Fig. 2.
The way of using the spherical rest is thus described by Mr. Hoblyn:—"When altogether, and the point of tool adjusted by means of a square exactly over centre of pivot, it is evident, if the top plate moves round, the point of the tool will still be in the same place; but if we retire the point one half-inch, on moving the plate right round it would describe an inch circle, so that if the centre of pivot be exactly under the centre of the lathe, and we move the rest the half-circle, it will cut a perfect ball (or any part of one, if the cut be less than half a circle) of such size as the distance doubled of point of tool from centre of pivot. Therefore, by adjustment of the lower plate on the lathe bed according to the size of material (this, of course, does not allude to the act of turning a ball, when the centre of pivot must be exactly under centre of lathe), adjustment of carriage from centre according to the size of circle required, and adjustment of tool for depth of cut, we are enabled to turn any convex curve. To turn the concaves, instead of retiring from centre of pivot, it is only requisite to advance the point beyond the centre to the distance required, when the same rules apply as to the convex curves. In turning the concaves, however, it is necessary to turn the plates half-round, so that A and B are in the same straight line, instead of over each other. This enables you to work on any sized piece of wood, the object of the longitudinal slot in the lower plate being to enable you to adjust your previously arranged curve to any sized wood or ivory that your lathe will take, less 2 in., the height of machine. The desired curve having been turned, take out the chisel, and place in the receptacle any ornamenting instrument, drill, eccentric, &c.; advance the point till it touches the work, then set your screw for depth of cut, and work according to fancy. Whatever it is it must be mathematically correct on the curve, as you have not altered it in any way. You then remove your rest to cut the next desired curve, and proceed as before. The best way to execute a piece of work is first to make a sectional pattern of your design, either by drawing or cutting it out with scissors from a double piece of paper, when, of course, both sides come the same, the line where the paper is doubled being the line of centre of lathe. You can then, with a pair of compasses, ascertain with accuracy the necessary size of circle, and the position of centre of pivot to procure the desired curves. You may also produce a very good effect by a combination of two different curves in this way:—Let your ornamental instrument be the universal cutter; working horizontally follow up your curve with it, but, instead of cutting right out on your curve, let the instrument finish of itself in wood previously turned so as to fit its curve, when you get the lesser and greater curves following in unbroken succession. With the eccentric chuck numberless effects of the most curious description may be produced, even work supposed to be only possible with a rose engine. We believe the tool is to be seen at the shop of Mr. Evans, 104, Wardour-street.
To turn a sphere by means of a template attached to the slide rest as described, the following adjustment of the rest and mode of proceeding should be followed. A, B, [Fig. 225], is the chuck containing the material, H, to be worked into a sphere. Upon this the length, or diameter (equal to that of the template, as will be explained), is to be marked at K, K, which being divided equally by the line L, L, will give the dimensions of the ball as if it were about to be turned by hand. The corners are to be turned off with the gouge, as far as shown, K being equal to e L, and K f equal to K e. The outline of the ball will with these measurements not be touched. The angles left may also carefully be removed, but (as shown by the figure) this operation must be conducted with great care. A template must now be made containing a full semicircle, every part of which can be traversed by the stud or screw upon the under part of the slide, or the ball will not be severed by the final cut. It is evident that the traverse of the slide during the operation will be the full radius of the ball, and in this, and indeed all cases of deep recesses, and greatly projecting mouldings, the ordinary tool-holder with tailpiece had better be removed, and replaced with a slide like M, having a pin straight through it to rest in contact with the template. This will preclude the necessity for the long stud or screw spoken of before as necessary when the slide with the tailpiece is used, but the tool cannot be advanced independently of the template as when the other form is used. Fix the rest so that when the top slide is at its central position the tool may stand as in the sketch exactly upon the central line of the ball. Take care that thus placed the tangent-pin of the slide is on the central mark of the template. The long frame of the rest must likewise be parallel with the bed of the lathe, keeping the top slide pressed against the template with the left hand, while the top part traverses the frame under the action of the screw moved by the right hand, the ball will be correctly cut.[16] One or two cautions must, however, be given here, to ensure a satisfactory result. In the first place the cylinder from which the ball is to be made must be exactly of the diameter of the semicircle on the template. H, H is the cylinder to be turned to a sphere, G, G, B shews the position of the tool at starting, the dot on A, the templet, the tangent pin of the slide, [Fig. 226]. As the work proceeds the tool will take the several positions shown, the dotted lines, D, being equal and parallel. The tool will thus repeat the form of the template. Let the latter remain as before, but let a smaller cylinder be inserted in the lathe, or, which is the same thing, let the tool be now lengthened so as to start at C on the inner dotted line. When the pin, F, has reached K, which should be the axis of the ball, the tool will be at M, quite out of cut. [Fig. 227] represents three forms of tool in contact with the ball at two points. The first two will evidently be out of cut at the axial line, as the side of these bevels will then touch the piece to be turned. C is a form that will remain in contact from the diameter to the axial line. The left side of the edge is slightly overhanging the side line of the tool, D.
[16] See Appendix
Fig. 225.
Fig. 226.