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Copyright © 2020 Quality Machine Tools, LLCPM-1440GT v9 2020-10
USING THE THREADING DIAL
Referring to Figure 3-23, the general rules are:
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. (This equates to the
16 choices on the dial plate, Figure 3-24.)
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.
4. If the TPI value is fractional, but becomes a whole number
when multiplied by 2, example 4-1/2, re-engage only on
the start line, or its diametrical opposite.
If in doubt, re-engage on the start line!
dial comes into alignment with the datum mark. If not, the sec-
ond and subsequent passes will be out of sync. In some cas-
es, Figure 3-23, 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 ap-
plicable to metric threads].
Typical depths of cut per pass vary from an initial 0.005” or so,
to as little as 0.001”, even less. A nishing pass or two with
increments of only 0.0005” — or none at all, to deal with the
Figure 3-22 Threading dial
Figure 3-23 Threading dial visualization for selected U.S. threads
Minimize wear by swinging the dial indicator assembly away from
the leadscrew when not in use
spring-back eect, can make all the dierence 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.
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