Dismantling the Pratt Burnerd Myford 3 jaw chuck

[iconParticipant @icon]

I’ve have this chuck which came with the lathe many years ago, and was a good few years old even then. I’ve had a couple of goes at dismantling it to give it a clean – there’s a few screws on the back holding it together, plus friction that I’ve never been able to overcome. After a few whacks with a hide-faced mallet, my heart quails and I put the screws back in. The time has come for another attempt, to clean a ton of cast iron dust out of it. Give me some ideas, if you would, as to how I can get this apart without damaging it.

[duncan webster 1Participant @duncanwebster1]

Mine had jacking screws to get the back off, but it’s a long time ago. It’s even possible I added them, memory fades after 20+years

[Dave HalfordParticipant @davehalford22513]

Heat, warm it, the oil returns to being more like oil than glue – about as hot as your CH radiators get (60C)

[Martin KyteParticipant @martinkyte99762]

Does it say GripTrue on it. If so you need to remove the 3 adjusting tapers as well as the screws on the back I think. These are found between the chuck key squares and are Allen head.

regards Martin

[Charles LamontParticipant @charleslamont71117]

If you can, a photo or two might help, as chucks are not all the same construction.

[iconParticipant @icon]

[iconParticipant @icon]

There we are – I added the holes for indexing purposes.

[DiogenesParticipant @diogenes]

Take the screws out replace them with some old studs or threaded rod and keep tapping round them in sequence..? Maybe some heat as Dave says, a bit hotter, even?
<p style=”text-align: start;”>There was a time when Myford mounted them ‘b*****d fit’..</p>
Probably worth marking before you split it so you can reassemble in the same relationship.

[DC31kParticipant @dc31k]

Loosen the capscrews 1-2 turns, leaving them still engaged with their threads and then tap on them. That should open up a gap at the back and start things moving. Work your way around evenly.

Remove capscrews altogether and reinsert from the back, fully engaging with the tapped threads. Go in from the front with a brass punch.

Remove capscrews completely, open jaws until they overhang body and prop up on three pieces of timber. Screw dummy spindle nose into chuck and tap gently on that. That gives a more central application of pressure.

As manufactured, they are a snug fit and as above, petrified lubricant can exacerbate this.

Put some sort of witness mark on the two parts so they go back in the same relative position.

[iconParticipant @icon]

Thanks, I shall give some of these a try. I’m loathe to damage the spindle screw thread, or the jaw ‘ways’, as you can imagine, which limits the violence I can inflict!

[Dennis RParticipant @dennisr]

Remove the jaws then take out the three cap head screws.

Put the three cap head screws into the rear holes , tighten them in a little at a time to jack the back off.

[old martParticipant @oldmart]

As DC31k mentions, the three capscrews are the answer, I would get some longer ones so they can be loose but also fill the threads in the rear of the chuck. The joint shows next to the indexing holes and the rear joint will be just outboard of the spindle thread register. Lots of gentle tapping on the screw heads should start things moving, and you should mark the body and rear plate first to ensure they go back together the same way. The lubrication and heating will also help. I assume that the second picture shows the ends of the screws just below the surface.

[Dennis R]

On Dennis R Said:

Put the three cap head screws into the rear holes, tighten them in a little at a time to jack the back off.

They would be jacking against fresh air and would be ineffective.

Why do the jaws need removing?

[Charles LamontParticipant @charleslamont71117]

Sorry Dennis, as the holes in the body are presumably clearance holes, I don’t think that is going to work.

I would try taking the bolts out, mounting the chuck on the lathe, gripping a 1/2″ bar in the chuck, long enough to stick out of the back of the mandrel, and start patiently tapping the end of the bar with a brass bammer, oops, hammer (but bammer sounds good too, so typo left in place).

[Dennis RParticipant @dennisr]

Sorry to confuse the issue, but on my earlier Myford chuck the back plate has six holes three for bolting the back plate on and three for jacking the back plate off.

My later chuck is same as the Icons, so withdraw bolts half out of threaded holes and tap  heads to get backplate off.

[Howard LewisParticipant @howardlewis46836]

Before separating the back from the front of the chuck, centre pop both halves so they go back in the same relationship

Once separated, it may be worth drilling and tapping the back (Deburr afterwards!) for jacking screws

(1/4 BSF, 1/4 UNF or M6 will be adequate) against the next time that there is a need to strip, clean and lubricate.

Howard

[DC31k]

On DC31k Said:

On Dennis R Said:

Put the three cap head screws into the rear holes, tighten them in a little at a time to jack the back off.

They would be jacking against fresh air and would be ineffective.

Why do the jaws need removing?

On DC31k Said:

On Dennis R Said:

Put the three cap head screws into the rear holes, tighten them in a little at a time to jack the back off.

They would be jacking against fresh air and would be ineffective.

Why do the jaws need removing?

From the photos, the holes in the backplate seem to have iron at the bottom, so I wouldn’t be at all surprised if they are tapped for jacking screws

It looks like a screwed body chuck to me, the backplate is not the bit that screws onto the mandrel, so attempting to lever it off with a bar in the chuck is unlikely to work

 

[FulmenParticipant @fulmen]

It’s not entirely obvious from the photos but the holes does not look like they line up with the screws. That should be easy enough to check for the OP.

[iconParticipant @icon]

The holes are indeed just through holes, tapped in the back plate and clearance in the body. It’s not obvious which bit the spindle thread ‘belongs’ to. I’ll find out when I get it apart! I think buying longer (1/4″ BSW) screws, propping the jaws on some bits of wood and tapping the screw heads is probably the best bet.

[FulmenParticipant @fulmen]

Guess you just need a bigger hammer.

[Michael GilliganParticipant @michaelgilligan61133]

This includes some useful shots of a dismantled chuck:

https://www.youtube.com/watch?v=axSbgFfMZyA

MichaelG.

.

[duncan webster 1Participant @duncanwebster1]

That’s not a screwed body chuck, I gave up watching after a couple of minutes, applying a clock gauge to a rotating bar is not good, and he should have had some means of loading the jaws inwards before attempting to grind them.

[Michael GilliganParticipant @michaelgilligan61133]

All true, Duncan … but as the OP said his chuck had never been dismantled, I thought perhaps he might be interested.

Quote: This includes some useful shots of a dismantled chuck:

Do feel free to post something more appropriate, if you can find such

MichaelG.

[duncan webster 1]

On duncan webster 1 Said:

applying a clock gauge to a rotating bar is not good

Bit of a digression, but I use a plunger type dial indicator to centre work in a 4-jaw chuck all the time. I would only rotate by hand though. Is this wrong?

[Dave HalfordParticipant @davehalford22513]

BMS bar isn’t very round unless machined.

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