14
PM-1440E-LB 11-13-17 V1
Copyright © October 2017 Quality Machine Tools, LLC
Gear shifters
R-4
R-1
S-3
T-4
V-1
R-3
T-1
V-3
V-3
Upp
er
gear
56
60
60
30
60
60
30
60
56
Low
er
gear
60
60
60
60
60
60
60
60
63
A-D
7.0
6.0
-
5.0
-
4.5
4.0
-
-
B-D
3.5
3.0
-
2.5
-
2.25
2.0
1.8
1.6
A-C
1.75
1.5
1.4
1.25
1.2
-
1.0
0.9
0.8
B-C
-
0.75
0.7
-
0.6
-
0.5
0.45
0.4
Figure 3-14
Metric thread pitches (mm)
For all other 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 sec-
ond and subsequent passes will be out of sync. In some cas-
es, Figure 3-13, there is a choice of lines for re-engagement,
but in every case the process calls for careful timing.
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.
GENERAL RULES FOR THE THREADING DIAL
1. Divide the TPI value by 2: If this gives an
EVEN whole
number
, example 12/2, re-engage at
any line
on the dial,
also
mid-way
between the lines.
1. If the ÷ 2 result is an
ODD whole number
, examples 10/2,
14/2, re-engage at any
line
on the dial, but not mid-way
between the lines.
2.
If the TPI value is a whole number not divisible by 2, ex
-
amples 7, 13, re-engage on the start line, or any line at
right angles to it.
3. 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!
Figure 3-13
Threading dial visualization for selected U.S. threads
GEAR SELECTIONS
FOR METRIC
THREADS
The threading dial
cannot be used
for metric thread
cutting — leave the
split nut engaged!
