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

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

Widely advertised on eBay.

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

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

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 Jenkins:

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

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 🤔

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.

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

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

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 👍

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