Fig. 228.
Fig. 229.
The guide ring here alluded to is shown at E, and also at G, fixed in its place upon the poppet. It is in the form of a raised ring with arms, B, C., which are turned at right angles near their ends, and through which pass adjusting screws with conical points. This plate is flat at the back and bears against the face of the poppet, the mandrel nose falling into the central opening E. It is kept in place by the points of the screws falling into conical holes at the sides of the poppet head. At F is a side sectional view of this plate, with its raised and accurately turned ring, H, and at G is seen the poppet with the plate attached, the left arm being dotted to show the position of the adjusting screw. It is this ring and plate which regulate the movement of the slider, and, with it, of the work, the latter being attached to the screw in the centre of the sliding plate, which screw is a counterpart of that upon the nose of the mandrel. Suppose the chuck A screwed to the mandrel, and the ring accurately concentric with the mandrel, in which position, the pallets must touch at two opposite points. In the best chucks there is an adjusting screw to each, by which the contact can be regulated. In this position any object of a circular form can be turned, for the slider remains in one position, and its screw, upon which the work is fixed, is a continuation of the mandrel. But if now the adjusting screws of the part E are turned, the one being loosened and the other tightened, the guide ring will no longer be concentric with the mandrel, and, as the screws of the slider bear upon it, the slider will during its revolution be moved to and fro to a distance regulated by the eccentricity of the guide ring. The combination of this circular motion of the chuck and rectilineal movement of the slider will produce an ellipse, and a stationary tool applied to the work will cut it, into that form.
The above simple arrangement of oval chuck suffices only for plain work. The only figures that can be described by its means, upon the cover of a box, for instance, being a series of ellipses of which the longest diameters fall in the same right line, and of which the centres are coincident with the axis of the mandrel, as [Fig. 229].
Fig. 229.
Even these, however, cannot be done without some compensating arrangement, as the minor axis does not diminish in length at the same rate as the major—hence the ellipses get narrower and narrower until the central one becomes a mere right line. This is referred to again in the ornamental section of this work.