cross-feed screw and the cross-slide nut. If the
backlash is not eliminated, a straight portion will be
Much of the machine work performed by a
turned on the work. You can remove the backlash by
Machinery Repairman includes the use of screw
moving the carriage and tool slightly past the start of
threads. The thread forms you will be working with
the cut and then returning the carriage and tool to the
most are V-form threads, Acme threads, and square
start of the cut.
threads. Each of these thread forms is used for
specific purposes. V-form threads are commonly
Methods of Boring Tapers
used on fastening devices, such as bolts and nuts, as
well as on machine parts. Acme screw threads are
Taper boring is usually done with either the
generally used for transmitting motion, such as
compound rest or the taper attachment. The rules that
between the lead screw and lathe carriage. Square
apply to outside taper turning also apply to the boring
threads are used to increase mechanical advantage
of taper holes. Begin by drilling the hole to the
and to provide good clamping ability as in the screw
correct depth with a drill of the same size as the
jack or vise screw. Each of these screw forms is
specified small diameter of the taper. This gives you
discussed more fully later in the chapter. We will also
the advantage of boring to the right size without
discuss cutting threads on an engine lathe.
having to remove metal at the bottom of the bore,
There are several terms used in describing screw
which is rather difficult, particularly in small, deep
threads and screw thread systems that you must know
before you can calculate and machine screw threads.
For turning and boring tapers, set the tool cutting
edge exactly at the center of the work. That is, set the
EXTERNAL THREAD: A thread on the outside
point of the cutting edge even with the height of the
surface of a cylinder.
lathe centers; otherwise, the taper may be inaccurate.
INTERNAL THREAD: A thread on the inside
Cut the hole and measure its size and taper using
surface of a hollow cylinder.
a taper plug gauge and the "cut and try" method.
RIGHT-HAND THREAD: A thread that, when
viewed axially, winds in a clockwise and receding
1. After you have taken one or two cuts, clean the
LEFT-HAND THREAD: A thread that, when
2. Rub the gauge lightly with chalk (or prussian
viewed axially, winds in a counterclockwise and
blue if the taper must be highly accurate).
3. Insert the gauge into the hole and turn it
LEAD: The distance a threaded part moves
SLIGHTLY so the chalk (or prussian blue) rubs from
axially in a fixed mating part in one complete
the gauge onto the surface of the hole. If the
workpiece is to be mounted on a spindle, use the
tapered end of the spindle instead of a gauge to test
PITCH: The distance between corresponding
points on adjacent threads.
4. Areas that do not touch the gauge will be
shown by a lack of chalk (or prussian blue).
5. Continue making minor corrections until all,
or an acceptable portion, of the hole's surface touches
the gauge. Be sure the taper diameter is correct
before you turn the taper to its finish diameter.
and a taper socket gauge. They not only give the
proper taper, but also show the proper distance that
Figure 6-80.--Morse taper socket gauge and plug gauge.
the taper should enter the spindle.