to fit the chamfered bars, on the end of which is a small piece of steel with a rectangular hole and set screw, as seen in the drawing marked X. This plate has a tailpiece of rather thicker metal, through which pass two set screws, which regulate the depth of the cut to be made, their points bearing against the end of the brass plate on which this tool receptacle works. No screw is attached to move this upper slide, but a wooden handle projects from it at right angles to be moved by hand, or a lever Y is made use of. This has two projecting pins, the front one taking one of the holes, 1, 2, 3 of the slide, the other one of a set of similar holes in the top of the chamfered bars. By this the slide can be advanced with ease and great steadiness. We now come to the pattern plates or templets F, alluded to. These are formed of sheet iron about1/20in. to1/16in. thick, and must be long enough to reach from one end to the other of the iron frame A, underneath which at each end are two holesa, a.,Fig. 224 B, to receive screws by which the templets are attached by means of the slots,b, b. The outer edge,a, of templet is the pattern to be copied. The dotted lines inFig. 224show a plate in position. Thetailpiece of the tool holder to be used has a projecting stud, or is made with a screw with or without the roller seen at H, and this is kept in contact with the templet by the hand or by a spring, so that when the slide traverses the lower frame the tool-holder necessarily follows all the curves of the pattern plate. An inspection of K will make it evident that the projecting screw must be so regulated as to length as to allow the tool-holder to go to the extremes of the projections and hollows. It must therefore in the pattern shown be at least as long as the linea, b, Fig. K, and a tool-holder of sufficient length must for similar reasons be selected, or the tailpiece might touch the fixed plate on which it works before the guide pin had penetrated the deepest hollow of the pattern. With these precautions nothing can exceed the ease with which this clever adaptation of the slide rest is made and used, the greatest advantage being that the workman can make his own templets to any curve or series of curves that may be desired.

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