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Myford super 7B Chuck threads

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Steviegtr01/01/2020 22:19:18
2422 forum posts
336 photos

I wonder if anyone can help here. I recently bought a super7B which I am delighted with, it came with a box of bits,, cutting tools, nice milling head. & some back plates along with a 4 jaw chuck. The 4 jaw chuck has no thread on the back so the backplate is wrong. I have taken my 3 jaw off & offered the backplates up. Out of 4 only 1 fits & the 3 others are binding on the threads after only 1 1/2 turns. Wrong thread. I had this once before with a Boxford I had. A careful look seems to offer no solution as they look identical. I have no thread gauges so cannot check. Does this mean they are metric or am I missing something. Anyway I will have to get a backplate from e-bay or even myford Halifax & fit the 4 jaw to it. But don't want to buy the wrong one. Any help gratefully received.

not done it yet01/01/2020 22:56:35
6733 forum posts
20 photos

Am I missing something?

You have a 3 jaw that fits OK and one other back plate that fits OK? How many do you need? Or is the back plate, that fits the lathe, the wrong size for the chuck?

There could be other reasons why the other back plates don’t fit - you just need to check out the details. Might even be the thread form.

Steviegtr01/01/2020 23:58:14
2422 forum posts
336 photos

Yes the lathe came with a 3 jaw & a 4 jaw chuck. The 4 jaw has the wrong backplate to fit my lathe. With it there was also 4 backing plates of various forms. The ones suitable to fit to the 4 jaw chuck will not screw onto the lathe threads. My question was are there different thread types. They look perfect & are not damaged. Even the pitch looks correct, but they will not go on.

Windy02/01/2020 00:39:18
887 forum posts
189 photos

I bought a S7 a few years ago with various chucks.

Most were a very good but tight fit.

Have you checked a bit of swarf or damaged thread is not causing binding.

Sometimes swarf can become embedded in the metal.

Robert Atkinson 202/01/2020 07:11:14
1195 forum posts
20 photos

Take an impression of the threads (spindle as well as back plates) with plasticine or similar and measure the pitch across several threads with a magnifying glass and digital callipers. This will give you a good idea of the pitch. Then slice through across the threads with a sharp blade and compare them end on for thread form.

Robert G8RPI.

bill ellis02/01/2020 08:35:19
71 forum posts
2 photos

I bought a S7 backplate from one of the well known suppliers a few years ago. When attempting to put it on the lathe it showed exactly the same characteristics (screwed on a little then bound up tight). I measured it with a pitch gauge and all seemed OK size wise. Eventually I borrowed a 1 1/8* x 12 Whitworth tap from a work colleague and ran that through. Worked a treat and the chuck with the new plate screwed on well just like all the others. Whilst the tap did not seem to remove a great deal of metal it was quite tight to get through.

Hopper02/01/2020 09:35:31
6195 forum posts
321 photos
Posted by Steven Edwards 1 on 01/01/2020 22:19:18:

... I have taken my 3 jaw off & offered the backplates up. Out of 4 only 1 fits & the 3 others are binding on the threads after only 1 1/2 turns. Wrong thread...

What you need to do is screw the 1 backplate that fits onto the lathe spindle, (assuming this is NOT the same one attached to your three jaw chuck) then machine that backplate in situ to match the recess in the back of your four jaw chuck. Usually there is a recess in the back of the chuck body and you machine a matching step on the backplate, after truing up the face with a light cut. Try to make the shoulder a snug fit so the chuck fits on without slopping around. Make sure the chuck seats on the backplate on the face area outside of the step too. The inner area of the face is left at clearance so there is no contact there between chuck and plate.

Then take the back plate out of the lathe, mark up the four bolt holes to match those in the chuck and drill them through in the drill press. Bolt chuck to backplate.

This is the standard way of setting up a backplate to match each individual lathe so it runs true on that lathe. I don't think you will be able to buy a finished backplate to fit your lathe that also matches your chuck -- there are too many different size and shaped chucks.

To make the other backplates useable, buy a thread gauge and determine they are 12 tpi and the inside diameter is about the same as on the one backplate that fits. If so, you may just need to purchase a 12tpi x 1-1/8" BSW tap from Tracy Tools or the likes and run through to clean out the threads a bit. Then use the above procedure to fit a chuck to the plate.

Martin of Wick02/01/2020 10:04:39
249 forum posts
5 photos

To answer your question, I am not aware that Myfords produced S7 lathe spindles of any other form than W12 1-1/8" (unlike Boxford where an export version had a metric spindle nose).

If you purchase a new backplate from any of the regular UK vendors, it should fit your Myford (unless your machine spindle has undergone some from of damage or alteration). However, my experience with some new backplates is that they can be quite (very) stiff to get on and off initially and may need the crests in the plate relieving.

Regarding your existing non fitting backplates which I assume have not been machined to fit a chuck, purchase a combination Whit/metric thread gauge and measure up the thread and recess as instructed above.

If they appear to be of the correct dimension, then it may be that a good clean and oiling, or passing a tap through as suggested above will retrieve them. If the backplates were home shop made, it is possible that someone cut the 12 tpi with a 60 degree metric form instead of the correct 55 degree Whit form - In that case I am not sure if using a tap to correct the thread is possible - someone here will be able to advise...

If after remedials the backplates fit your spindle tolerably well, then machine  as required to fit your chucks.



Edited By Martin of Wick on 02/01/2020 10:12:48

Edited By Martin of Wick on 02/01/2020 10:19:08

Swarf, Mostly!02/01/2020 10:39:56
645 forum posts
70 photos

I'm sure I remember something in the literature put out by the original (Beeston) Myford to the following effect:

Chucks and backplates sold with an ML7 or Super 7 lathe were factory fitted to that particular lathe. Backplates sold as spares were sized to require a light easing of the parallel register to ensure a close fit to lathe spindles that had done some work and might consequently have, by then, worn a bit undersize.

Of course, this is separate from any thread issues.

It might be pertinent to add that, a few years ago, I bought a set of taps sold as being 'Myford nose thread'. On arrival, I found that they were clearly marked '1⅛" UNC' which is a 60° thead form. The suppliers then sent me a set marked '1⅛" Whit' but I've never been able to detect any difference!

Best regards,

Swarf, Mostly!

Howard Lewis02/01/2020 12:11:56
6013 forum posts
14 photos

If the backplates are only entering 1 1/2 threads onto the Myford spindle, it looks as if the thread pitch / form does not match that of the spindle.

The Myford spindle should be a Whitworth form (55 degree ) x 12 tpi .. 1.125 diameter.


Check that the Myford spindle thread and register are correct, and undamaged or dirty.

Check the female threads in the backplates that do not fit, that they are the correct pitch and form. The internal diameter of the thread should be 1.0182 ignoring the radius on the top of the thread; so may be a few thou larger.

The backplates sold by suppliers, as being suitable for Myford should screw on. The OD of each backplate will need to be machined to be a tight fit in the register of each chuck. It would be worth lightly facing each backplate before turning the OD


Georgineer02/01/2020 12:58:10
564 forum posts
32 photos

When I bought my ML7 the 3- and 4- jaw chucks that came with it fitted perfectly, but nothing else would go on more than about one turn before jamming. On close examination I found that the mandrel thread was slightly deformed - goodness knows how - and a few minutes spent with a small file cured it.

If your other backplates go on as far as 1½ turns I would conclude that they are the correct pitchbut with a problem of fit. As others have said, a tap may cure it, if the problem is in the female thread. It's worth remembering that chuck threads should be a bit slack, to allow the register (which should be very accurately machined) to align the chuck as it is screwed home. So even if you bung the correct tap through a slightly wrong thread, the result will probably be perfectly satisfactory. You won't strip the thread unless you are really brutal with it.


P.S. If anybody wonders how I got ½ instead of 1/2, the secret (in Windows) is to hold down the Alt key and type 0189 on the number pad. There's a whole world of alternative and useful characters out there!

Steviegtr02/01/2020 13:34:18
2422 forum posts
336 photos

A great big thank you for all the above posts. I will inspect more carefully & measure to see if I can work it out. My original thoughts were that some of the backplates had some sort of metric thread on them, but I think the replies above are more logical & likely. Thanks again. Will report back on the subject when sorted out.

Mike Poole02/01/2020 13:57:49
3305 forum posts
73 photos

The critical parts of a backplate are the register surfaces, the face surface and the accurate diameter of the spindle nose, a tight thread is likely to conflict with the registers so up to a point a loose fitting thread is desirable.


Steviegtr02/01/2020 14:07:38
2422 forum posts
336 photos

Thanks for that.

Harry Wilkes02/01/2020 14:17:22
1325 forum posts
65 photos

Hi Steven when I purchased my S7 not all the chucks that came with it screwed on the spindle some I purshased a tap ***link***

I used the tap to clean out the threads and all was fine, however if you do purchase a tap do not use force


Swarf, Mostly!02/01/2020 14:39:10
645 forum posts
70 photos

Posted by Georgineer on 02/01/2020 12:58:10:



P.S. If anybody wonders how I got ½ instead of 1/2, the secret (in Windows) is to hold down the Alt key and type 0189 on the number pad. There's a whole world of alternative and useful characters out there!

A useful tip, George. I use another method - go into 'all programs', then into 'accessories', then into 'character map'. I keep it pinned to the task bar. That's how I got '1⅛" Whit', well the '⅛' anyway!

Best regards,

Swarf, Mostly!

ega02/01/2020 14:51:23
2496 forum posts
200 photos

Early in my S7 ownership I solved the tight chuck problem by very carefully "screwcutting" the spindle nose thread with a tool that closely matched the correct form.

On the rare occasions that the lathe is used with the spindle nose not in use I protect it with a plastic cover.

Steviegtr02/01/2020 23:55:13
2422 forum posts
336 photos

Parts aquired with latheCross sectionBackplatesSome pictures of what I have to work with. Going to Rufforth jumble on Sat morning so maybe pick up a 1 1/8" bsw tap. Bottom photo of rear of 4 jaw chuck with backplate undone. To discard. The slotted faceplate fits great, a keeper. The Large undrilled one is no good as it has a nose on the front which has the threads on it for the headstock. The only one that would possibly go on is the one with peg on but don't want to destroy that one. Looks like i'm looking for a backplate. No rush as the lathe is in bits awaiting the new motor, belts & control gear.

4jaw chuck

Roderick Jenkins03/01/2020 08:44:33
2176 forum posts
608 photos

Please be aware that the Myford nose thread is not a standard Whitworth thread. A 1 1/8 BSW tap will be 7 tpi not 12. I've bought a backplate from Arc that was fine and cheaper than buying a raw casting.



Journeyman03/01/2020 09:47:14
1147 forum posts
230 photos

Interesting and useful collection of parts for the lathe.


Don't know what the items bottom right are and the plate in the centre seems to be the only candidate for a chuck back-plate but too big for the 4-jaw. Down the left hand side are a collection of tool bits, holders and centres which may or may not be useful eventually! Can't ID the item with the cross handle. The block with two round bits just above and left of the catchplate appears to be for offset turning between centres (possibly).

As noted by Rod above the spindle nose thread is not BSW but simply uses the Whitworth thread form.


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