How to make a large ring gear?

I want to add a gearbox within the flat belt pulley on the spindle of my Denbigh mill. At the moment the hang up is how to make a MOD 4 38 tooth ring gear.

Im going to make a prototype on my 3D printer to check everything will work on the mill but with a worst case scenario of 1.5KW at 50 RPM I don't trust PLA much.

Can anyone recommend a way to build the gear? Tools I have are a lathe, mill and 3d printer. My current thought is using the printer to make a mould for some sort of plastic or epoxy? Not sure what or how though. Wishing I had a shaper. Might try and out source it but feel like it could be too expensive.

External or Internal?

Ah yeah, ring gear of a epicyclic gear so internal teeth

Laser cut from steel?

Ian P

Well I have got to make a 24T 1.75Mod one for an engine I am working on at the moment and will be planing it on the lathe, with some pre drilling to get rid of the majority of the waste there won't be a massive amount of metal to remove on your 4MOD one. Even easier if cut from a slice of Oilon or sheet Tuffnol

Edited By JasonB on 15/12/2017 20:48:08

Rainbows

Have you verified the tooth shape for that ring gear and its associated pinions?

As I understand it there can be interference problems with physically large internal gear teeth, especially at relatively low tooth counts, and modifications to tooth shape et al may be needed for proper running.

Seems to be some detail on calculations here :- **LINK**.

Looks as if off the shelf ring gears start at around 50 teeth and go upwards. Usually significantly larger. Could interference issues with standard gears be the reason why?

Clive.

I think Andrew Johnston has been looking for information on internal gears for some time. That link may be useful to him.

Neil

Gear Engineering by H.E.Merritt gives the following advice:

"A general-purpose rule, for the avoidance of tip interference at standard centres, is to make the difference between the tooth numbers not less than

(T-t)min =10 + t/5

By strict geometry, the minimum tooth number difference at which tip interference begins is approximately 8, for 20 degree full depth proportions, at standard centres. But this is a marginal condition."

Using this advice I designed a small reduction gear box for a governor drive in 10DP with a 21 tooth pinion and 36 tooth internal gear. Addendum modification was +0.25 for the pinion and -0.25 for the internal gear. This was about as close as I thought it prudent to go. The gear boxes have been in service on US Coast Guard vessels for over 30 years now, and I have not heard of any problems. So I would say it's a fairly good rule to follow.

As far as actually cutting the gear, personally I would follow JasonB's advice and use the lathe to plane the profile.

Neil

Found a thread on practical machinist where they were arguing over laser cut gears. Couldn't tell if they were ok or trash from it though.

Might well end up broaching it on the lathe, will mean adding indexing to it but thats useful anyway

The reason for 3d printing it was so I would have a final check that I hadnt bodged any calculations before making it properly, think I fudged the gears though and the bowden tube went pop so thats gone well. Might have to actually do research on it now instead.

The 3d printing sounds interesting, will get in touch with you in a bit

Hi Rainbows

A while back I made a few Epoxy timing gears....
**LINK**

Also made some for a clock....
**LINK**

If your application is slow speed they work fine

For this sort of application use more Epoxy maybe 25% by volume
And plenty of release agent (Wax) so you can get the belt off

Bore the mould and bearing bore in one setting as shown and the result will be very accurate.

Belts are available with hundreds of teeth so high ratios can be achieved with one pair of gears.
Small pinions are available quite cheaply ready made, its not really worth making them unless you want the exercise.

Regards
John

Hi Rainbows
Hmm I missed that you want an internal ring gear, I was thinking an external gear train.
Regards
John

To recap, what you are looking to make is an internally toothed gear with 38 teeth and Module 4?

If this is for an epicylic gearset, unless I've got the wrong end of the stick I cannot see how you will have enough room for the sun and planet gears inside. A 38 tooth gear with external teeth is about 6" diameter, presumably an internal one would be a similar size.

Mod 4 has quite large teeth compared to gears which are common to typical small workshop machinery, even a 12" swing lathe changewheels are a lot smaller than mod 4.

Maybe I have misunderstood the application.

Ian P

I have a note which says "Extracted from Dave Lammas' article in ME 20.09.91 page 338". This appears to be one of a series of articles on Gear Cutting by Lammas. My extract concerns a method of creating an internal gear. The basic idea (check with the original article(s)) is that a tool steel cutter is made identical to the corresponding pinion. The cutter is then hardened & tempered, followed by the removal of several teeth leaving one tooth at say 12 o'clock, and sufficient teeth to cover an arc of 120 degrees centered on 6 o'clock. The single tooth is first used to carve, or gash, out the internal gaps, followed by the arc of teeth which then trim the sides of the internal teeth.

Apparently aluminium & brass cut quite readily, whilst cast iron & mild steel require more effort and some care.

Peter G. Shaw