Dismantling a Pratt Burnerd Grip Tru Chuck

[Andrew TinsleyParticipant @andrewtinsley63637]

[Andrew TinsleyParticipant @andrewtinsley63637]

I have been given two Pratt Burnerd 3" grip tru chucks. They have done very little work, but are neglected and seized up.

Now I do know how to strip these down and the first one was straightforward after a good soaking in Plus Gas. The second one freed up and I took out the adjusting screws, to find one of them had sheared off in the middle of the taper section. Thus the rear of the chuck cannot be taken off. You get so far and the broken adjuster stops it coming out.

The first chuck definitely needed the strip down and clean, so I expect the second chuck to needs the same treatment. I have run out of ideas to get the broken threaded remains of the adjuster, out of the chuck..

So any good ideas? I am still thinking of ways to do the job while I drink some red wine in the garden

Thanks,

Andrew.

[DC31kParticipant @dc31k]

What size is the broken off bit? Is it hardened (check the piece that did come out)? How deep inside is the broken bit?

Drill it with a left hand drill. If that fails to shift it, tap it left hand thread. Screwing in the tap will screw out the broken bit. Left hand drills and taps from Tracey Tools. If it is hard, carbide mill and drive in a Torx bit.

[old martParticipant @oldmart]

Can you post pictures of one of the unbroken ones with an indication of where the break is? There are diagrams of the chuck in an earlier thread, back on page 4, or thereabouts.

[Andrew TinsleyParticipant @andrewtinsley63637]

Not sure about photos, but will give it a try!

Andrew

[egaParticipant @ega]

Any chance of "sticking" an arc welding electrode to it?

[DC31kParticipant @dc31k]

The thread to which Mr old mart refers is here:

https://www.model-engineer.co.uk/forums/postings.asp?th=124264

and it is Mr ega who kindly posted the diagrams.

[egaParticipant @ega]

DC31k:

Thanks for the credit!

The fact that the adjuster screw sheared suggests that someone was attempting to move the immoveable and, perhaps, lends some support to the suggestion that the retaining screws need to be slackened.

[old martParticipant @oldmart]

The design of the adjuster screw seems to invite snapping off the small diameter thread if the screw is turned clockwise until it bottoms. The taper section would seem to be the largest diameter of the screw, so the reason for it snapping there is a mystery.

[Andrew TinsleyParticipant @andrewtinsley63637]

Hello Old Mart, the screw can't bottom as the hole comes out on a tangent. I assume it has broken half way along the taper as this is where it bears on the scroll disc!Must have been a bit of a monster whoever managed to break the taper section with just an Allen key!

Still trying to find a digital camera to take a shot of the adjusting screws. I don't have (or want) a smart phone.

Andrew.

[egaParticipant @ega]

Don't forget that the Allen key (hex driver) is on the end of the chuck key ie lots of leverage!

[Andrew TinsleyParticipant @andrewtinsley63637]

Hi Ega,,

The chuck keys are simple square section, conventional keys, no hex drivers are present. Hence my marvel at breaking one of the adjusters with an Allen key. Wished I was that strong!

Andrew.

[Swarf MakerParticipant @swarfmaker85383]

Andrew Tinsley, the 'as supplied' standard chuck key for these chucks has the hexagon key on one end of the 'T' bar, so as 'ega' says, plenty of leverage!

[Bernard WrightParticipant @bernardwright25932]

Hi Andrew, just a suggestion, measure the broken end of the tapered screw.

Find a good bolt of that diameter with enough shank to reach the remainder of the screw in the chuck body.

Cut off the excess shank, place in your lathe chuck/collet and drill the largest through hole without weakening too much the bolt shank.

Providing the hole is big enough to accept a 0.6/1.0 MIG wire or even a 1.6/2.5 welding rod with a bit of insulation on it (the reason for the insulation is to provide positioning ease without premature arcing, probably some very small plastic tubing (windsreen washer pipe)).

If the previous procedure is attainable, insert the insulated welding medium into the hollow bolt and with a gloved hand position onto the broken screw and weld the hollow bolt to it.

I would think the heat shock would hopefully loosen the seized screw.

 

Hope that may be of help.

Edited By Bernard Wright on 12/04/2020 17:29:54

Edited By Bernard Wright on 12/04/2020 17:32:12

[Andrew TinsleyParticipant @andrewtinsley63637]

Hello Swarf Maker,

The chuck key that came with the chucks didn't have a hex on the end of the tommy bar. If you say there is a hex there, then obviously the key that came with the chucks is not the original one.

I am not surprised that one sheared if that is the normal adjusting tool! Plenty of brute force and ignorance to be had with it!

Andrew.

[old martParticipant @oldmart]

I assume you have removed the other unbroken adjuster screws. And the front mount screws which hold the body onto the rear part. You have a second chuck which does come apart, with that you should be able to see exactly where the snag is.

[Andrew TinsleyParticipant @andrewtinsley63637]

Yes indeed Old Mart! I already said that it was the broken adjuster that is still in the chuck that is stopping the rear of the chuck coming off.

Andrew.

[old martParticipant @oldmart]

All the rest of us have to go on are the Pratt drawings posted by ega. Can you measure the depth of the obstruction from each end to estimate how long the broken part is? Measuring would ascertain whether the end of the screw with the hexagon socket has broken up. That is more likely than the fat part of the taper, which is the strongest part. Pratt have a tendency to heat treat parts like that adjuster, so drilling could be difficult. The Pratt instructions refer to "tangential grooves of circular form" in the rear part. This must be the cause of the jamming.

Edited By old mart on 12/04/2020 20:49:40

[John Hall 7Participant @johnhall7]

The Tommy bar did come with a hex on one end…I have one from new and it has the hex for the adjusting screws…

maybe they were not provided on all models…mine is a 4”

Itis actually shown in use in the instruction manual

Edited By John Hall 7 on 14/09/2023 17:18:42

[John Hall 7Participant @johnhall7]

[MacolmParticipant @macolm]

A long shot if desperate might be to contrive friction welding a steel rod, assuming you have direct axial access to the broken part. As you may know, the parts to be joined are rotated with pressure until the interface melts. There must be provision to stop the rotation dead by a clutch and to hold in position till the interface solidifies. Results are impressive if carried out properly, with nearly full strength due to the inherent complete and clean metal to metal contact.

But you would indeed need to be desperate, a botch would not succeed.

[SteviegtrParticipant @steviegtr]

Hi Andrew not sure if this will help but i did a full stripdown video of the griptru.

Steve.

[Author]

Posts