Thread: Tyzack lathe
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Jim Wilkins[_2_] Jim Wilkins[_2_] is offline
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Default Tyzack lathe

"Stephen Dixon" wrote in message
...
Hi, I've just been looking at the questions above and notice someone
may have gear chart for screw cutting imperial & metric. Any chance
you could send it to me or point me in the right direction. Thanks Ste

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http://www.lathes.co.uk/zyto/

You can figure out the gearing and make your own chart, as I did using
a spreadsheet which computes compound gear ratios and inch pitch to
metric modulus conversions, and provides many formatting options for
the chart printout.

The key is the pitch of the leadscrew. When the spindle and leadscrew
are geared 1:1 the lathe will duplicate the leadscrew pitch. For other
pitches the gear reduction (or increase) ratio equals the leadscrew to
desired pitch ratio, for example to cut 32 TPI with an 8 TPI leadscrew
the gear ratio is 1:4, such as 16 and 64 tooth gears, or 20 and 80, or
whatever you have that fits. 13 TPI would require 8x4=32 teeth at
spindle speed and 13x4=52 teeth at leadscrew speed. The leadscrew
turns and advances slower to cut finer pitches. British thread pitches
are mostly the same as US ones.

Lathe change gears traditionally used the 14-1/2 degree tooth pressure
angle which made the teeth easier to lay out by hand on a wooden
casting pattern, as the sine of that angle is 1/4. Modern power
transmission gears commonly have a stronger 20 degree pressure angle
and don't mesh correctly with 14-1/2 degree "change" gears.
https://www.bostongear.com/products/...s/change-gears

The inch-to-metric conversion is a fraction with 127 as the
denominator, corresponding to a 127 tooth gear, because 1.00000 inch =
25.40000 mm.

jsw