Watch gear gearboxes - Page 6 - RC Groups
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Jun 27, 2006, 02:48 PM
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jberg's Avatar
Jon, you have to count. The "working diameter" (as I called it) is not the outer diameter but the diameter at about (not exactly) half of the height of the teeth. For getting the count of the teeth by measuring the diameter of the gear you need special mathematics or special tables and exact micrometer calipers. In the case of watch gears you also need a loupe. And you might fail anyway because you don't know to which standard the gear is manufactured. A watch gear can easily be based on cycloidic curves or on involute curves, both are difficult to calculate, and you don't know which amount of addendum modification was applied.

At the end of the day counting is the easiest way. The exact dimensions of gears and the shape of the teeth are a complex matter, as you can easily see on that page MegaTesla pointed to.

And, I wrote "(50+10)/2*0.2 = 6mm", you interpreted it as "(50+10)/(2*0.2) = 150", which is, I'm sorry, something different.

Regards, Jochen (mechanical engineer)
Last edited by jberg; Jun 27, 2006 at 02:55 PM.
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Jun 27, 2006, 05:55 PM
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billystiltner's Avatar
You can use my gear center calculator to calculate the gear centers and mod.
It is pretty close or close enough for our purposes to use the following formulas

Mod = module = the lenght in mm, of the pitch circle diameter per tooth
PCD = Pitch Circle Diameter
PCD(s) = PCD of spur
PCD(p) = PCD of pinion
C = Center Distance, this is the calculated value of the center to center distance
MA = Mesh Allow Distance, this is how much you want to add to C to allow the gears to run smoothly
Didel recomends 0.02mm for 6.67:1 and lower ratio 0.3mod gears
For the 0.2 module gears the mesh allow distance is 0.01mm

Mod = OD /(N+2)
PCD = N x Mod
C = (PCD(s) + PCD(p)) / 2

There was a problem with the javascript that caused errors in the calculator results on my site but it is fixed now.

Jun 27, 2006, 05:57 PM
Registered User
Wow, thanks Billy. that will make it much easier. For some reason, I didnt see that on your site. thanks though. It kind of sucks that i will have to count the teeth by eye to find the mod. oh well
Jun 27, 2006, 06:08 PM
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billystiltner's Avatar
Jonathon your PM box is full.
Jun 27, 2006, 07:22 PM
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rguillot's Avatar
Jon, you have to count the teeth with your naked eye. Using any kind of magnification is cheating

Jun 27, 2006, 08:25 PM
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billystiltner's Avatar
It really is cheating just like using your prop instead of carving my own.

Oct 08, 2006, 09:04 PM
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billystiltner's Avatar

I looked for the 1/4 bag of watch part soup at and cant find it.
Does anyone know if you can still get a 1/4 lb bag of assorted watch parts?

Oct 09, 2006, 12:37 AM
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for gears, etc.

Oct 09, 2006, 09:33 PM
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rguillot's Avatar
great gears???

EDIT: sorry just saw the gears
Oct 09, 2006, 10:04 PM
Registered User
they call them

train wheels (spur gears)
, clutch wheels ( crown gear...)
, cannon pinions (pinions...)
Oct 10, 2006, 05:07 PM
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billystiltner's Avatar

Thanks for the link.
The bulk watch parts are no longer available from clockworks or slarose.
clockworks supplier was slarose and they are out of business it seems.
The phone message at slarose says closded for inventory for 2 weeks.

Check out page 2 of there bulk items.
They have the ruby bushings 70 for $40.00.


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