Myford 33t and 34t gears for metric threads

[Mike DonnerstagParticipant @mikedonnerstag]

I’ve just read the article in MEW August 2018 on metric overlays for the Myford gearbox. This uses 33 and 34 tooth gears to replace the existing gear, to enable most of the common metric thread pitches.

Can anyone tell me how this compares with using the metric conversion set, as it certainly seems simpler and cheaper.

Can anyone tell me where I can source the 33t and 34t gears?

Many thanks, Mike

[Mike DonnerstagParticipant @mikedonnerstag]

[norm nortonParticipant @normnorton75434]

Mike

A google search for Myford 33T, Myford 34T comes up with lots of links.

Read this past thread for more information as well

https://www.model-engineer.co.uk/forums/postings.asp?th=125630

Norm

Edited to add that the John Stevenson mentioned in that thread is now no longer with us. But the gears are now made by others.

Edited By norm norton on 27/01/2019 09:58:32

Edited By norm norton on 27/01/2019 10:00:52

[Thor 🇳🇴Participant @thor]

Hi Mike,

It seems the 33T is available from RDG.

Thor

[Brian WoodParticipant @brianwood45127]

Mike,

The results are almost equivalent to the values you would get using the expensive conversion set, in practice you will see no sensibly measurable errors. However, the metric and BA pitches you can get with just these two gears is rather limited in extent.

Regards

Brian

[Mike DonnerstagParticipant @mikedonnerstag]

That's great Thor – I'll buy a 33T from RDG.

Does anyone know where I can get the 34T?

Mike

[egaParticipant @ega]

Posted by Mike Donnerstag 1 on 27/01/2019 10:44:30:

That's great Thor – I'll buy a 33T from RDG.

Does anyone know where I can get the 34T?

Mike

Widely advertised on eBay.

Brian Wood's book Gearing of Lathes for Screwcutting is a good read.

[peak4Participant @peak4]

Can't see any on ebay at the moment.
Try HPC Gears, who offer 34t 20dp in metal and Delrin.
OK would need modifying to fit, but probably easier than making from scratch.

Bill

[Roderick JenkinsParticipant @roderickjenkins93242]

Mike,

Just for interest I calculated the actual pitch achieved using the official Myford metric conversion set for the gear box. The actual figures are in the right hand column:

A similar calculation for the 33/34 option is shown below

So, the official set is an order of magnitude more accurate but for practical purposes the difference is negligible. For the finer metric threads some additional gears to the 33 and 34 are useful. I have the metric conversion set, it came withe my S/H lathe but when I needed to cut a 16mm x 2 thread last week I broke out the 34T gear. Much more convenient.

HTH,

Rod

[Rod RenshawParticipant @rodrenshaw28584]

Hi

I bought both 33 and 34T gears from the present Myford brand owner last year at one of the shows. I don't have any information about current stocks.

Rod

[Mike CrossfieldParticipant @mikecrossfield92481]

Take care if buying gears from HPC. Most of their standard gears are 20 degree pressure angle, whereas Myford change gears are 14.5 degree pressure angle.

[Roderick JenkinsParticipant @roderickjenkins93242]

Posted by peak4 on 27/01/2019 10:59:37:

Can't see any on ebay at the moment.
Try HPC Gears, who offer 34t 20dp in metal and Delrin.
OK would need modifying to fit, but probably easier than making from scratch.

Bill

We need to be a little careful here, Myford change gears are 14.5 degrees pressure angle.

Rod

[egaParticipant @ega]

Roderick Jenkins:

Thanks for the comparison; I regularly consult your stud gear table.

[Neil WyattModerator @neilwyatt]

Posted by Roderick Jenkins on 27/01/2019 11:08:12:

So, the official set is an order of magnitude more accurate but for practical purposes the difference is negligible. For the finer metric threads some additional gears to the 33 and 34 are useful. I have the metric conversion set, it came withe my S/H lathe but when I needed to cut a 16mm x 2 thread last week I broke out the 34T gear. Much more convenient.

Some of those Myford errors are pretty huge, approaching the wavelength of light…

And one of the other lot is almost six microns out!

Lean on the lathe while you are screwcutting and it may introduce a bigger error

Neil

[Peter SpinkParticipant @peterspink21088]

Hmm, was thinking of going this route but 33T and 34T changewheels not on the Myford or RDG site anymore.

At £263 for the metric conversion kit I wonder why 🤔

[Thor 🇳🇴Participant @thor]

Posted by Peter Spink on 27/01/2019 23:59:42:

Hmm, was thinking of going this route but 33T and 34T changewheels not on the Myford or RDG site anymore.

At £263 for the metric conversion kit I wonder why 🤔

The 33T seems to be available.

Thor

Edited By Thor on 28/01/2019 05:12:40

[ianjParticipant @ians]

When this subject came up in the past(10/2016) I bought a pair of gears from the late John Stevenson for £25 inclusive of UK postage. I'm sure John would have produced a "stick" of gears and parted them off to width. Maybe worth an email to Adam to see if he can lay his hands on any.

[Mike DonnerstagParticipant @mikedonnerstag]

Hi Ian,

Is Adam related to John? How can I contact him?

Many thanks,

Mike

[Roderick JenkinsParticipant @roderickjenkins93242]

I don't think 33 and 34 tooth gears were ever part of the old Myford inventory. I got mine from the late, great John Stevenson. Shortly before his sad demise Sir John mentioned on this forum that he was making a batch of gears – I wonder if these were for RGD Myford? If so it seems that they are running out. Perhaps there is an opportunity for a little entrepreneurship here.

Rod

[LambtonParticipant @lambton]

Mike,

No lathe with an imperial lead screw can cut a truly accurate metric thread unless a 127 tooth gear is used. Despite this fact both the official Myford conversion set and the 33, 34 toothed input gear method are capable of producing metric threads accurate enough for all practical purposes.

The Myford conversion kit is both very expensive, is inconvenient and time-consuming to set up and then take down. The 33, 34 method only takes a few moment to set up.

I cannot understand why anyone would find the need to screw cut BA threads as a good set of taps and dies together with a tailstock die holder will do a very good and accurate job. In almost 60 years of model (and full size ) engineering I have never known anyone screw cut a BA thread.

Eric

[Roderick JenkinsParticipant @roderickjenkins93242]

Posted by Mike Donnerstag 1 on 28/01/2019 09:23:25:

Hi Ian,

Is Adam related to John? How can I contact him?

Many thanks,

Mike

Adam runs the homeworkshop.org.uk website. A request on there for 33 and 34 tooth Myford change gears might do the trick.

[Mike DonnerstagParticipant @mikedonnerstag]

Fantastic – I'll give it a go.

Many thanks chaps!

[Peter SpinkParticipant @peterspink21088]

Posted by Thor on 28/01/2019 05:12:04:

Posted by Peter Spink on 27/01/2019 23:59:42:

Hmm, was thinking of going this route but 33T and 34T changewheels not on the Myford or RDG site anymore.

At £263 for the metric conversion kit I wonder why 🤔

The 33T seems to be available.

Thor

Edited By Thor on 28/01/2019 05:12:40

Many thanks Thor – that drop down box wasn’t working last night but is now so I’ve ordered a 33T for a tenner.

Already have a 34T 👍

[Roderick JenkinsParticipant @roderickjenkins93242]

Posted by Lambton on 28/01/2019 09:32

I cannot understand why anyone would find the need to screw cut BA threads as a good set of taps and dies together with a tailstock die holder will do a very good and accurate job. In almost 60 years of model (and full size ) engineering I have never known anyone screw cut a BA thread.

Eric

I agree. I use dies for anything smaller than M6/0BA but die cut threads can be wobbly so the information is out there for anybody needing it.

Rod

[SillyOldDufferModerator @sillyoldduffer]

Posted by Lambton on 28/01/2019 09:32:19:

…

No lathe with an imperial lead screw can cut a truly accurate metric thread unless a 127 tooth gear is used. Despite this fact both the official Myford conversion set and the 33, 34 toothed input gear method are capable of producing metric threads accurate enough for all practical purposes.

…

Being in pedant mode this morning, I have to insist Eric's opening sentence should be 'No lathe can cut a truly accurate thread'. Never mind if it's imperial or metric.

The reason is, no matter how expensive and well-maintained, the way a lathe works causes unavoidable errors – they are imperfect machines. Making a lathe that can cut accurate to rather better than a thou isn't hard. Making a precision lathe to cut accurately to tenths is far more difficult, and the machine has to be maintained in top condition and operated with extreme care. Achieving accuracy surpassing tenths on a lathe is so difficult it's better to dump the lathe in favour of some other technique. Highly accurate threads are made by Grinding and Lapping, not by turning.

Also in pedant mode, I don't believe it's true that a 127 tooth gear is literally essential, though it's certainly by far the most straightforward way of producing suitable ratios. But there are many other combinations of gears that can produce metric pitches on an imperial lathe. However, it's a tad theoretical unless you like hard sums – it's easier to have a 127 toothed gear rather than dozens of other gears and a computer!

I completely agree with Eric's main point though – very often you can get 'close enough' by putting 63, 33, or 34 in the chain. Or some other combination! Although it's reasonable to strive for the best result possible, it does seem quite a few hobbyists lose their sense of proportion and put a lot effort into trying to do better than necessary. Cherry Hill makes top-quality models – I polish turds…

Dave

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