Hello Dave,

Yes, I can help you.

There are two possibilities, one involving a 23 tooth driver matched to 12 tpi on the gearbox. Pitch = 2.029 mm

The other uses a 25 tooth driver matched to 13 tpi on the gearbox. Pitch now a near perfect 2.035 mm

12.5 tpi has a pitch of 2.032 mm; your 2 mm pitch gearing would actually do perfectly well over a short threaded section, the error is only 32 microns which is approximately 1.25 thou per inch of thread.

The choice is yours

Regards

Brian

One way to take out the gearbox is to set it to 8 tpi and add change wheels to achieve what is needed. The gearbox is then 1:1. Or 4 or 16 tpi etc if that helps with the change wheels you have.

What may be an easier way of looking at it is that 0.64 is 16/25 so tooth counts that are based on that are needed.

There is a program on lathes co uk where you enter the gearbox ratios and what gears you have and then tell it what you want. It will sort it out for you.

John

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Is the quadrant aspect a Myford thing Harold? The quadrant on my boxford is identical to the ones on the none gearbox lathe as far as I am aware. The problem with the gearbox really is that it can add a lot of other factors to even a compound gear train. What I did initially was to generate a spread sheet that included the basic gearbox ratio's account for the stud and tumbler arrangement and tried various gear arrangements on it, The program on lathes co uk is a lot easier but I think that the preset gearbox ratios it comes with are for a boxford. It doesn't take long to work out what these are and add them anyway.

John

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If it is 11.5 then swap the 24 tooth driver gear to a 25 and use 12 tpi on the box.

If it's 12.5 they again used a 25 tooth gear and select 13 tpi on the box.

Simples.

The situation with the Myford John WI is that when fitted with a gear box the quadrant is used primarily to switch from thread cutting to fine feed. A clip is swung out of the way, a pair of gears removed and replaced the other way round and the clip replaced, this is very quick and easy as the gears are linked together. The setup is then in fine feed mode. However, unfortunately, the quadrant does not lend itself to fitting a conventional gear train and an alternative quadrant has to be obtained. Also gears for the purpose are not therefore supplied with a lathe fitted with a gear box. I do not know if any other lathe makes have this limitation.

In my case, I purchased a gear box, with the necessary quadrant, etc, to fit to the lathe, as a result I still had all the gears in the standard set together with the quadrant for non gearbox lathe. The quadrant did not fit the lathe, now fitted with a gearbox, and wanting to set up for a worm gear, it was a case of make or purchase a quadrant to suit the gear box, I actually decided to make one.

The program on Lathes co uk sounds interesting, I must find time to look at it.

Harold

Hello Dave,

Unfortunately the quadrant that is used in combination with the gearbox has two fixed position pins that carry the 57/19 cluster gears, one of which, the one nearest the wide 72T wheel, is reversible for fine feed.

It is I believe a unique arrangement and it isn't possible to mount other gear trains on it. The alternative quadrant which Myford sold to cut metric threads has variable position changewheel studs that allow many gearing arrangements to be set. You can of course set the gearbox to 8 tpi as a 'neutral' and arrange changewheels to give you 11.5 TPI gearing, but in doing so you are ignoring completely the option of exploiting the gearbox to better advantage.

By fitting a 25 T driver instead of the usual 24 T gear and selecting 12 tpi on the gearbox, the combination will cut a thread of 11.5 TPI with a very small error of 0.18% and you will have saved yourself all the hassle of changing quadrants, building up a gear train and meshing the wheels, maybe 1/2 hour or so of fiddly work.

I hope this helps you decide on the way ahead

Regards

Brian

Edit. Just to add a little further information. Setting 8 tpi on the Myford gearbox to give a 1:1 'neutral' drive  doesn't always coincide with an 8tpi setting on other lathe/gearbox combinations that are fitted with 8tpi leadscrews. The Machin version gearbox that can be fitted to this lathe gives a 1:1 ratio when the tpi setting is 16 TPI; this is a result of the way it is built.

Another example, the Little John lathes with 8 tpi leadscrews give a 1:1 ratio through the gearbox when it is set for 20 tpi

Edited By Brian Wood on 01/09/2015 09:59:49

Ian, when you say "converted", what do you mean? Even the "new, metric" Super 7 has the standard 8 tpi leadscrew and can only cut approximations to metric threads unless you have the 127 tooth gear.

However, the approximations can be perfectly fine for all practical purposes at least for shortish fasteners, and a short search on this site will find a number of threads with tables, or pointers to tables, for cutting metric threads on the Myford 7 lathes. When you say "just under" 1.25 how much is "just"?

For example see this: http://www.model-engineer.co.uk/forums/postings.asp?th=86738&p=1

It would be a great service to us all if the people who operate this site added an FAQ facility as this is a topic that comes up time after time.