Cutting 60º External Threads Using a Thread-Chasing Dial
What
are the steps to apply external right-hand threads?
1.
Set the spindle speed at
one-quarter the normal turning speed.
2.
Set the quick-change gearbox
for the desired pitch using the index plate or change wheels.
3.
Set the leadscrew to turn in
the forward direction. When engaged, the carriage will move from right to left.
4.
Set the tool height on center
and the compound slide angle to 30º.
5.
Mount the work (between
centers, in a chuck or in a chuck plus a center, in a collet or in a collet
plus a center) and insure it is secure and there is no end-to-end play.
Lubricate the lathe centers, if they are used.
6.
Set the tool at right angles
to the work using the thread center gage or fishtail, Figure 7–132 and Figure 7–136.
7.
Measure the diameter of the
work. Good practice is to have the work diameter 0.002 inch (0.05 mm)
under
the
thread major diameter.
8.
Start the lathe and chamfer
the right end of the work to just under the thread final minor diameter using
the left edge of the cutting tool.
9.
Measure and mark the work
where the threads will end by cutting a light ring with the threading tool bit.
Crank the tool bit away from the work so the carriage can be moved. The spindle
will still be turning. See Figure 7–138.
10.
Manually position the carriage
toward the right end of the workpiece and, using the compound rest, move the
tool bit towards the work so it just lightly marks it.
Set both the cross slide and compound micrometer
collars to zero
. See Figure 7–138
.
11.
Back the tool bit away from
the work using the cross slide and position the carriage so the tool bit tip is
⅛
inch or more to the right end of the work, the
starting point.
12.
Using the compound slide, move
the tool bit into the work between 0.001 and 0.003 inches with your left hand
on the compound crank. Engage the half-nuts lever with your right hand when the
thread dial reaches the right graduation. This will make the first, or scratch
pass.
13.
When the tool reaches the left
end of the cut, quickly crank the tool away from the workpiece using the cross
slide crank, and disengage the half nuts.
14.
Move the carriage manually back to the starting
point and set the cross slide back to the zero position.
15.
Stop the spindle, and using
either a thread pitch gage or a ruler, check that the cut is the desired pitch.
See Figure 7–139. Correct the gearbox setting and try again if the thread count
is wrong.
16.
Restart the spindle.
17.
Manually move the carriage to
Position 1 as in Figure 7–140. Then, using the collar calibrations, move the
cross slide to the zero position, Position 2. With the compound, advance the
tool bit in preparation for the next thread cut, Position 3. Use Table 7–9 as a
rough guide for the depth of each pass. The idea is to use cross slide motion
to pull the tool away from the workpiece at the end of the thread and move the
tool back to the zero position for the next pass. Once the compound collar has
been set to zero, the compound is incremented for each pass and is never used
to pull the tool away from the work.
18.
Engage the half-nuts lever with your right hand
when the threading dial reaches the correct graduation and the tool will move
from Position 3, through Position 4, and to the end of the threads at Position
5. When the tool reaches Position 5, use the cross slide to quickly withdraw
the tool to Position 6. Then manually move the carriage back to Position 1. You
are now ready to begin the next cutting pass.
19.
Tables in
Machinery’s Handbook
show the final thread depth for all common thread
forms. This depth can be read off the compound collar and can be checked using
a threadingmicrometer. Also, inch thread center gages have the thread depth
for various tpi engraved on them. See Figure 7–132. Not having this information
at hand, the operator can watch the thread develop until the width at the
thread bottom (root) just about equals the width of the thread top (crest).
Then take additional small cuts until the mating part or master nut fits
properly. Figure 7–141 (a through d) shows the thread development. Repeat steps
17 and 18 until the threading reaches full depth and is completed, as shown in
Figure 7–141 (e).
20.
Use a file to remove burrs on
the top thread edge.
Cutting
60º external threads on a lathe equipped with a thread-chasing dial is the most
common and least complicated threading operation.
Copyright © 2004 Metal Arts Press