These works may be constructed either in wood or ivory, the latter being preferable, from its uniformity of texture, which avoids some difficulties incidental to the former; all require an accurate sphere as the gage or foundation from which they are worked, and it is always supposed that this has been first produced. Some, require tools made to the size of the sphere in which the work is to be executed, or else, the sphere must be reduced to the diameter for which the tools are intended; while it is also a great advantage to the operator, to be himself capable of making and replacing the more delicate of the tools, some of which are rather easily broken.

The following examples, which will suffice to illustrate the different processes, may all be turned by hand without much difficulty. Many among them, together with some others still more elaborate, are ably described by Bergeron; but in setting out the platonic solids, rather easier expedients have been adopted, which are yet sufficiently good for the degree of accuracy that may be obtained by hand turning. The platonic solids, may be produced with comparative facility with the slide rest and apparatus for ornamental turning; but the description and manipulation of these, are foreign to the present volume. A complete and interesting series of these curious specimens of plain turning, may be seen in the collection of the Conservatoire des Arts et Metiers, Paris.

The popular ivory Chinese ball, made in the district of Canton, consists of several thin spherical shells, pierced with large holes, and contained one within the other, enclosing a small solid ball at the center. The shells are separated in the lathe with tools introduced through the holes, their surfaces being carved and ornamented subsequently, also through the holes, with appropriate small tools and drills. In the native specimens the turning is usually rather roughly executed, but the carving effectually hides all imperfections.

The Chinese ball shown in the section, fig. 560, has three shells pierced with six holes, the centers for which, fig. 561, are first carefully set out upon the sphere, turned to the diameter A. B. The circumference C. D, is determined from the edge of the chuck and marked with a pencil line, in the manner described page 416 ; it is then divided into four equal parts by the division plate and marked across with short pencil lines. The edge of the rest, exactly at the height of center of the lathe being placed close against the ball, or in preference, one of the accurate scribing tools, being used in marking them. The centers for the remaining two holes, are upon the axis A. B; one center is turned with a point tool at B, the ball is then reversed in the chuck, the line marking the circumference adjusted to again run true, and the other center at A, turned in like manner.

The plain wood chuck, that employed in turning the sphere, is now exchanged for another similar to that described by Bergeron, but with the addition of a metal flange screwing on the mandrel, fig. 562 ; it is formed of three parts shown in section. The back E, is of wood, hollowed exactly hemispherical to the diameter of the ball. The wooden ring F, hollowed within to the same curve, is carried on the back by a plain cylindrical fitting; the two are closed upon each other by a metal ring, G, screwing upon E, thus grasping the ball, of which about one third is accessible by the aperture in the face of the wooden ring. The ball can be readily fixed with any point of its surface in the axis of the lathe, and remains undisturbed when G. is tightened. The ball being placed in fig. 562, the four centers upon the circumference C. D, are successively adjusted to the center and struck with the point tool; previously to turning all the six radial holes, A. B. C D. shown by the sections, figs. 560 and 562.

Fig. 560. Fig. 561. Fig. 562.

Section III Various Works Formed Within The Sphere 400333

The inner ends of these apertures form portions of the surface of the solid central ball, and to render them all similar, they are fitted to a template fig. 567; the tools used being the drill, the right side, and a narrow flat tool for the bottom, or otherwise, a shallow hollow tool ground to the section of the central ball. It is more convenient to replace the template, by the taper halfround bit, fig. 563; the holes are then prepared with the hand tools and finished with the bit, which like the template, is prevented entering to too great a depth by two non-cutting projections that are arrested by the surface of the ball. The tools for separating the shells, figs. 566, 568., are analogous to inside parting tools, with the blades curved to the radii of the circles for which they are required. They are sharpened only upon the end, a, the cutting edge being slightly wider than the rest of the blade, which, like the blade of the inside parting tool, is also ground taper vertically to give it clearance within the grooves cut. The stem of the tool lies in a rectangular groove in the shaft of a metal guide, fig. 566, the front of the guide being filed exactly to the curve of the ball. The tool is attached by two screws passing through elongated holes, and is adjusted to the requisite depth within one of the radial holes in the work, with the entire curve of the guide bearing upon the surface of the ball.

The solid central ball, fig. 560, is first completely detached and then every shell seriatim, from the most central outwards. The sphere is placed in the chuck, fig. 562, with the radial aperture B in front, the grain therefore running in the direction of the axial line. The smallest tool placed in the guide, is then applied within the hole and adjusted and fixed, so that the corner of the blade a, just touches the surface of the central ball, a c, with the back of the stem close against the side of the radial hole c d, and the guide, in contact with the exterior surface. The tee of the hand rest is fixed just sufficiently high to place the surface of the tool both horizontal, and radial to the sphere. Thus arranged the tool is made to cut around the curve of the ball in the direction B to D, until it is arrested, by the front of the stem arriving in contact with a b, or the opposite side of the radial hole.

The first cut completed, the brass ring is slackened, and the ball reversed with the fingers, is refixed with the aperture A running true in front; and the tool remaining fixed as before, is then used at this hole. The four holes upon the line C. D. are then successively adjusted to the front to run true, the tool gradually separating the central ball from the surrounding material by cutting at each hole, and setting it entirely free at the termination of the sixth cut. The innermost shell is separated by the next larger tool, used in like manner from all six centers, followed by larger tools according to the number of the shells; the two first cuts being always made at B and A, fig. 560, or across the direction of the grain.

Section III Various Works Formed Within The Sphere 400334

The path of the tool is completely under control, so long as the entire curve of the guide is retained in contact with the surface of the ball; but the cutting action, not only resembles that of the inside parting tool, previously referred to, but the liability to an accumulation of shavings arising from the enclosed position of the tool, is now also increased by its curved form. Increasing the width of the groove to give the shavings facility of escape, as with the inside parting tool, not being allowable, the tools require using with great gentleness and caution; and the formation of any hard collection of shavings is carefully watched for and immediately reduced, to avoid the otherwise almost certain breakage of the tool or work. The tool also requires to be frequently withdrawn, to have its sides and cutting edge cleansed from adhering shavings, and its edge slightly set upon the oilstone, that it may cut freely and leave the surface of the shells smooth; as any surface roughness cannot be conveniently removed subsequently.

The diameter of the radial apertures depends upon their number and arrangement, but the smallest diameter that will suffice, produces the most effective result. The tool has to cut through rather more than half the material lying between neighbouring holes, and the diameter of the hole is required so much larger than this, as will allow space for the width of the stem of the tool in addition to the length of the blade. Balls pierced with six holes, which have been selected for simplicity of illustration for this, and the two following examples, show a radial diameter of one eighth of the circumference to be rather more than sufficient, while one ninth barely suffices; but, when the apertures are more numerous, as eight, or twelve, they may be of proportionately smaller diameter. In making a set of balls from the six apertures as described, it is obvious, that the least accessible and therefore the most difficult part of the turning upon each shell, is at the last, and lies at the point where the three circles cut by the end of the tool intersect. The difficulty experienced in cutting this portion of the work, may however if necessary be considerably reduced, by boring smaller additional conical apertures at these points of intersection, to remove a portion of the material. This expedient also allows of a small reduction in the length of the blade of the separating tool, with a corre-. sponding reduction in the diameter of the original six apertures; and may be considered to improve the appearance of the result. The additional apertures have been omitted from figs. 560. 562. to avoid confusion.

The edges of the holes in the shells may be left square, or turned to a bead or moulding with an internal tool, fig. 401; a bead or other narrow ornament may also be cut around them on the surface of the shell with ordinary tools, introduced at a considerable angle to A. B. Such enrichments are turned concurrently with the separation of the shells; the dividing tool is allowed to cut a short distance and withdrawn, the moulding is then cut upon the edge while the shell is still solidly attached to the surrounding material, and the separation is then resumed.

The intervals or surfaces between the apertures upon all the different shells, may also be covered with ornament formed of circles, small hemispheres, beads and cones, turned upon them, and by small pierced holes; all admitting of arrangement in groups or patterns. Every interval is brought successively to the aperture at B, and temporarily fixed by a conical wood stopper, inserted in the opposite aperture at A. The head of the stopper fits the hole in the external shell, its outer surface being turned to the curve of the ball. The stopper fig. 564, is sufficiently long to reach the central ball, and to press it and the entire contents of the external shell together against the side B; holding them with sufficient firmness for the shallow turning required in producing the ornament. The shells further from the center, may be held by corresponding stoppers of less length.

The pattern is first sketched out upon the surface with a pencil, every center is then adjusted fairly to the mandrel axis, seriatim; the outer shells by the fingers, and the innermost by aid of a sharp steel point; the screw ring of the chuck being slackened previously to every change of position, to lessen the pressure of the stopper; the enrichments are easily executed and may be very elaborate, but they then require both time and patience.