Tc 9-524 fundamentals of machine tools

Offsetting the Tailstock
The oldest and probably most used method of taper turning
is the offset tailstock method. The tailstock is made in two
pieces: the lower piece is fitted to the bed, while the upper
part can be adjusted laterally to a given offset by use of
adjusting screws and lineup marks (Figure 7-63).
Since the workpiece is mounted between centers, this method
of taper turning can only be used for external tapers. The
length of the taper is from headstock center to tailstock center,
which allows for longer tapers than can be machined using the
compound rest or taper attachment methods.
The tool bit travels along a line which is parallel with the
ways of the lathe. When the lathe centers are aligned and the
workpiece is machined between these centers, the diameter
will remain constant from one end of the piece to the other. If
the tailstock is offset, as shown in Figure 7-64, the centerline
of the workpiece is no longer parallel with the ways; however,
the tool bit continues its parallel movement with the ways,
resulting in a tapered workpiece. The tail stock may be offset
either toward or away from the operator. When the offset is
toward the operator, the small end of the workpiece will be at
the tailstock with the diameter increasing toward the
headstock end.
The offset tailstock method is applicable only to
comparatively gradual tapers because the lathe centers, being
out of alignment, do not have full bearing on the workpiece.
Center holes are likely to wear out of their true positions if the
lathe centers are offset too far, causing poor results and
possible damage to centers.
The most difficult operation in taper turning by the offset
tailstock method is determining the proper distance the
tailstock should be moved over to obtain a given taper. Two
factors affect the amount the tailstock is offset: the taper
desired and the length of the workpiece. If the offset remains
constant, workplaces of different lengths, or with different
depth center holes, will be machined with different tapers
(Figure 7-65).