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Copyright © 2020 Quality Machine Tools, LLC
PM-1440GT v9 2020-10
THREAD CUTTING
External change gears
The large gears in Figures
3-18 are
transposing gears
, 120T
and 127T. They allow a standard-thread leadscrew, in this
case 4 TPI, to cut metric threads. The transposing gears are
keyed together.
KEY FACTS TO REMEMBER ...
TPI threads
When configured for inch thread cutting (30T upper, 60T low
-
er), with just one exception the lathe cuts
all UNC and UNF
threads
from 1/8” (#5) to 4” diameter, 44 TPI to 4 TPI, without
the need to change gears. The exception is 13 TPI, which re-
quires 65T as the lower gear.
For inch thread cutting, the 127T larger gear is simply an
idler
,
transferring the drive from the upper gear to the lower gear.
In this configuration, the spacer bushing is
outside
the lower
gear, as Figure 3-18.
Figure 3-16
Configuration for TPI thread cutting
For all TPI threads (U.S.A.) the 127T gear is an idler between upper
and lower gears.
Gear swapping
Any change to the drive train typically calls for one or both
of the upper and lower gears to be exchanged for a larger or
smaller gear. This will require the transposing gear pair to be
repositioned. The procedure for this is:
Figure 3-18
External change gears
Gear shifters
P-X
P-X
Q-X T-Z
R-X R-X Q-Z
R-X
R-W
Low
er
gear
60
65
60
60
57
60
60
69
60
A-D
3
3-1/4
4
4-1/2 4-3/4
5
5-1/2
5-3/4
7
B-D
6
6-1/2
8
9
9-1/2
10
11
11-1/2
14
A-C
12
13
16
18
19
20
22
23
28
B-C
24
26
32
36
38
40
44
46
56
Figure 3-17
Threads per Inch (TPI)
1. Remove the M6 socket head screws from the upper
and lower gear shafts.
2.
Remove the gears, washers, keys and bushing (lower
gear only).
3.
While holding the gear support casting (quadrant) with
one hand, use a 19 mm wrench to loosen its anchor
nut. Allow the casting to swing downward.
4. Loosen the 19 mm hex nut securing the transposing
gears to the support casting.
5.
Install the lower gear (for TPI threads the lower gear
spacer is outside, for metric threads, inside).
6. 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.
7. Tighten the transposing gears in position, then re-
move the paper. Check for working clearance be-
tween the gears.
8.
Install the upper gear.
9. Swing the gear support casting upward to mesh the
127T gear with the upper gear, again using a paper
scrap for separation.
10. Tighten the gear support casting.
11. Lubricate the gears.
Metric threads
With the change gears supplied the lathe cuts all COARSE
metric threads from M3 to M36 (pitches 0.5 to 4.0 mm) and all
FINE metric threads from M4 to M100 (pitches 0.5 to 6.0 mm).
For metric thread cutting, the lower gear is driven by the 120T
transposing gear. In this configuration, not shown, the spacer
bushing is
inside
the lower gear.
