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Copyright © 2020 Quality Machine Tools, LLC
PM-1228VF v3 2020-10
Figure 3-35
General setup for Metric threads
In this setup the lower gear is driven through the
translating gear pair, 86T + 91T.
Upper
gear
30
30
30
35
35
35
50
50
50
60
60
60
Gearbox
A
B
C
A
B
C
A
B
C
A
B
C
1
2
0.75 1.5
1.75
1.25 2.5 0.75 1.5
3
3
0.5
1
2
4
0.5
1
2
1
2
4
5
0.6
0.7
1
2
4
METRIC THREADS
METRIC THREAD CUTTING
Pitch Upper
gear
Gearbox
0.5
30
A4
0.5
35
A3
0.6
30
A5
0.7
35
A5
0.75
30
B2
0.75
60
A2
1.0
30
B4
1.0
35
B3
1.0
50
A5
1.0
60
A4
1.25
50
B2
Pitch Upper
gear
Gearbox
1.5
30
C2
1.5
60
B2
1.75
35
C2
2.0
30
C4
2.0
35
C3
2.0
50
B5
2.0
60
B4
2.50
50
C2
3.0
60
C2
4.0
50
C5
4.0
60
C4
Metric threads sorted by pitch
0.8 mm pitch
For 0.8 mm use the setup for 32 TPI,
Figure 3-33. For practical purposes
the two pitches are interchangeable:
8 mm = 0.31496”, 1/32 = 0.31250”
How a non-metric leadscrew cuts metric threads
This is done by “translating” the leadscrew drive, running it slower than
for US threads by the factor 86T/91T = 0.945. To see how this works,
compare the US setup for 8 TPI (0.125” pitch) with the metric setup
for 3 mm pitch. Both use the same 60T upper gear and the same C2
gearbox setting, but the leadscrew turns more slowly — so 0.125“ be-
comes 0.125 x 0.945 = 0.1181” = 3 mm.
Figure 3-36
Setup for 13, 19 and 26 TPI
This is like the metric setup, but with 56T as the middle gear. On the gearbox select
C & 2 for 13 TPI, B & 2 for 26 TPI. For 19 TPI use the 30T upper gear with C & 2.
How the 13 TPI setup works ...
The usual metric setup, Figure 3-35,
assuming a 60T upper gear, gives an
overall shaft ratio of 60/91 x 86/90 =
0.630. With gear selections C and 2 this
cuts a 3 mm pitch thread.
The 13 TPI setup uses the same 60T
upper gear, but a smaller middle gear
(56T), giving a shaft ratio of 60/91 x 56/90
= 0.4102. This turns the leadscrew slower
by the factor 0.4102/0.630 = 0.6511.
What would have been 3 mm pitch
becomes:
3 x 0.6511 = 1.9533 mm
= 0.0769” pitch = 13 TPI.
SPECIAL CASE: 13TPI
