Part 15

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 being provided, a very simple apparatus will answer the purpose. To commence with Bergeron's, which, though venerable, is by no means inefficient. This is represented in Figs. 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 plate 214, on which a side sectional view is given in Fig. 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 in Fig. 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 projections _a_, and _b_, in B, represent the projecting rim, _a_, in the Fig. 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 that this 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 in Fig. 217 on 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.

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 in 219, and detached in 220. 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 to denote 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 diagram Fig. 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 hang down 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 taken _quantum 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. 222 represents 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. At _a_, 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 at _b_, 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 hole in 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 in 223. 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.

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 in Fig. 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 one V-piece similar to D, the second E being removable at pleasure, or both V-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 marked _e, e_, in the drawing of the rest. On the top of the brass plate is fixed in a like manner 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 B shows 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 1/4 in. thick and two in width the bars may be 3/8 in. stuff before being bevelled, the attaching screws can then be 1/3 in. or 3/16 in. 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 highest skill. 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 about 1-20 in. to 1-16 in. 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 holes _a, 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 in Fig. 224 show a plate in position. The tailpiece 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 line _a, 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.

At a later page is described a slide rest with the arrangements of the tool-holder somewhat improved, and calculated for the reception of larger tools and apparatus. The little rest here described is, however, very light and useful.

HOBLYN'S COMPOUND SLIDE-REST.

A compound spherical slide-rest, for ornamental turning lathes, which has been patented by Mr. Hoblyn, Rickling-green, Essex, is capable of turning and ornamenting accurately spheres and any segments of circles convex or concave, either on the surface or cylinder; also, it is stated that by the addition of templates any other curves, not segments of circles, can be turned out, in addition to the work performed by an ordinary compound slide-rest. In our illustrations, Fig. 1 is a side view, and Fig. 2, an elevation of the rest. A is the lower, and B the top plate, C the carriage, and the saddle as usual. The lower plate, of iron, is planed on bottom and sides, and has a longitudinal bevelled slot in the bottom to receive the fastening bolt, so that the plate can be tightened up at any point on the lathe bed. At one end of this bottom plate is a piece raised the whole of its breadth, accurately turned and faced, with a stout turned pin, _b_, for a pivot in the centre. A wheel, _c_, is placed around this raised piece, having any specified number of cogs, and on its top any specified number of small holes drilled to receive two pins as segment stops. The top plate is slightly narrower than the bottom one, and has on its underside a corresponding surface to that raised on the plate A, turned and faced with countersunk hole in the centre to fit on the pivot _b_, on which it is tightened by a screw and washer. The top of B is placed to receive two parallel bars of brass, bevelled on the inner edge, so as to form sliding bars for the carriage C, one bar being a fixture, the other capable of adjustment by tightening screws. The sides are also accurately planed, and on one a tangent screw, _e_, is so fixed as to be put in and out of gear with the brass wheel, _c_, for the purpose of driving the top plate round the pivot _b_. In the raised surface of B, is a countersunk slot forming a quarter circle, with screw passing through it into the bottom plate; by this means the top plate can be firmly fixed at any angle to the bottom plate for the purpose of slide-rest turning. A small pointer, _f_, works clear of the brass wheel in the capacity of segment stop--two small pins being placed in the specified number of holes in _c_. On the opposite side of the screw, _e_, on the top surface, a screw, _g_, works in two standards screwed into the plate through slots in the bar, which works through a traveller, _h_, firmly attached to the carriage, C, and therefore capable of driving the carriage in a rectilinear direction--for slide-rest purposes. This carriage, C, consists of an iron plate _i_, with standard _k_, top plate _l_, with parallel brass bars to receive tool receptacle _m_, the bottom plate and standard being in one piece with the plate, planed side and bottom, so as to slide truly between the parallel bar on B. The standard has its surface accurately turned with a pin in centre to form pivot. The top plate is planed true, bottom, sides, and top, and has a countersunk hole in the exact centre, so as to work on the pin, and tightens up with a large nut. In the plate is a countersunk slot forming a quarter-circle, with screw passing through into the standard, to set the tool receptacle either parallel with B for spherical turning, at right angles to B for slide turning, or at any angle for thick and thin cuts, and similar patterns when used either as a spherical or slide rest. The parallel bars are to be similar to those on B, to enable the tool receptacle to be advanced or retired by screw or lever. This part of the machine can be adjusted for height of centre thus:--The standard _k_ to be a hollow cylinder with fixing screw on one side; the plate _i_ to be made with turned pin on bottom, to fit into standard; elevating screw; saddle as usual, but with the addition of a small hole drilled in one of the sides, and a corresponding one in the side of the lower plate A, so placed that when a pin is fixed into these holes the pivot is exactly central to the lathe centre.