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Copyright © 2021 Quality Machine Tools, LLC
PM-1228VF v3 2021-05
CUTTING PROCEDURE FOR TPI THREADS
This procedure assumes that a single point thread cutting tool
will be used, and that the worm wheel on the threading dial
assembly properly engages the leadscrew, Figure 3-32.
The threading dial cannot used for metric threads! The
split-nut on the apron must not be disengaged until the
threading operation is completed.
For metric and UNC/UNF threads the tool is ground to 60
o
(in
-
cluded angle). It is installed so that its flanks are exactly 30
o
either side of the cross axis, ideally with the compound offset
as Figure 3-31. Single-point threads are cut in as many as
10 successive passes, sometimes more, each shaving a little
more material off the workpiece.
To make the first thread-cutting pass the leadscrew is run at the
selected setting (tables on following pages), 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 workpiece, the split-
nut lever (Figure 3-26) is lowered to engage the leadscrew.
This can be done at any point,
provided
the split-nut remains
engaged throughout the
entire multi-pass thread cutting
process
.
When the first pass is completed, the tool is backed out clear
the workpiece (using the cross slide), and the spindle is re
-
versed to bring the saddle back to the starting point. The cross
slide is returned to its former setting, then the tool is advanced
a few thousandths by the compound for the next pass. Each
successive pass is done in the same way, each with a slightly
increased infeed setting of the compound.
Many users working on U.S. threads save time by disengaging
the split-nut at the end of each cutting pass, reversing the sad-
dle quickly by hand, then re-engaging, usually by reference to
the threading dial.
Figure 3-32
Threading dial (US threads only)
For most TPI numbers every engagement,
including the first
,
must at the point where a
specific
line
on the threading dial
comes into alignment with the datum mark. If not, the second
and subsequent passes will be out of sync. In some cases,
see the “visualization” Figure 3-34, there is a choice of lines for
re-engagement, but in every case the process calls for careful
timing.
GENERAL RULES FOR THE THREADING DIAL
1. Divide the TPI value by 2: If this gives an
EVEN whole
number
, example 12/2 = 6, re-engage at
any line
on the
dial, also
mid-way
between the lines.
2. If the ÷ 2 result is an
ODD whole number
, examples 10/2
= 5, 14/2 = 7, re-engage at any
line
on the dial, but
NOT
mid-way between the lines.
3. If the TPI value is a whole number not divisible by 2, ex-
ample 7, re-engage on the start line, or any line at right
angles to it.
If in doubt, re-engage on the start line!
Figure 3-33
General setup for UNC/UNF (TPI) threads
In this setup the 91T gear is only an idler.
[NOTE: Disengagement and re-engagement of the split-
nut is not applicable to metric threads — leave the split-
nut engaged throughout the entire process]
Typical depths of cut per pass vary from an initial 0.005” or so,
to as little as 0.001”, 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 difference 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 approximately 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.
Table of threads per inch
For 13, 19 & 26 TPI, see the page following Metric Threads.
Upper
gear
30 30 30 49 49 49 60 60 60
Gear
box
A B C A B C A B C
1
72 36 18 44 22 11 36 18
9
2
64 32 16
32 16
8
3
56 28 14
28 14
7
4
48 24 12
24 12
6
5
40 20 10
20 10
5
UNC/UNF THREADS
