Gear chart for inch threads
Each box along the top of the chart shows the
number of TPI: 8, 9, 10, etc. Below each TPI
setting is a chart of four gear positions: A, B, C
and D. The smaller numbers below each TPI
setting designate the number of teeth on the
gear at each position.
Thus for a 12 TPI thread, the gears would be
A:50, B:100, C:127, D:60.
The next exercise will cut a ¼"-20 thread, or 20
threads per inch. The gear chart under the
heading 20, shows that the following gear train
A:30, B:120, C:127, D:50.
The letters A, B, C and D correspond to four
positions on the gear shafts to indicate which
gear goes in each position. In the photo below,
the gear shafts have been labeled to make it
easier to visualize the position for each gear.
PDF created with pdfFactory Pro trial version
Note the spacer to the right of the D position.
For some gear trains, the D gear may go
where the spacer is shown, with the spacer
moved to the outer position on the shaft.
The 30-tooth gear goes on the end of the shaft
labeled A, the 120-tooth gear goes on the inner
position of the shaft labeled C-B, the 127-tooth
gear goes on the outer position of that shaft
and the 50-tooth gear goes on the shaft
marked D. The next photo shows the gears in
their respective positions before they are
locked in place with the lock washers and nuts.
Refer back to the gear chart above. Note that
no gear is used in the C position for a 36 TPI
thread. In that situation, the spacer ring on the
D shaft is removed and the gear at D occupies
the position where the spacer had been. Then
the spacer is replaced where the D gear
normally would go. In some cases, a small
gear such as the 30-tooth may be used as a
spacer without engaging any other gears.
The frame on which the B-C gears are
can be rotated to accommodate