After the crosshead end is finished, the rod is reversed in the lathe for turning the piston end. The dog is clamped to the finished end, preferably over a piece of sheet copper to prevent the surface from being marred. When turning this end, either the piston reamer or the finished hole in the piston can be calipered. The size and angle of the taper are tested by driving the rod into the piston, and the end should be fitted so that by driving tightly, the shoulder will just come up against the finished face of the piston. When the taper is finished, the attachment is disengaged and a finishing cut is taken over the body of the rod, unless it is to be finished by grinding, which is the modern and most economical method.
Fig. 13. Tool Point should be in same Horizontal Plane as Axis of Work for Taper Turning
Height of Tool when Turning Tapers.—The cutting edge of the tool, when turning tapers, should be at the same height as the center or axis of the work, whether an attachment is used or not. The importance of this will be apparent by referring to [Fig. 13]. To turn the taper shown, the tool T would be moved back a distance x (assuming that an attachment is used) while traversing the length l. As an illustration, if the tool could be placed as high as point a, the setting of the attachment remaining as before, the tool would again move back a distance x, while traversing a distance l, but the large end would be under-sized (as shown by the dotted line) if the diameters of the small ends were the same in each case. Of course, if the tool point were only slightly above or below the center, the resulting error would also be small. The tool can easily be set central by comparing the height of the cutting edge at the point of the tool with one of the lathe centers before placing the work in the lathe.
Fig. 14. Plan View showing Method of Turning a Taper with the Compound Rest