Author Topic: Small Bevel Gears Wanted  (Read 410 times)

Offline pgp001

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Small Bevel Gears Wanted
« on: September 11, 2019, 08:34:15 AM »
I am looking for a pair of small bevel gears for the governor of a model engine, ideally they need to be 64DP 2:1 ratio.
That would give a wheel of 36T and 0.572" OD and a pinion of 18T and 0.319" OD

Other very near sizes would be considered, but I have some space constraints on the engine.

Thanks
Phil

Offline jadge

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Re: Small Bevel Gears Wanted
« Reply #1 on: September 11, 2019, 11:29:30 AM »
Wow, those are pretty dinky bevel gears. Not sure how I'd make them. I normally make bevel gears on a 3 or 4-axis CNC mill, depending upon the pitch cone angle, with ballnose cutters. The smallest I've made have been 16DP with a 1mm cutter. It'd be a pretty small cutter for 64DP.  :o

I might be tempted to try and make an involute gear cutter and cut the gears the old school way with three passes on a horizontal mill plus tilting dividing head. At this scale I suspect the errors inherent in this method would be very small.

By the way I calculate the OD values to be 0.5765" and 0.3092" respectively.

Andrew

Offline gbritnell

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Re: Small Bevel Gears Wanted
« Reply #2 on: September 11, 2019, 11:51:21 AM »
Here in the  States we have a company, Stock Drive Products, that stocks 64 DP bevel gears. They are a little pricey but then comparing to the time spent trying to make your own they might not be that bad.
gbritnell
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Offline jadge

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Re: Small Bevel Gears Wanted
« Reply #3 on: September 11, 2019, 12:00:01 PM »
Mystery solved! Davell Gears in the UK list the exact parts needed in brass or steel. The ODs they quote agree with Phil's figures. But their gears are cut using the Gleason system. In this system the gear addendum is decreased and the pinion addendum increased to aid smooth running. This accounts for the discrepancies in the calculated ODs.

Andrew

Offline pgp001

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Re: Small Bevel Gears Wanted
« Reply #4 on: September 11, 2019, 04:15:24 PM »
Hi Andrew

I had a quote from Davall yesterday, that is my reason for asking on here...................................................They quoted me 84.67 for one pair inclusive of postage @15 plus VAT  :'(

Phil

Offline jadge

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Re: Small Bevel Gears Wanted
« Reply #5 on: September 12, 2019, 08:38:59 AM »
Postage seems a bit high, but not a bad price all things considered. Still a fair wedge to hand over though.

I had a quick look at the CAD model for my governor gear. I reckon you'd need a 0.2mm ballnose cutter to form the root radius at the small end of the teeth. Drill Service have a 0.2mm HSS ballnose cutter at around 18. My experience is that if you buy one cutter you break it; buy two and you don't. I'm not sure if my CNC mill is up to running such small cutters. The smallest I've run so far is 1mm.

Andrew

Online Jasonb

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Re: Small Bevel Gears Wanted
« Reply #6 on: September 12, 2019, 11:42:01 AM »
Could you not buy a MOD 0.5 cutter (0.4 seem hard to find)and reduce the teeth to something like 15/30 to keep the overall size the same or even grind up your own cutter. At such a small size I should think you would only need one pass for parallel depth method.

Offline pgp001

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Re: Small Bevel Gears Wanted
« Reply #7 on: September 12, 2019, 01:20:28 PM »
Hi Jason

I have thought about making my own parallel depth bevel gears for this job, but having made a few sets in the past I know how time consuming it is on a manual milling machine.
I made a set of diff gears for my 3" Ruston Proctor tractor which were fairly big, and at the other end of the scale I made a tiny wee set for the compensating cross shaft on the 2" Clayton Wagon brakes.

My problem is I only seem to get the odd 1/2 hour stints in the workshop these days, and making these would be probably a fortnights worth of time when I could be making something more bespoke to the engine.

I have now actually found a pair of ever so slightly smaller bevel gears that might just do the job with some modifications, my plan is to rig these up on some adjustable mounting centres and try them out before I dust off my wallet off  ;D

Phil

Offline jadge

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Re: Small Bevel Gears Wanted
« Reply #8 on: September 12, 2019, 02:14:27 PM »
I'd add a caveat to using the parallel depth method. The design of straignt bevel gears uses the DP at the outer edge, while the parallel depth method uses the DP at the inner edge. Given that one is normally constained to a limited range of DP (if using commercial cutters) it can be messy to get close to the required OD at the outer edge. I originally intended to use the parallel depth method for my governor gears but it got so messy, and wouldn't fit the existing casting, that I gave up and machined them on the CNC mill.

Personally I've not used the parallel depth method, although decades ago I did use a similar method using a special involute cutter marked "BEVEL" which is narrower than standard.

Andrew