Myford chuck not gripping

[RoParticipant @ro]

So, I’ve recently acquired an old Myford super 7 and I’m loving using it, with one exception.

It came with a 4 inch Pratt Burnerd 3 jaw chuck, and I’m finding that if I take a cut of 1mm or more on steel (EN1A-PB) the work will move in the jaws.

I have tightened up the jaws fairly tight, but after reading warning about over tightening I don’t want to really crank it down.

Having had a mini lathe for the last 15 years or so, I’ve got used to how much I need to tighten the chuck on that, and I’m definitely applying at least as much torque on the Myford, and probably a bit more.

So, should I just crank down on the chuck key to hold the work, or is there something I can do to the chuck to make it grip better?

[RoParticipant @ro]

[John HaineParticipant @johnhaine32865]

Just give it some welly…

[JasonBModerator @jasonb]

When you say it moves doe it get pushed back into the chuck or spin? If it spins where do the jaws leave a mark. If the mark is deeper towards the back of the jaws they are probably worn and bellmouthed

[Bill Davies 2Participant @billdavies2]

Perhaps the scroll needs some lubrication.

[RoParticipant @ro]

Jason: It spins and moves back. Just tried it and the mark is towards the back of the jaws. Can I just regrind them with a dremel held in the toolpost? Or is it not that simple?

Bill: I have lubed the scroll with the jaws taken out, but I haven’t stripped it down yet.

Edited By Ro on 10/07/2020 19:38:22

[J HancockParticipant @jhancock95746]

As long as you only use the chuck key and your hands to tighten the work it is most unlikely to break anything.

UNLESS you are using it near to fully open capacity, then all bets are off.

[RoParticipant @ro]

J Hancock & John Maine: I’ve just tried giving it some welly, and I really had to fight it to get it to grip. I had to go so far that I was getting visible imprints of the jaws in the steel.

And to add info to Jason’s question, yes the imprints were from the rear of the jaws.

[JasonBModerator @jasonb]

You need to find a way to put tightening load onto the jaws when you grind them, there are various jigs or a simple washer at the back of the jaws can be used, a search of the forum will throw up a few results

[old martParticipant @oldmart]

These chucks are very easy to dismantle, clean and regrease, yours may benifit from a little tlc.

If you have some silver steel. of other ground stock like the plain shank of a 1/2" drill in undamaged condition, then you could try chucking it up lightly so you can just turn it by hand, and then wiggle it about. You should be able to tell if the jaws are looser at the front or not. If the are not touching at the front, they are bellmouthed.

Perhaps a cut of 1mm is expecting too much from a little chuck in a Myford.

Edited By old mart on 10/07/2020 20:22:16

Edited By old mart on 10/07/2020 20:25:51

[BazParticipant @baz89810]

Oh please! Perhaps a cut of 1mm is expecting too much, I regularly rough out EN1a with cuts 1/8 deep, 1/4 inch off diameter on a Super 7.

[RoParticipant @ro]

Ok, cool. Thanks for all the advice.

I’ll give the chuck a bit of TLC tomorrow and then check if it needs the jaws ground. Sounds like a fun Saturday project.

oh, and I’m glad to know I should be able to take even deeper cuts

[Harry WilkesParticipant @harrywilkes58467]

Ro there are a few videos on youtube showing how it's done LINK Removed, See Code of Conduct

and there are others a search should find them

H

Edited By JasonB on 11/07/2020 06:54:39

[Enough!Participant @enough]

Since no one has mentioned it, you are going around the chuck tightening all the jaws evenly – not tightening from a single position?

[not done it yetParticipant @notdoneityet]

That video nicely demonstrates how not to improve the runout on a GRIPTRUE chuck?

[The Novice EngineerParticipant @thenoviceengineer]

A tip given to me by an old machinist to true up the jaws is as follows

Select a grinding stone that has a diameter so it can pass through the rear of the chuck.

Turn up a ring that will fit over the jaws so that when they are opened against the ring ,the grinding stone will pass through.

Now gently stone the jaws till true.

Not done it myself, but he did sort out an old chuck for me.

An alternative might be to source some replacement jaws Pratt Burnerd use to do spares .

[HopperParticipant @hopper]

Posted by The Novice Engineer on 10/07/2020 23:25:28:

A tip given to me by an old machinist to true up the jaws is as follows

Select a grinding stone that has a diameter so it can pass through the rear of the chuck.

Turn up a ring that will fit over the jaws so that when they are opened against the ring ,the grinding stone will pass through.

Now gently stone the jaws till true.

Not done it myself, but he did sort out an old chuck for me.

An alternative might be to source some replacement jaws Pratt Burnerd use to do spares .

You have that bass-ackwards. If you open the jaws against a ring around the outer edge of the jaws, as some YouTube videos wrongly show, you will push the outer edges of the jaws inwards and thus grind MORE off the already worn ends of the jaws.

Issue 267 of MEW has an article on the right way to do it, on a Myford ML7. Secret is to make a clover leaf, aka spider, or even three flat spacers to push the ends of the jaws OUTWARDS when the chuck jaws are tigthened against them.

Here's my version of it:Not my original idea. Copied from an ancient Duplex article in ME years ago.

[RoParticipant @ro]

Thanks all for your help.

this morning I stripped the chuck down and found that the jaws were very tight in a couple of places in the scroll.

A bit of work with a file sorted out burrs that had formed in those places. Then after reassembly, the chuck was a lot easier to tighten, but still had the same problem wit regards to gripping.

I made up a quick top hat and trued the jaws with a carbide boring bar and the top hat held in the back of the jaws (then reversed the top hat to the front and did the same for the very rear of them).

The jaws now grip fully along the length and don’t slip at all!

I was slightly worried about the work on the scroll affecting the runout, but after testing at multiple diameters I seem to be getting a maximum of about 3 thou.

thanks again for all your advice

ro

[old martParticipant @oldmart]

That runout you now have is about par for the course, if you want better, a four jaw independent chuck is the way to go. They take more getting used to, but worth it.

[JasonBModerator @jasonb]

I'd have thought you would have ended up with better being the jaws were trued in place.

As for par for the course I'll stick with my less than 1/2thou out of the box 3-jaw and Old Mart can keep his wonky ones

[not done it yetParticipant @notdoneityet]

Posted by old mart on 11/07/2020 15:36:55:

That runout you now have is about par for the course, if you want better, a four jaw independent chuck is the way to go. They take more getting used to, but worth it.

Or a griptru – like the chuck on the video! i just luuv videos like that. Correcting bell-ended jaws Is one thing, but talking about improving run-out by grinding the jaws of a griptru chuck seems rediculous.

Edited By not done it yet on 11/07/2020 16:25:04

[RoParticipant @ro]

Posted by JasonB on 11/07/2020 16:15:27:

I'd have thought you would have ended up with better being the jaws were trued in place.

yeah, to be fair, I was hoping for a bit better. At some diameters I’m getting a couple of tenths, and other diameters a bit more, with the worst being about 3 thou.

I’m blaming it on the scroll, which wasn’t in the best of conditions (and also maybe my file work when trying to repair it)

half a thou sounds fantastic. Have you got any recommendations if I decide to invest in a new chuck at some point?

[JasonBModerator @jasonb]

Scroll could be worn/deformed and it is only true at one position per full rotation, probably previous owners giving to too much wellie.

Mine is a Bison 3274 picked up at a good price but about the same as a Pratt

[SillyOldDufferModerator @sillyoldduffer]

Well done Ro, 3 thou isn't too bad for a 3-jaw. Run-out is mostly a pain when work has to taken in and out of the chuck because each time the job takes up a slightly different position and the turnings aren't concentric. Cutting stays concentric when the job is only set in the chuck once, in which case a bit of 3-jaw run-out probably won't matter.

The main advantage of 3-jaws is speed and convenience gripping round objects, as a breed they're not hot on accuracy. If low run out is essential, switch to an independent 4-jaw because these can be set up very accurately with a DTI. Their disadvantage is the extra time taken to set them up, though some use nothing else and say – with practice – they can be as quick as a 3 jaw. They're also required for holding odd shapes. Collets are good for speed and accuracy, their disadvantage is cost!

Just as a matter of interest, how exactly do you chaps measure run-out? My Chinese certificate calls for a 20mm diameter test bar and the run-out to be measured 50mm out from the jaws. This, I think, is a rather tougher test than finding run-out as close to the jaws as the DTI can get. Rude things said about Chinese certificates, but my chuck was about 0.01mm better than the 0.05mm @ 50mm claimed for it. Not quite so good now unless I fuss with it on the register. Wear and tear on an inexpensive item I expect.

Dave

[HopperParticipant @hopper]

Posted by Ro on 11/07/2020 15:26:30:…

…I made up a quick top hat and trued the jaws with a carbide boring bar and the top hat held in the back of the jaws (then reversed the top hat to the front and did the same for the very rear of them).

The jaws now grip fully along the length and don’t slip at all!

I was slightly worried about the work on the scroll affecting the runout, but after testing at multiple diameters I seem to be getting a maximum of about 3 thou.

 

Possibly because when you held the top hat ring in the rear of the jaws and tightened the jaws inwards, the front tips of the jaws were pushed inwards, the opposite of the way they are pushed when in use holding a piece of round bar etc.

That is why the clover leaf plate I posted above works better. It pushes the front tips of the jaws outwards while they are being machined, replicating the position they are in during normal operation.

If there is a bit of slack between the jaws and the slots in the chuck body they run in, it can make quite a difference.

I got the chuck on my 1937 lathe to run within one thou using this method.

Edited By Hopper on 12/07/2020 02:43:19

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