Author Topic: Cutting "mis-shapen" gears  (Read 3075 times)

Offline hopefuldave

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Cutting "mis-shapen" gears
« on: June 20, 2011, 02:09:25 PM »
I have in mind an attempt at making one or more dual-dials for t'lathe (similar to those one some Colchesters), reading in Imperial on one section, metric on t'other, and would need to make a pair of internal gears, one 127 tooth and one 120 *but both the same pitch diameter* to mesh with a (f'rinstance) 20-tooth pinion to give 125 thou" and 3mm per rev.

 :proj:

My thought is that if I make a pinion-shaped cutter with relieved teeth (e.g. 20T 48DP 20*PA) and use 127 and 120 tooth gearwheels coupled somehow to the cutter to index the gear blanks (a poor man's gear shaper?), I should be able to cut an "accurate" 127-tooth and a 120-tooth with somewhat mis-shapen teeth? They'd be under next to no load, and rotating slowly so I  suspect they'd probably stand up to the task, but any feedback / suggestions / outright criticism would be appreciated!

Thanks,
Dave H.
Rules are for the obedience of fools, and the guidance of wise men.

Offline djc

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Re: Cutting "mis-shapen" gears
« Reply #1 on: June 21, 2011, 04:11:35 AM »
I have in mind an attempt at making one or more dual-dials for t'lathe (similar to those one some Colchesters), reading in Imperial on one section, metric on t'other, and would need to make a pair of internal gears, one 127 tooth and one 120 *but both the same pitch diameter* to mesh with a (f'rinstance) 20-tooth pinion to give 125 thou" and 3mm per rev.
 
My thought is that if I make a pinion-shaped cutter with relieved teeth (e.g. 20T 48DP 20*PA) and use 127 and 120 tooth gearwheels coupled somehow to the cutter to index the gear blanks (a poor man's gear shaper?), I should be able to cut an "accurate" 127-tooth and a 120-tooth with somewhat mis-shapen teeth? They'd be under next to no load, and rotating slowly so I  suspect they'd probably stand up to the task, but any feedback / suggestions / outright criticism would be appreciated!

Sounds good. Have you studied the patents for the Gamet dials? They are available from www.freepatentsonline.com Nos. 3536031, 3696776 & 3996878. As you have realised, the non-127 gear differs if your imperial leadscrew is 8 or 10 tpi (120 for 8 tpi; 125 for 10 tpi). The Chipmaster manual contains an exploded view of these which I could send you if you like.

As for your gearcutting question, ask over at the Gearotic forum (http://gearotic.com/ESW/FavIcons/). It's not unusual to cut on a modified pitch circle; I have seen pictures by John Stevenson of 20, 21 & 22 teeth all cut on the same blank OD (similar to http://homepage.ntlworld.com/stevenson.engineers/lsteve/files/hob%20indexer15.jpg).

If I were doing this, I would generate the gears in Gearotic and then find a wire EDM shop to cut them for me.

If you cut them yourself, a shaper with an indexer is the easiest. You'd need a good magnifier to check your tool profile. Just be aware that for normal gears you cut the spaces between the teeth, hence the width of your normal cutter is not 50% of the circular pitch (Ivan Law quotes 52%). Clearly for an internal gear, this is reversed.