the gage, as shown somewhat exaggerated at _D_; in that case the
center would be shifted a little more towards the front, whereas if the
taper were too steep, the adjustment would, of course, be in the
opposite direction. A light cut would then be taken, to be followed by
another test. If the plug should fit the gage so well that there was no
perceptible shake, it could be tested more closely as follows: Draw
three or four chalk lines along the tapering surface, place the work in
the gage and turn it a few times. The chalk marks will then show whether
the taper of the plug corresponds to that of the gage; for example, if
the taper is too great, the marks will be rubbed out on the large end,
but if the taper is correct, the lines throughout their length will be
partially erased.

[Illustration: Fig. 5. Setting Work for Taper Turning by use of Caliper
Gage]

Another and more accurate method of testing tapers is to apply a thin
coat of Prussian-blue to one-half of the tapering surface, in a
lengthwise direction. The work is then inserted in the hole or gage and
turned to mark the bearing. If the taper is correct, the bearing marks
will be evenly distributed, whereas if the taper is incorrect, they will
appear at one end. Tapering pieces that have to be driven tightly into a
hole, such as a piston-rod, can be tested by the location of the bearing
marks produced by actual contact.

After the taper is found to be correct, the plug is reduced in size
until it just enters the gage as at _C_. The final cut should leave it
slightly above the required size, so that a smooth surface can be
obtained by filing. It should be mentioned that on work of this kind,
especially if great accuracy is required, the final finish is often
obtained by grinding in a regular grinding machine, instead of by
filing. When this method is employed, a lathe is used merely to
rough-turn the part close to size.

[Illustration: Fig. 6. Side View showing Relative Positions of Gage and
Work]

When the amount that the tailstock center should be offset is determined
by calculating, as in the foregoing example, it is usually necessary to
make slight changes afterward, and the work should be tested before it
is too near the finished size so that in case one or more trial cuts are
necessary, there will be material enough to permit this. When there are
a number of tapered pieces to be turned to the same taper, the
adjustment of the tailstock center will have