12.5 TPI myford super7

Can anyone help? I have a myford super7. I seem to be able to find more information on cutting metric threads than imperial ones. perhaps thats because the common imperial ones are easy to select on the quick change gearbox.

I want to cut a 12.5 TPI NTP thread but it's not listed on the chart on top of the gearbox. Should it be easy to select this pitch?

I was thinking that its close enough to 2mm pitch and for my requirement I could probably make that fit, I have the gears that I'd need to cut a 2mm thread but thought that since its an imperial lathe and I want to cut an imperial thread there is perhaps something that I'm overlooking?

Dave

Hi

Do you have 25 tpi on the gearbox, if so select change wheels to speed up the gearbox input 2:1 and use the 25 tpi setting.

Trevor

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

I don't know what threads are available on the QC box, but 12.5 tpi is 0.08 pitch. The leadscrew is .125 pitch so you want the leadscrew to rotate .08/.125 revs per headstock rev which is a ratio of 0.64. I'm sure that would be easy with the standard set of change wheels, but may not be available fro the ratios available in the box. In many ways change gears we more versatile.

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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If you go with John's idea of setting the gearbox to 8TPI then you may find the lists on my website of interest. I have calculated (well the computer has) every combination possible using 14 gears and an 8TPI leadscrew. This results in over 66000 combinations, extracting only those between 6TPI and 50TPI still leaves over 37000.

If you go to my page here and click on "12+ to 14" and scroll down to 12.5 you will find there are 17 combinations that achieve this number. Thirteen of which use 3 drivers and 3 driven but four combinations only 2 drivers and 2 driven which are obviously the ones to consider first.

The reason for quoting all is that the user can then chose a combination that can be set up using the available gears. One, two or three missing gears will rarely be a problem therefore. Unfortunately, most lists only list a single set up for a given TPI, this can easily give the beginner the idea that there is only one setup suitable and buying a missing gear to make the TPI required possible.

Having a gear box on the lathe unfortunately means that the fitted quadrant is unsuitable for setting up special combinations and a quadrant specifically for the purpose has to be purchased or made. If the later then this page on my web site may be worth looking at.

Incidentally, the list of combinations is particularly useful when worm gears are being machined as these almost always have very complex pitch value.

Harold

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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It may not help much but I think you will find that 12.5 is not a pitch used in the NTP thread series. Do you mean 11.5 which is a standard NTP pitch?

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.

JS's solution to the problem is probably OK for all practical purposes but it will produce threads with small errors of 0.16% & 0.17%. Try it and see if gives the desired result as it is an easy procedure as a 25 tooth change wheel is a standard Myford item.

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

Thank you all for the replies, It does seem that it should have been 11.5TPI not 12.5. I seem to have a fair collection of gears so I should be able to find a combination of gears to do the job. The information about the leadscrew pitch and setting the gearbox to 8 TPI to make it 1:1 is very helpful. armed with that information I can tackle the job with a bit more confidence. The information about the charts and tools to calculate rations is great as well.

I've not looked yet at changing the gears in the drive train but would I be right in thinking that the quadrant and gears that presumably came as standard with the lathe would be used in combination with the quick change gearbox for cutting the common imperial threads. For anything else I should change the quadrant to the one that I had previously assumed was for cutting metric gears. I assume that this quadrant gives a bit more flexibility for using different combinations of gears and so is best for metric or unusual pitches?

Dave

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

I have a Myford Super 7 which has been converted to metric and I need to cut an M10 thread. I appreciate I am being obtuse but I cant seem to work out the exact combination of change gears and gearbox setting to use. The Myford Metric Chart specifies a 45 tumbler stud but mine seems to be 35 and 60 Change Gear which I have and selecting Position 6 and Lever A on the gearbox is giving me a pitch of just under 1.25. I am using the 45T middle change gear and I wonder if this is what I am doing wrong. If any one could give me the combinations I should be using for each of the Tumblers and Change gears I would be very grateful

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.