Myford lathe chuck issue

[Jules]

Hi all, I've been on this forum long enough now to know that there is a wealth of knowledge out there from around the world, so HELP please if you can  ....
I inherited an old Myford (ML7?) lathe from Dad when he passed, he was a toolmaker so pretty much made up whatever he wanted, anyway I had to remove the chuck from the lathe last weekend to fit a plate to make up an index plate, and when I tried to screw the chuck back on it jams up now after only a few threads. I've fitted another 4 jaw chuck on fine and the plate fits fine so I know the thread and shoulder on the lathe spindle is good, and looking at the female thread inside the 3 jaw chuck it doesn't look "good", so not sure why it spun off and on freely in the past, but not now  ...
I believe the thread is 1 1/8" (?) but unsure of the thread, its coarse and maybe tapered on the spindle (?), so maybe BSP (?), anyway can anybody help with this please ie confirm thread or how to clean up the thread in the chuck etc etc???
also, if anybody in Australia is aware of somebody in/around Victoria/Melbourne that maybe able to help??? appreciate any ideas/suggestions, dont hold back either    cheers

[terryg]

1 1/8”, 12 tpi, right hand Whitworth I think you’ll find.

Google ‘Myford spindle tap’ and you might find one for sale - although a large tap wrench or substitute may be required. Possibly not so much if you’re tidying an existing thread.

For example…
https://www.chronos.ltd.uk/product/1-1-8-x-12-tpi-second-tap-myford-thread/

Please let us know how you get on.

[Jules]

thanks Terry, good prompt, I cant find anybody else selling those taps, only Chronos it seems and they have nil stock of a plug tap, so I've asked Tracy Tools, waiting for response. Problem is shipping costs are currently extraordinary overseas, Chronos want 12 GBP (exc vat) but 22 GBP incl. shipping....Tracy actually just replied while typing this and offer it in UNF form of 60 deg ilo 55, do you know if that would work ok to clean up the threads (Tracy says they sell them for Myfords and they work ok)?? cheers

[terryg]

Jules - I saw a number of UN spec taps but I’m afraid I can’t comment on how advisable their use would be to try to clean that chuck thread.
Another thought was that the dies appear to be available in Whitworth - although I’ve not confirmed that. With a die, making up a thread chaser would seem possible. But, at the same time I’m thinking (if you have thread cutting leadscrew and gearing) why not turn up a thread chaser.
Just a few thoughts!

[RDfella]

Before you spend money on a tap, try a toothbrush and solvent (eg petrol) to clear any dirt / swarf from those threads. If it fitted / came off previously there's no reason for it not to go back on.

[Rex]

And if that doesn't work there's the guaranteed method of a smidge of fine grinding paste and 15 minutes of to-ing and fro-ing at the point where it gets stiff to turn.

[Jules]

thanks for the thoughts, I have done the cleaning bit already RD and its made no difference whatsoever, and I'm reticent to use grinding paste to marry the two parts together Rex because I really dont want to damage the spindle thread in any way (I figure that grinding the 2 pieces together will effectively match just those 2 pieces, whereas I need to be able to swap various other bits onto the spindle as well).
When I look closely at the thread there are small sections broken out and I'm sure that there is a section that is pulled inwards somehow ie its not round!! and when I screw it on thats just how it feels, as if its come up against a stop, really odd, considering its been on/off a few times before without issue.
I think Terry's idea of a tap is probably the best solution but certainly not the easiest, nor cheapest (for me)! I dont have the capability of chasing the thread Terry , however, I asked an old model engineer club member about it and his comment was "thats easy, I can make up a tool and chase the thread, no problem", so I'm heading down that path, but it will be after christmas now because he lives a few hours away from me and he is busy coming up to christmas....I'll post a closing note after seeing him, just fyi, cheers

[groily]

How about a 2MT tapered mandrel, chuck tightened over the parallel nose, and 55° tool in the toolpost, 12 tpi on the change gears and wind it by hand?. A quick mock- up here (please excuse filth, busy couple of days in the shed  . . .)

[Rex]

, and I'm reticent to use grinding paste to marry the two parts together Rex because I really dont want to damage the spindle thread in any way (I figure that grinding the 2 pieces together will effectively match just those 2 pieces, whereas I need to be able to swap various other bits onto the spindle as well).

The idea is to just put the paste onto the (presumably damaged) area that's causing the issues rather than the whole spindle. You just want to ease off the "high spot" that's causing the stiffness.

[Jules]

Hi Groily, I suspect that that maybe what the model engineer guy will do, he was talking about using a 55 deg tool to "chase" the thread. I'm afraid my skills are nowhere near that level unfortunately, I'm just a user of the lathe not a toolmaker, that was dads' expertise  . and Rex, I dont think its just a high spot, I understand your idea and its good for a high spot I agree, but in this case I think the thread has become distorted somehow - I've attached some photos, hard to see really but you can see that a piece of the thread has broken out and I suspect that its no longer "round", but I really do not know why??? Its interesting that the thread actually has a cavity behind it at the bottom, its not solid material, like its been relieved behind the thread....

[groily]

Can see that the crest of the thread has been damaged in one place or more, but there should be plenty there to hold it on I'd have thought. Can't see how 'out of round' it might be, but it can't be all that very off, as it was on there and was behaving until it didn't. Maybe it was the damaged bit that prevented it coming off in fact, if it was sat there as shards of debris? - in which case, it's good that the headstock spindle thread is still good.

If the chuck was reasonably true for normal ops, it's probably recoverable I'd have thought.

It's not actually that tricky a thing to chase it as described (I'm a totally self-trained geezer with no properly-acquired skills and 300% definitely not a toolmaker even if I flatter myself I can make the odd servcieable weapon!).

If I were doing it I'd use a chunky threading tool owing to the coarseness of 12 tpi, and I'd turn by hand using the lead screw or a handle on the back of the headstock. Very light cuts. You'd find out how out of round it imight be straight away (as long as the jaw side is close to true), because the tool wouldn't bite all the way round to start with.
It wouldn't be hard to get the tool to pick up the thread squarely at that size - it's the small stuff it's hard to get aligned - jiggling the top slide to find the sweet spot in the thread after engaging the leadscrew.

If you do it, don't disconnect the leadscrew until you're done - wind the tool in and out using the leadscrew only, don't touch the top slide, just adjust depth of kiss with the cross slide and retract tool on 'exit' after each run through, before adding a thou or two on the next pass. Unless you have a reliable thread indexing gizmo - I haven't.

That cavity is quite useful in a way because you know when you have run through the thread to full depth.
It would be good if not essential to have another item to hand with the male spindle thread on it, to do regular checks for fit as you go along, as you don't want to have to de-chuck and re-chuck once you're under way if you can help it. And you don't want to take out more than is needed. It's what your model engineer mate would do, I'm pretty sure.

Whatever, Good Luck!

If no go, then the only real option is to replace the chuck, but as we all know, they are quite pricey.

[terryg]

Would a replacement backplate for the chuck be a cost effective option? Perhaps as a fallback plan at this stage.
I’ve not had to replace one but I can imagine there might be some fettling, trueing and balancing to do. Or maybe one would just bolt straight on - although that sounds a bit optimistic to me.

[groily]

I fitted a backplate to a geared 4 jaw (bought from Chronos) for a Myford. Can't recall the cost, but it was a bit of a fiddle to set up to drill the holes in exactly the right place for the three fasteners involved in that case. Embarrassed myself by making a pig's ear of one hole, and so started again 60° round and got it right. Been good but tell-tale signs  of 'stupid eejit' are there for ever!

[terryg]

Bill - surely the extra holes are the result of you finely balancing the assembly!

[RichardL]

What I'm curious about is how that bit of thread got broken off. When the chuck is on the spindle, it's protected. When it's off the spindle, what's to get in there and break it? Maybe it was a foundry flaw when the original iron ingot was formed from which the chuck was turned, where some random carbon deposit laid in wait behind where the thread was cut until enough years of force and vibration let the thread loose. Or was it a chip that got in there and laid against the side of the thread, waiting to break the thread away when the sepindle was spun on. Says something, I guess, for cleaning the internal threads before turning a chuck slowly onto its spindle when mounting.

While the broken thread does seem the obvious cause of the original problem, I'm not sure it completely rules out a different cause. It may be worth counting threads and turns to calculate if the interference starts when the lead of the spindle thread reaches the anticipated position of the broken internal thread.

Richard L.