in relation to _B_ when bolts _C_ are loosened. The
driving pin for the lathe dog is attached to plate _A_. When one groove
of a multiple thread is finished, bolts _C_ are loosened and plate _A_
is turned around an amount corresponding to the type of thread being
cut. The periphery of plate _A_ is graduated in degrees, as shown, and
for a double thread it would be turned one-half revolution or 180
degrees, for a triple thread, 120 degrees, etc. This is a very good
arrangement where multiple thread cutting is done frequently.

[Illustration: Fig. 14. Correct and Incorrect Positions of Tool for
Taper Thread Cutting]

=Taper Threading.=--When a taper thread is to be cut, the tool should be
set square with axis _a--a_ as at _A_, Fig. 14, and not by the tapering
surface as at _B_. If there is a cylindrical part, the tool can be set
as indicated by the dotted lines. All taper threads should be cut by the
use of taper attachments. If the tailstock is set over to get the
required taper, and an ordinary bent-tail dog is used for driving, the
curve of the thread will not be true, or in other words the thread will
not advance at a uniform rate; this is referred to by machinists as a
"drunken thread." This error in the thread is due to the angularity
between the driving dog and the faceplate, which causes the work to be
rotated at a varying velocity. The pitch of a taper thread that is cut
with the tailstock set over will also be slightly finer than the pitch
for which the lathe is geared. The amount of these errors depends upon
the angle of the taper and the distance that the center must be offset.

=Internal Threading.=--Internal threading, or cutting threads in holes,
is an operation performed on work held in the chuck or on a faceplate,
as for boring. The tool used is similar to a boring tool except that the
working end is shaped to conform to the thread to be cut. The method of
procedure, when cutting an internal thread, is similar to that for
outside work, as far as handling the lathe is concerned. The hole to be
threaded is first bored to the root diameter _D_, Fig. 15, of the screw
that is to fit into it. The tool-point (of a tool for a U. S. standard
or V-thread) is then set square by holding a gage _G_ against the true
side of the work and adjusting the point to fit the notch in the gage as
shown. The view to the right shows the tool taking the first cut.

[Illustration: Fig. 15. Method of setting and using Inside Thre