(very) Early Colchester Bantam change gears

[Paul Relf-DaviesParticipant @paulrelf-davies37806]

Hi all,

I’m working towards bring an pre-war (perhaps 1932, based on the serial mumber) Colchester Bantam lathe back into service.

As part of the initial assessment I gave the machine I measuerd all the change gears and discovered they are all 14DP, with an 8-splined 13/16″ bore.

However, I bit of rooting round round the internet suggests that Bantams (post-war I assume) use 16DP gears…?

Assuming pre-war Bantam change gears are now effectively unobtainium, are there any other lathes who’s change gears might be compatible?

cheers

[DC31kParticipant @dc31k]

A few early Colchesters used 14DP gears, including ‘larger’ models than the Bantam.

Drummond lathes use 14 DP, Harrison L5 and 140 use 14DP, Chipmaster uses 14DP. I saw one reference that Raglan use 14 DP (but do not know this for sure).

Of these, Drummond gears are most easily available, but the centre detail is different.

This might be an option: buy some modern 16DP gears to get the splined centre and 3D print a 14DP gear around the outside.

https://www.thingiverse.com/thing:1808219

 

[Paul Relf-DaviesParticipant @paulrelf-davies37806]

Thanks for rour reply, DC31k

I had been thinking (assuming I couldn’t find any ‘off the shelf’ gears to work) of sacrificing one ofmy existing duplicate gears – for some reason I seem to have 4 x 35T for some reason – to get the correct splined bore and buying a generic 14dp 28 spur gear. I could then machine them both to an interference fit & press them together.

However, it hadn’t occured to me that I could 3D print some or all of the required gear…would be be strong enough? I know change gears get machined from nylon/delron, but I haven’t heard of them being printed before… I’ll give it a try.

[Paul Relf-Davies]

On Paul Relf-Davies Said:

…would be be strong enough?

Give it a try as you say.

Search for ‘3d printed change gear’ to learn of other people’s experiences.

In subtractively-machined gears, the tooth root is a byproduct of the cutter shape. In additively manufactured gears, you can increase the root fillet to the maximum before interference takes place, thus greatly increasing the tooth’s bending strength. As long as your CAD program offers this facility… FreeCAD does offer this.

If you are making something like the Thingiverse item, do not be tempted to make the inner profile as an internal gear as the fit will not be good – you  have to subtract an external gear to make the internal shape.

Make a small gear to start with (possibly including the centre spline detail) and place it in the drivetrain such that it is the most highly loaded one in the train.

My guess is that would be directly on the leadscrew with the overall ratio being set up for a very coarse thread (i.e. 1:1 or greater ratio between spindle and leadscrew). For testing, it does not need to be a ‘real’ thread – if the ratio happened to  3.268 tpi, that would be OK.

Give it a torture test and see how much abuse is necessary to make the gear fail.

A loctite fit is easier to achieve than an interference fit. Back it up with a Dutch key if paranoid.

[Paul Relf-DaviesParticipant @paulrelf-davies37806]

Thanks again, DC31k.

My current project is to add a reverse tumbler mechanism to the lathe.

To this end, the trickiest gear I need to come up with is the output gear, as it needs to be both 14DP and have the correct 8-splined bore. The other gears will just need smooth bores, so should simpler to acquire.

 

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