[Turning Spheres.]
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. 211] gives 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 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.