09-02-2012, 12:44 PM
Looking at the pics, it's pretty definitely 2 tpi... A lot of older lathes had relatively coarse leadscrews as most of the threads they had to cut were pretty coarse too :)
If you're making a nice new numbered dial with a friction fit to replace the old centre-punched one, maybe it'll be worth knurling the edge for adjustments?
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If you're making a new gear too it's probably going to want to be 16 teeth, as an 8-tooth gear will have some funny tooth shapes and quite a pronounced helix... even a straight-cut 16-tooth gear gets tricky with a 14.5-degree pressure angle!
Some calculations for the gear (if you need to):
Assuming you have a 29-degree Acme leadscrew so want 14/5-degree pressure-angle gears - bear in mind this is a "rough" gear, you'd have some errors creeping in from setting it over to match the helix angle of the leadscrew... Using something soft like brass, aluminium, delrin or nylon would be a good idea, the gear would wear instead of the leadscrew...
The gear won't be a particular whole-number diametral pitch (the pitch will be 1/2" *circular*), you could calculate it as Pi/(a half) or 2 pi DP - not an off-the-shelf gear!
the initial diameter of the gear blank would be (teeth + 2)/2 pi - so for a 16-tooth, 18/(2 pi), OD = 2.865" - you could reduce this a touch for clearance without much of a problem.
If your leadscrew has a keyway and the gear's made of something *soft*, you could then "gash" the gear blank for the 16 tooth spaces and slowly feed it (rotating on a bearing) in against the rotating leadscrew, it'd cut its own tooth profile?
If not, a hob copying the leadscrew (diameter, pitch and thread form) and fluted like a tap will do a very good job if run in the lathe with the (gashed) gear blank free-running against it.
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Interesting piece of vintage iron, that - the back-gear cover looks very familiar, but I can't place it...
Dave H. (the other one)
If you're making a nice new numbered dial with a friction fit to replace the old centre-punched one, maybe it'll be worth knurling the edge for adjustments?
================== Probably unwanted info starts here ==========
If you're making a new gear too it's probably going to want to be 16 teeth, as an 8-tooth gear will have some funny tooth shapes and quite a pronounced helix... even a straight-cut 16-tooth gear gets tricky with a 14.5-degree pressure angle!
Some calculations for the gear (if you need to):
Assuming you have a 29-degree Acme leadscrew so want 14/5-degree pressure-angle gears - bear in mind this is a "rough" gear, you'd have some errors creeping in from setting it over to match the helix angle of the leadscrew... Using something soft like brass, aluminium, delrin or nylon would be a good idea, the gear would wear instead of the leadscrew...
The gear won't be a particular whole-number diametral pitch (the pitch will be 1/2" *circular*), you could calculate it as Pi/(a half) or 2 pi DP - not an off-the-shelf gear!
the initial diameter of the gear blank would be (teeth + 2)/2 pi - so for a 16-tooth, 18/(2 pi), OD = 2.865" - you could reduce this a touch for clearance without much of a problem.
If your leadscrew has a keyway and the gear's made of something *soft*, you could then "gash" the gear blank for the 16 tooth spaces and slowly feed it (rotating on a bearing) in against the rotating leadscrew, it'd cut its own tooth profile?
If not, a hob copying the leadscrew (diameter, pitch and thread form) and fluted like a tap will do a very good job if run in the lathe with the (gashed) gear blank free-running against it.
======================= End of unwanted info ==============
Interesting piece of vintage iron, that - the back-gear cover looks very familiar, but I can't place it...
Dave H. (the other one)
Rules are for the obedience of fools, and the guidance of wise men...
(Douglas Bader)
(Douglas Bader)