Disassembling a Pratt 10 inch 3 jaw

I have just bought a Lang Junior lathe (that I'm looking for a manual for) with a 10 inch (242mm to be precise) Pratt steel 3 jaw chuck that has jammed with rust. I am trying to strip it down to clean out any debris, but I have been unable to split it apart so far. I have removed the 6 cap head screws from the back, the 3 from the front, and removed the grub screws and pins that hold the gears that drive the scroll. I have been able to get the back plate to move out about 2 mm, but can't get any further.

The only markings on the chuck are The Pratt Chuck. LT387. Steel. Made in England, and on the back A39 and B39.

Could someone give me some pointers as to how to get the back plate out?

I've uploaded some images to an album showing the chuck and name, but I can't seem to get them into this message.

Nick you dont seem to have an album of pictures to view

Does it have threaded holes in the backplate that you can screw some bolts into to jack the backplate out of the chuck evenly?

Usually the chuck key barrels with the bevel gears on them should come out once you remove their retaining pins. But that should not stop the back plate from coming off.

In the absence of any [hoped-for] jacking arrangement … intelligent/gentle/careful, use of the recently-derided dead-blow mallet is probably the best bet.

MichaelG.

Nick,

Refit three of the rear cap screws at 120 degree intervals around the chuck to get a decent number of thread turns engaged and with a dead blow hammer or regular hammer and a block of timber tap them each in a pattern around the chuck to knock the front part of the chuck away from the rear. Once you have a gap of say 2-3 mm between them you can use a pair of broad bladed screwdrivers on opposite sides to carefully prise the two halves of the chuck apart.

Try lathes.co.uk for a handbook on your lathe, Tony Griffiths has some but maybe not your model. They are not cheap being quality digitally enhanced reprints, but the material is clear, clean and nicely presented.

Regards Brian

The "jammed with rust" is probably contributing to your difficulties.

There is minimal clearance between the scroll and the two parts of the chuck body and between the two body parts themselves.

Any corrosion in this small space will make separation challenging.

Submerge the chuck in a bucket of diesel for a week. Remove it and go around it with a wood mallet or nylon hammer. You can buy an attachment for an SDS drill to rattle the sludge from domestic radiators. Applying (a home-built) one of those will also assist. You need lots of gentle, sustained vibration, not heavy blows.

Possibly better than diesel would be Evaporust or one of the unbranded products that do the same job, e.g

https://www.ebay.co.uk/itm/374293981667

https://www.ebay.co.uk/itm/115253455158

(Do compare the text, and especially the supplier's address, on the two items above).

The hassle is that the rust removal products require a clean item as they do not cut through rust covered in oil or grease, so you might have to degrease, hot detergent wash and water rinse before dropping into the solution.

And if it is rust, sometimes if you move the backplate out as far as you can, then tap it back in then repeat a number of times, it can clear the rust sufficiently to let it come out. Liberal lashings of your favourite releasing fluid, WD40 etc along the way of course.

Trouble with rust is it expands taking up more room than the original metal.

Plus Gas Formula A is said to reduce the volume of rust reducing jamming effects.

Given time to penetrate it appears to work. Regular squirting with an aerosol over a week or two interspersed with careful knocking as advised above may be effective. Might help if you can support it so gravity acts to pull the main casting off, its quite heavy.

That method shifted a well jammed turret ram on a Bridgeport for me. Removed the ram and set it up pointing down at an angle, anointed the joint with plus gas and knocked it with a dead blow 3 times a day, reversed it every 3 or 4 days. After about 3 weeks I went to give it its morning anointing and found the turret and slid down all on its ownsome overnight.

Clive

Lob it in a bucket of diesel. Go on holiday for a fortnight. Try again.

Martin

Thanks for all the suggestions, guys. I have loaded the photos (which I hadn't done correctly before)

It appears the main ingredient I need is patience; I will order a large jar of it (with some diesel) and start the process.

Regards,

Nick

Good morning all.

A bit late on this one. I have two 250mm iron Pratt & Burnerd chucks. On one of mine there's a single locating dowel on the back plate that goes into the main body. If your chuck has such a dowel you'll need to virtually fill it with penetrant in order to free it off.

Mine was a complete bugger to take apart the first time I did it, due to said dowel being slightly corroded.

Morris oil do an excellent grease for chucks, (or anything else for that matter). It's called K323 blue grease. It's only available in big tubs, or 400g grease gun refills. I buy the refills by the case & decant one into a convenient pot & use it for most applications. I found it out performs the proper chuck grease that's available.

Cheers.

Stewart.

Brian Wood's helpful suggestion reminds me that if you haven't got a pair of broad-bladed screwdrivers you could always use a pair of tire levers instead - more effective, kinder to the work and frees up the screwdrivers (if you have them) for their proper purpose of opening paint tins and stirring the paint!

Chemicals are better than diesel, some of that rust needs to be out of the way

Good luck.

Nick , Suggest you consider consulting such sources as Pratt Burnard 600 or Rotagrip or Colchester Machine Tools.