The threads are cut to fit ½ inch and ⅝ inch nuts having United States
Standard threads. These threads are flattened at the top and bottom to
the amount of ⅛ of the pitch instead of being sharp pointed as in the
case of Standard V-threads.
=Pitch.=—The pitch of the thread is the distance from the center of one
thread to the center of the one adjoining. On the end of the exercise
having 13 threads per inch the pitch is ¹⁄₁₃ of an inch so that the
width of the flat at the top and bottom of this thread should be ⅛ of
¹⁄₁₃ of an inch, or about .009 of an inch.
=Lead.=—The lead of the thread is the distance a nut on the screw will
travel in making one complete turn. For single threads the pitch and
lead are the same, but for double threads the lead is twice the pitch.
=Grinding Tool.=—The sides of U. S. S. threads form an angle of 60
degrees. To cut this thread in a lathe, a tool the same shape as the
threads is used. A gauge for grinding this tool accurately is shown in
Fig. 14.
[Illustration: Fig. 14]
[Illustration: Fig. 15]
[Illustration: Fig. 16]
If a U. S. S. thread gauge is not available, the tool can be ground
with the aid of a regular thread and center gauge, shown in Fig. 15.
With such a gauge the angle can be ground accurately, but it will be
necessary to measure the flat point with a rule.
The top of the tool should be ground so that it will be approximately
in a horizontal plane when set in the lathe.
Where the thread to be cut is as fine as 13 per inch the flat surface
at the point of the tool is so small that the extreme point can be
oil-stoned off instead of being taken off with the grinding wheel. The
flat point should never be wider than the standard size, but if it is a
little too narrow it will make very little difference in ordinary lathe
work.
=Setting Tool.=—To set the tool so that both sides of the thread will
have the same angle, the thread gauge is used as shown in Fig. 16. The
tool should be set on a level with the center of the lathe.
=How Lathe is Geared.=—To cut 13 threads per inch the work must make
13 revolutions while the carriage, which carries the tool, travels one
inch. For this purpose the lathe spindle is connected to the lead
screw with the proper size gears and the lead screw to the carriage by
a split nut. This split nut is back of the carriage apron and is opened
and closed by the lever E, Fig. 17.
[Illustration: Fig. 17
A. Index Plate
B. Stud Gear
C. Screw Gear
D. Intermediate or Idle Gear
E. Lever for connecting Carriage to Lead Screw
J. Lever for Disconnecting and Reversing Feed
K. Adjustable Stop for Thread Cutting
]
[Illustration: Fig. 18]
If the lead screw of the lathe has 6 threads per inch, the gearing to
cut 13 threads per inch must have the same ratio as 6 is to 13. To cut
16 threads the ratio would be 6 to 16.
It is not necessary to figure the size of gears for the different
threads as all lathes are provided with an index plate that designates
the proper size gears to be placed on the stud B and screw C, Fig. 17,
for the desired thread.
=To Set Change Gear.=—To change these gears, first loosen the nuts
holding the stud and screw gears B and C. Next loosen the nut G. This
will allow the intermediate gear to drop away from the stud gear B.
Then loosen the nut H so that the intermediate gear can be drawn back
away from the gear on the lead screw C.
When the gears are put together, they should be set so that there will
be a little slack, or lost motion, between the different gears. If they
are set too close together, they will make a great deal of noise when
running and there is also danger of breaking the teeth.
While all lathes are not designed alike the method of changing the
gears is very much the same on all machines except those having the
quick change-gear device. With a lathe having such a device, instead of
changing the gears on the stud and screw the same result is obtained by
shifting a combination of levers.
=Why Feed Should Be Disconnected.=—The mechanism that controls the
feed, or travel, of the tool when cutting threads is independent of
that used for the feed when doing plain turning. The two feeds usually
run at different speeds so that if they are both in action at the same
time the gears in the carriage will break. For this reason all lathes
are provided with some means of disconnecting the feed used for plain
turning when cutting threads.
To disconnect the feed on the lathe shown in Fig. 17, move the lever J
to the central, or neutral, position. This should always be done before
starting to cut the threads.
=Speed of Lathe.=—The lathe should run slower for cutting threads than
for plain turning. With most lathes if the belt is on the largest step
of the cone it will give about the right speed for cutting the threads
in this exercise.
The object of running the lathe slow is to give the operator time to
draw back the tool at the end of the cut and to obtain a smoother cut.
If the speed of the lathe is too fast, the cutting action will be so
quick that the tool, instead of cutting clean and smooth, will tear out
the metal leaving a rough surface.
The slower the lathe runs the easier it is to cut the threads, but it
will also take longer to do the job. It therefore requires practical
experience to determine the proper speed to be used for cutting the
different size threads.
=Chamfering.=—After the lathe and tool are properly set, chamfer off
the sharp corners where the threads begin and end with the side of the
thread tool. The depth of this cut should be about the same as that
of the threads when finished. If the corners are not chamfered, the
threads, when cut, will form a very thin edge, or fin, at the ends.
=Use of Adjustable Stop.=—To regulate the depth of each cut an
adjustable stop is used as shown at K. First move the tool so that the
point just touches the work, then adjust the screw on the attachment
K so that the cross-rest will not go in any farther. Now move the
carriage by hand until the point of the tool is a little past the
tail-stock end of the work; close the split nut on the lead screw with
the lever E; and turn the screw on the attachment K so that the tool
can be moved in just enough to take a very light cut.
Start the lathe and when the tool has reached the end of the cut
back it out and reverse the lathe. By reversing the lathe the tool
is returned to the starting point without disconnecting any of the
gearing. The object of drawing the tool back is to prevent it from
dragging on the work during its return.
The tool will never travel over the same path on the reverse as on the
forward movement of the lathe on account of the slack, or lost motion,
in the gears.
This first cut is taken to prove that the lathe is properly geared, so
the work should be measured with a rule, or screw pitch gauge.
Adjust the screw at K until the tool can be moved in deeper for the
next cut and repeat the operation until the thread is nearly finished.
Then the tool should be reset so that it will cut on only one side at a
time.
=Finishing Side of Thread.=—When roughing out the threads, the tool
cuts on both sides of the point since it is fed straight into the work.
It is much easier, though, to finish the threads smooth if the tool
cuts on one side only. This is done by rapping the end of the tool
holder so that it is turned in the tool post just enough to change the
position of the point of the tool about .01, or .02 of an inch.
To prove that the tool is set over the proper amount, turn the lathe
forward by hand a few revolutions, to take out all the slack, or lost,
motion in the gears, then move the tool into the groove of the thread
until one side just touches the side of the thread. The other side of
the tool should then be about .01, or .02 of an inch away from the side
of the thread.
After the tool is properly adjusted, set the stop K. The tool is then
drawn back and the lathe reversed until the tool is at the end of the
work ready for a cut. It usually requires several finishing cuts to
take out all the rough marks left by the roughing cuts.
When this side of the thread is finished, the other side is finished in
the same manner.
If the lathe is provided with a compound rest, a somewhat different
procedure is usually followed since the rest can be set at an angle of
30 degrees with the work, as in Fig. 18.
In this case the tool is moved in by turning the small hand-crank M
until the side at O has been cut to the proper depth. While making
these first cuts, the stop K is merely used to bring the cross-rest to
the same position each time. The tool is then drawn back slightly with
the hand-crank M and the stop K adjusted so that the tool can be moved
straight in by means of the hand-crank Q. This will finish the other
side of the thread at P.
To determine when the thread is cut to the proper size the work is
removed from the lathe and tested with a standard nut having U. S. S.
threads.
After the threads are cut on this end of the exercise, it is reversed
in the lathe and the other end threaded in a similar manner.
To prevent the screw of the dog from marring the portion already
threaded two nuts should be screwed on and the dog fastened to the nuts.
=How to Reset the Tool.=—When cutting threads of this size and larger,
the tool usually becomes dull from taking the heavy roughing cuts. It
is then necessary to resharpen it before taking the fine finishing cuts.
To reset the tool in the lathe first get the angles correct, as shown
in Fig. 16. Then revolve the lathe forward by hand to take up the slack
in the gears and move the tool in close to the threads. If the tool is
in a position so that it will cut too much off one side of the thread,
it may be changed by disengaging the reversing gears with the lever R
and turning the lathe by hand. When the tool is in the proper position
relative to the groove of the thread, the reverse gear lever R is reset.
In a case where the tool is off the desired position only a very
little, it may be corrected by the rapping process.
If the lathe has a compound rest the tool may be brought to the correct
position by turning the hand-crank M.
It would be well for beginners to practice thread cutting on a piece of
scrap steel before trying to cut them on the exercise.
EXERCISE No. 3.