Chucking out

[fastdaveParticipant @fastdave]

Hi folks,

I tried very hard to get to the bottom of a chuck wobble – I knew it wasn't seating, and pursued various efforts to grind the jaws, but the whole thing was badly worn – back to the guy who sold me the 1966 ML7 – another chuck at a price – Toolmex this time, not the original Pratt Burnard – but at least the jaws met, and it was in good condition.

I have to say that what made me certain the chuck was at fault, was the fact that I could true the same steel bar in the 6" 4 jaw chuck – all this with no tailstock support.

New chuck – at the jaws, pretty true – deflection at 4" out – wobble again.

Buy 2MT parallel bar – 11" long – also, since mouth of spindle looks pretty bashed, a 2MT reamer, and hey, RGD, I'll just have a 2MT tailstock spindle chuck adapter – extra check for the chuck – now wait

– Ahh, good, one day delivery – ream headstock spindle, insert parallel – and absolutely true – wind my way through all the tests I can muster – bur when I put the parallel in the wrong way round – into the chuck, – there's a wobble.

I now put the steel bar in, wobble still present – slide up the tailstock, dead centre – true as a die.

Am I being unreasonable in expecting this ole' ML7 to be true on a 4" 3 jaw chuck without support – I never had any of these problems on my old Clarke 430 – and it's not going anywhwere fast until my confidence in this increases.

Anyone got anything other than strong drink or vallium?

[fastdaveParticipant @fastdave]

Continued really, from my ML& thread – now going quite insanne

[HopperParticipant @hopper]

The essential key is how much wobble? Two thou? Twenty? Fifty?

You can only expect an old 3 jaw chuck to run true to about two or even three thou right at the jaws. That can then be amplified by leverage as you move further away from the jaws.

If you can't regrind the chuck using the correct clover leaf ring or similar to hold the jaws tensioned outwards while grinding, you a probably best off to buy a brand new chuck. There are plenty of good ones out there for not too much money. As your main "go to" workholding device it's worth the investment.

[JasonBModerator @jasonb]

Is this a threaded body chuck or one on a backplate? if a backplate one then it will need the backplate skimming on your spindle as it is unlikely to match that of the other lathe.

[fastdaveParticipant @fastdave]

Thank you Hopper and JasonB, – run out at 4" = 0.14 mm –

the jaws on this one really don't need reground, and I sent the old chuck back to the seller.

The chuck is threaded, yes, and on a backplate, which I imagine just keeps the guts together – I did think about skimming the bush at the back of the thread, but the other chucks work.

I think I'll take comfort in Hopper's phrase about an old chuck – I'll keep it right with the tailstock.

Thanks guys – your usually helpful selves!

Dave, Fife, Scotland MM7ASN

[Dave HalfordParticipant @davehalford22513]

You could try clocking the chuck body first, don't go grinding the jaws until everything else has been exhausted.

I have a 5" chuck that is worn out for anything less than 3/4" – 15 thou wobble, over that size it's like new.

[Howard LewisParticipant @howardlewis46836]

I have only once found a three jaw chuck with Zero runout. Most have been within about 0.005".

One very old one gave 0.036"!

If you are turning the OD, the finished surface will be concentric to the axis of the spindle. One assumes that the bearings are in good condition, and wear is not contributing to the problem. If concentric diameters are a necessity, they should all be turned without disturbing the work in the chuck.

Perfection is unlikely to be found in old or hobby machines. Even very costly industrial machines have their problems, so we will most certainly. Ditto for our measuring facilities, (Instrument quality, ambient conditions not temperature or humidity controlled )

Recently, I had dealings with an industrial gauge – a new replacement would have been over USD 5,000 with a 8 month lead time to delivery, so not for a hobbyist without very deep pockets

Howard

[JasonBModerator @jasonb]

Posted by fastdave on 31/07/2020 12:19:28:

The chuck is threaded, yes, and on a backplate,………….

Dave can you confirm which.

Either the body of the chuck has the thread cut directly into it in which case you can't do much about it.

Or it is screwed to a threaded backplate either from the face or from behind in which case the two should be separated, the backplate mounted on the spindle and skimmed true then the chuck refitted

Well looks like my chuck is 10 times better than most, 0.015mm or 6/10th of a thou and thats mounted on the unskimmed chinese backplate that came with the lathe, even the Warco chuck was just over a thou but scroll got tight.

[duncan webster 1Participant @duncanwebster1]

Posted by fastdave on 31/07/2020 12:19:28:

Thank you Hopper and JasonB, – run out at 4" = 0.14 mm –

…………

That's pretty good for a three jaw in my book, if I wanted better I'd use the 4 jaw independant

[Howard LewisParticipant @howardlewis46836]

+1 for Duncan and the 4 jaw independant. With care you can adjust to less than 0.0005" run out.

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