Material

A sphere turned as in the ninth exercise.

Exercise

To turn a ball inside the one already turned.

Fig. 117.

Over the wooden chuck used in turning the ball fit a wooden disk, and turn a hole in the centre of it, so that it may be fastened to the chuck with screws, and have a portion of the ball project through the front, as shown in Fig. 117. The fit to the ball must be close, so that the ball will be held firmly in any required position.

Fig. 118.

Procure two tools 6f the form shown in Fig. 118 and Fig. 119. The curvature of the end must be the same as that of the inside ball. These may be made from old files by grinding.

Mark the extremities of the three axes of the sphere that are perpendicular to one another. This may be done by setting the sphere in a corner of a square box and marking the points of contact, and then finding a point on the sphere opposite each one of the three so found by means of the calipers. When the points are found, fasten the sphere in place in the chuck in such position that one of the points is exactly in the centre of the opening in the disk that covers the chuck, and with the tool shown in Fig. 118 cut a hole into the ball in the manner shown in Fig. 117. The depth of the hole should be 1/2 inch, and this may be regulated by making a mark on the corner of the chisel before starting the cut. Next, loosen the front part of the chuck and move the ball until a second mark is in the centre of the opening, and cut in again, and so continue until the six marks have been used and the ball has the appearance shown in Fig. 120. Now, using the second tool in the manner shown in Fig. 121, remove the material between the holes so cut until the inner ball is free from the outer shell, as shown in Fig. 122. The last cut taken must be made with great care, or the freed inner ball will catch on the tool and break the outer shell. The lathe-spindle would better be revolved by hand during the latter portion of the cut.

Fig. 119.

Fig. 120.

Fig. 121.

Fig. 122.

Fig. 123.

This is but one of many solids that may be turned inside the sphere in nearly the manner indicated, and the turning of which is most fascinating work for a skilful turner.