Try doing a site search to include “Andrew Johnston” and “bevel gears”. He was (still is?) building a couple of traction engines and posted a number of times about cutting bevel gears.  He is no longer an active participant I believe, though I suspect he takes peek now and then. You might get lucky and get some input from “Anonymous “.

John

I believe Andrew did the diff bevels on his CNC due to not wanting o compromise with parallel depth ones. Though still the font of knowledge when it comes to gear cutting, Julie Ann knows as much and is a member.

Also be aware there is an error in Laws book that does not show up in his 45degree example as at that angle Cos and Sin give the same result.

I suppose these days you could generate the gears in CAD, 3D print a pattern and then have the gears cast just needing the bores and faces machining. Several model engine suppliers do cast gears so they do work including my own Fowler.

On 11 December 2025 at 11:08 Michael Smith 15 Said:

I will do a practice run on a piece of aluminium first .

It would be possible to 3D model the gear (using the shape of your actual chosen cutter rather than the theoretically correct one) and then 3D print its inverse (i.e. the print would be the tooth space, the material removed during machining). That could be used as a gauge to assess your aluminium test piece.

The teeth sounded quite fine to me, my 2″ engine has 6DP final drive same as the OPS but somewhat less teeth

The bevel gears of my diff are 4.205″ OD so about 1/3rd of the OPS 12″ but are 8DP so probably 2.5 or 3DP on a 6″ model. But the 12″ diameter would be about right.

There are certainly quite a few model designs that use the same DP for all the drive gears possibly to keep the cost of cutters down and allow for smaller machines for the builder so that may be why the large tooth count and small teeth are specified. Mu Minnie is all the same but the Fowler has 10, 8 & 6DP to better represent the original.

Ah there is a build of one of those on TT but the guy used a Gleeson to do his diff

Equipment is similar to what I have, except I don’t have the bigger lathe.

Some pretty impressive setups and machining and the two cutouts answer my question about the number of pinions.

The drawing indicates that the bevel gears have been designed using the DP at the outer edge, which is the normal method but assumes tapered teeth. Converting to the parallel depth method while retaining the correct spacing of the gears is non-trivial. I’ll have a closer look at the maths tomorrow.

Julie

On 12 December 2025 at 09:05 Les Riley Said:

I have a spreadsheet that will do all the calcs for you….

That will definitely help a lot.

I’d select a DP at the inner edge (probably 5 or 6) and keep the number of pinion and gear teeth the same. Then one can use the spreadsheet to look at the values for the pitch circle diameters. Since the OP has clearly already machined the differential centre, and possibly other parts, it is important to keep the intersection of the pinion and gear PCDs the same. Finally the thickness of the gear blanks can be adjusted to give the correct distances, inside and outside, of the bevel gears so they fit with the other parts.

Julie