This casting is rough turned to within ¹⁄₃₂ of an inch of the finished
size before any part of it is finished. The 1⅛ inch end is then turned
until it fits the corresponding part of the hole in piece A as closely
as possible and yet not so tight that it cannot be freely rotated. This
kind of a fit is called a close running fit.
=Cutting Threads.=—The portion to be threaded should be turned a little
smaller than the diameter at the bottom of the threads in piece A. This
size is measured by means of the inside spring-thread calipers.
There is no recess, or groove, cut at the end of this thread so that
if the threading tool is allowed to travel farther than the end of the
preceding cut, either the point of the tool or the threads may break.
To prevent this, the lathe is stopped when the tool is within a half a
thread of the end and the cut finished by turning the lathe by hand.
In this way the lathe is kept under control and the tool may be drawn
back when it reaches the end of the preceding cut. Experienced lathe
operators do not, as a rule, turn the lathe by hand, but control the
lathe entirely by the shipper.
The speed of the lathe for cutting this thread will be about right for
beginners if the belt is on the second smallest step of the cone and
the back gears are thrown in.
=Finishing the Angle, or Taper.=—The 30 degree angle may be cut by
setting the compound rest to the correct angle and using a regular
turning tool. In case the tool leaves a few tool marks, they may be
removed by filing.
If the lathe is not provided with a compound rest, this angle may be
cut by setting a square-nose tool, as in Fig. 31, with the aid of a
thread gauge. Any other angle would have to be set with a bevel and
bevel protractor.
[Illustration: Fig. 31]
This tool is not as wide as the surface to be cut because one that will
cut the full width is very liable to chatter. It is therefore better to
make several cuts with a narrow tool. The surface can then be finished
smooth by filing.
The closeness of the fit of this taper with that in A can be tested
by rubbing black paint, which consists of lamp black and oil, on the
tapered surface in A. When B is screwed into A, marks will be made on
B indicating the high spots. If these are not too high, they may be
removed by filing.