I broke it.............

[slim_boy_fat]

As per the title, I'm pretty sure I've knackered my Pratt Burnerd 4" Griptru 3-jaw chuck.

I was working with a piece of round bar which just fitted into the chuck [yes, I should have changed the jaws ], when I went to release the work-piece the chuck was incredibly tight initially, then felt like it cleared a 'cam', became slack, then tight again and so on, ad nauseam........

I've removed the jaws and there's no obvious damage to the scroll, but am wary of going any further as a) being the Griptru model, I have NO idea what lurks behind the backplate/facade and b) even if I establish what the problem is I'm not sure I'll either be able to source the necessary parts for a repair.

I have phone numbers for both Myford [think...pricey] and Pratt Burnerd Service Departments, and will call them tomorrow to see if they have a repair/refurbish service.

A poke on the interweb shows replacement 3 and 4-jaw [even 6-jaw ex China ] self-adjusting chucks for around £100, what are the advantages of the greater number of jaws? I don't think I really need the adjustability of the Griptru" but it was nice to have - got it dialled in to +/- 0.002" If I go to 4" or even 5" then I'll need a new backplate as well,with the right spindle thread for the ML7...and so it goes on.

Luckily I have a 4-jaw 6" chuck to keep me going in case of 'emergencies', but I'm SOOOOO mad at myself at the moment.............. I know nobody died, but......

 

[Wallace]

First thought is the end of the scroll has a little damage on it and goes tight entering each jaw.

 

[Anguz]

What he said....

 

[zeppelin builder]

As per the title, I'm pretty sure I've knackered my Pratt Burnerd 4" Griptru 3-jaw chuck.

I was working with a piece of round bar which just fitted into the chuck [yes, I should have changed the jaws ], when I went to release the work-piece the chuck was incredibly tight initially, then felt like it cleared a 'cam', became slack, then tight again and so on, ad nauseam........

I've removed the jaws and there's no obvious damage to the scroll, but am wary of going any further as a) being the Griptru model, I have NO idea what lurks behind the backplate/facade and b) even if I establish what the problem is I'm not sure I'll either be able to source the necessary parts for a repair.

I have phone numbers for both Myford [think...pricey] and Pratt Burnerd Service Departments, and will call them tomorrow to see if they have a repair/refurbish service.

A poke on the interweb shows replacement 3 and 4-jaw [even 6-jaw ex China ] self-adjusting chucks for around £100, what are the advantages of the greater number of jaws? I don't think I really need the adjustability of the Griptru" but it was nice to have - got it dialled in to +/- 0.002" If I go to 4" or even 5" then I'll need a new backplate as well,with the right spindle thread for the ML7...and so it goes on.

Luckily I have a 4-jaw 6" chuck to keep me going in case of 'emergencies', but I'm SOOOOO mad at myself at the moment.............. I know nobody died, but......

Get it in bits .and mark it all first
Time you bought a DSG

 

[slim_boy_fat]

Get it in bits .and mark it all first
Time you bought a DSG

Thanks. But a DSG [or similar] is never going to happen, unless I move into the shed to live and use the living room in the house for my machinery......

The only thing preventing me from diving straight in is the Griptru set-up - I'm not sure if there are any loose bits in there in conjunction with its operation. #scaredeycat........ I'll see what PB and/or Myford have to say tomorrow.

 

[Barking Mat]

If you think its knackered, you've nothing to lose. Good luck!

 

[Hood]

Not much in them, just tapered pins basically. 6 jaws can be good for thin wall stuff or for holding drills on the flutes but are a pain otherwise as you have twice the amount of jaws to get choked up with swarf.
As has been said it is likely damage to the scroll, should be easy enough to clean up.

 

[Hood]

Pic from an old manual showing the pins.

 

[Hood]

BTW don't get a 4 jaw self centring chuck, they don't grip too well most of the time. Usually two jaws will contact and the other two will be close but not holding things tight.

 

[MCKDAVID]

search on tubalcains series on youtube, he does a strip down, a rebuild of the 3jaw, piece of cake. even if it's not the same model exactly, it's the same principals. Might give you a starting point. when it's apart it's be more obvious if it's repairable or not economic

no special tools required, just patience. Have at it.....

 

[slim_boy_fat]

Thanks guys. Spoke to a VERY helpful chap at 600 [formerly Pratt Burnerd] this morning and explained my predicament. He said they carry spares for 4-jaws back to 1932, but the 3-jaw stuff doesn't go back beyond 1990 - my chuck is from 1975 but at least it was made for the ML7 so must be original spec on this lathe and not something added later, which is nice.....

He's sending me the exploded schematic and said that the 'micro screws' alluded to by @Hood [] must be removed before attempting to take off the back plate - that's something I'd have got wrong had I jumped in. He reckoned the most likely culprit is a broken pinion, which are still available for c.£24. If it's the scroll, then it's scrap as they are no longer available.

I'm going to wait until I his email arrives and I can give this my undivided attention, which won't be today as I have other things I have to do.

The light at the end of the tunnel is either a fix...or a train! I'll update when I have something report.

 

[Richard.]

Mines identical.
I’ve had it apart a couple of times now don’t worry it’s not going to jack in a box on you. Remove the 3 tapered alignment screws in the body and the 3 fixing screws on the back. Inside youll have the 3 worm drives onto the big gear. Have a good check over of the teeth and give it a light oil.
Put the plate back on using the tapered screws to get the chuck halves aligned together. I put it back together loose and fit the chuck on the lathe. Then set up the dti on the chuck body and tighten those tapered screws a bit at a time till the run out is completely gone. Then take it off and nip up the back 3.
I think your issue is as Wallace said you’ve tightened up right on the end of the scroll thread and probably caused some light damage where the jaws first engage. Dress it up and make sure you use them with a bit of meat behind them in future and it will probably be ok.

 

[slim_boy_fat]

Thanks @Richard. The nice man has sent me the drawing in PDF format, but apart from d/loading and saving it I've not had a chance to examine it - just too much on elsewhere today.

As I said above, I'm going to clear space, mark everything and take pics as I go - never ventured inside a chuck before, even a 'basic' one, so it's "to infinity and beyond".....

P.S. Would you like a copy of the drawing?

 

[Richard.]

Thanks @Richard. The nice man has sent me the drawing in PDF format, but apart from d/loading and saving it I've not had a chance to examine it - just too much on elsewhere today.

As I said above, I'm going to clear space, mark everything and take pics as I go - never ventured inside a chuck before, even a 'basic' one, so it's "to infinity and beyond".....

P.S. Would you like a copy of the drawing?

No mate. It’s a simple thing when you take the back off you’ll see. I marked up the back so it went on in the same position but to be fair it’s probably not crucial. You’ll see when you get the back off it’s 3 tapered screws and 3 cap heads. The important thing is on re assembly when you tighten the tapered srews they are moving and aligning the two pieces up so get it some where near on the bench and finish the job on the lathe with a dti to get it spot on. If you don’t your two pieces will be out and your chuck will not run true. It’s easy. Mines had 2 brief services now

 

[slim_boy_fat]

I marked up the back so it went on in the same position but to be fair it’s probably not crucial.

I've noticed there are register marks [i.e. dots] stamped on both the back-plate and the chuck body, so that's a start.

 

[MCKDAVID]

maybe been apart already then...

 

[Wallace]

I've noticed there are register marks [i.e. dots] stamped on both the back-plate and the chuck body, so that's a start.

Definitely been broken, I mean apart before!

 

[Hood]

Yes, someone has had it apart but that is most likely a good thing especially as they have punched it. It is amazing how many people don't think or bother about doing that.
Chucks need stripped and cleaned every now and then.

 

[Richard.]

Mine wasn’t marked up. I should stamp it really. It fits on in 3 different positions but I mark it with a sharpie and put it back how it came off. Not sure if it makes any odds or not mind. I suppose it will of being finish machined in one position and should go back in that position every time.

 

[Hood]

When dismantling a chuck you should always put it back the same way, even the pinions should ideally go in to their respective locations. It is not just due to the original way it was but also because through time these parts will bed in and wear so putting back in a different position may upset things.