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.
=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.
[Illustration: Fig. 14. Plan View showing Method of Turning a Taper with the Compound Rest]
=Taper Turning with the Compound Rest.=--The amount of taper that can be turned by setting over the tailstock center and by the taper attachment is limited, as the centers can only be offset a certain distance, and the slide _S_ (Fig. 9) of the attachment cannot be swiveled beyond a certain position. For steep tapers, the compound rest _E_ is swiveled to the required angle and used as indicated in Fig. 14, which shows a plan view of a rest set for turning the valve _V_. This compound rest is an upper slide mounted on the lower or main cross-slide _D_, and it can be turned to any angular position so that the tool, which ordinarily is moved either lengthwise or crosswise of the bed, can be fed at an angle. The base of the compound rest is graduated in degrees and the position of these graduations shows to what angle the upper slide is set. Suppose the seat of valve _V_ is to be turned to an angle of 45 degrees with the axis or center, as shown on the drawing at _A_, Fig. 15. To set the compound rest, nuts _n_ on either side, which hold it rigidly to the lower slide, are first loosened and the slide is then turned until the 45-degree graduation is exactly opposite the zero line; the slide is then tightened in this position. A cut is next taken across the valve by operating handle _w_ and feeding the tool in the direction of the arrow.