8
PM-236 v4 -207.indd
STANDARD THREADS
Standard threads in the U.S.A., often referred to as “TPI”
(threads per inch), are mostly cut using the same exter-
nal gears as for the finer pitch metric threads — 24T up-
per, 48T lower. The table in Figure 3-9 lists all threads
available with that setup.
TPI
TYPICAL USAGE (standard threads)
16
3/4
3/8
18
5/8
9/16
5/16
20
1/2
7/16
1/4
24
3/8
5/16
#12
#10
28
1/4
#12
32
#10
#8
#6
36
#8
40
1/4
#6
#5
#4
48
#4
56
#3
Figure 3-9
Popular threads cut with the 24T/48T setup
and lower gear shafts.
Remove the gears, washers, keys and bushing (lower
gear only).
While holding the gear support casting (quadrant)
with one hand, use a 5 mm wrench to loosen the
M0 hex nut hidden under the 27T gear. Allow the
casting to swing downward.
Loosen the M0 hex nut securing the transposing
gears to the support casting.
Install the upper and lower gears.
Bring the transposing gears into mesh with the lower
gear, trapping a scrap of bond paper (letter stock) be-
tween the two to hold them at the correct separation.
Tighten the transposing gears in position, then remove
the paper. Check for working clearance between the
gears.
Swing the gear support casting upward to mesh the
27T gear with the upper gear, again using a paper
scrap for separation.
Tighten the gear support casting.
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CUTTING PROCEDURE FOR TPI THREADS
This procedure assumes that a single point thread cut-
ting tool will be used, and that the threading dial assem-
bly has been pivoted forward to engage worm gear with
leadscrew, Figure 3-4. Note that the threading dial is
not used for metric
threads.
For metric and UNC/UNF threads the tool is ground to
a precise included angle of 60
o
. It is installed so that its
flanks are exactly 30
o
either side of the cross axis, ideal-
ly with the compound offset as Figure 3-7. Single-point
threads are cut in 0 or more successive passes, each
shaving a little more material off the workpiece.
To make the first thread-cutting pass the leadscrew is
run at the selected setting, Figure 3-20, and the carriage
is moved by hand to set the cutting tool at the starting
point of the thread. With the tool just grazing the work-
piece, the split-nut lever is lowered to engage the lead-
screw. This can be done at any point,
provided
the split-
nut remains engaged throughout the
entire multi-pass
process
.
When the first pass is completed, the tool is backed out
clear the workpiece (using the cross slide), and the spin-
dle is reversed to bring the carriage back to the starting
point. The cross slide is returned to its former setting,
then the tool is advanced a few thousandths by the com-
pound for the next pass. Each successive pass is done
in the same way, each with a slightly increased infeed
settting of the compound.
Many users save time by disengaging the split-nut at the
end of each pass, reversing the carriage by hand, then
re-engaging, usually by reference to the threading dial,
Figure 3-2.
If the TPI number is divisible by 4 re-engagement can be
done at any point — forget the threading dial.
For all other TPI numbers every engagement,
includ-
ing the first
, must at the point where a
specific
line
on the threading dial comes into alignment with the da-
tum mark. If not, the second and subsequent passes will
be out of sync. In some cases, Figure 3-2, there is a
choice of lines for re-engagement, but in every case the
process calls for careful timing. [
NOTE:
Disengagement
and re-engagement of the split-nut is not applicable to
metric threads].
Typical depths of cut per pass vary from an initial 0.005”
or so, to as little as 0.00”, even less. A finishing pass
or two with increments of only 0.0005” (or none at all)
to deal with the spring-back effect can make all the dif-
ference between a too-tight thread and one that runs
perfectly. Assuming that the compound is set over at
between 29 and 30 degrees, the total depth of cut is ap-
proximately 0.69 times the thread pitch, P (this equates
to a straight-in thread depth of 0.6 times P). There may
be a need for a few thousandths more in-feed than
0.69P, almost certainly not less.
