Help needed with stiff Pratt Burnerd 3 jaw chuck.

Dear All,

I have a ‘new old stock’ PB 125mm 3 jaw with an inbuilt D1-3 mounting. It was far too stiff to use. After stripping, cleaning and drying, and without the scroll in place, the number 2 jaw slides nicely into its own and the other 2 slots. Jaws number 1 & 3 jam at the half-way point in their slots and need to be tapped with a nylon hammer to move them further and/or to get them out. The outside jaws show exactly the same problem.

I can’t measure any difference in the jaw or slot widths and am stumped by this. The chuck is unusable in its present state. As an engineering novice, any pointers on this would be much appreciated.

Thanks

JR

Look at the condition of the slots where they begin to stick, possible slight damage, but enough to stiffen.

I'd guess you're looking for minor corrosion or sticky grease in the horizontal slots on the side of each jaw, and the gooey bit may be out of sight. Try sousing it with a mixture of paraffin and thin oil and rubbing the jaw in and out with the scroll removed. Acetone and ATF oil makes a good penetrant. Don't be tempted to get the emery cloth out, though oily scotchbrite might help.

You probably only need a few tenths of a thou' irregularity to cause each jaw to catch.

HTH Simon

Is yours the type of chuck having the jaw guides made as separate steel inserts fitted into grooves in the body rather than the usual style where the guides are directly machined into the body?

My 160 mm 3 jaw that I bought around 30 years ago has that style of jaw guide.

If I recall correctly the instruction leaflet had some details on what to do if a jaw was a touch tight. I put the leaflet somewhere safe and might be able to find it if it would be of any direct use to you..

Clive

Is it a recent acquisition, or something that you have had for a while ?

What's the provenance?

Check all the corners. If they are the correct jaws then often damage to sharp edges will throw the whole thing out. You can stone the corners without touching the mating faces.

regards Martin

There is some argument for leaving it as it is, lightly stone all surfaces to be sure there are no burrs then just lubricate and use.

Make a temporary extension for the chuck key or eat another Weetabix.

As you use it, over the years it will wear. It is never going to wear tighter, so it truely has been supplied so it will wear in before it wears out.

You can compare this to when you scrape a lathe bed. You should always scrape the tailstock to the highest point of the tolerance as it will never get any higher in use.

Thanks for all the helpful responses. To answer the specific points raised:

Clive: it's a standard cast iron body with the slots directly machined, but thank you for the kind offer;

Diogenes: I bought the chuck years ago from a reputable dealer as 'new old stock' but put it aside when I discovered how stiff it was in operation. I assumed the stiffness was disuse/congealed grease but I was too busy professionally to investigate further. Last year year, having by then retired and having researched how to open a 'native' D1-3 chuck, I stripped, cleaned [inc ultrasonic cleaning] and reassembled it. No better, so I put it aside again in frustration until today.

Taking onboard all the other suggestions on cleaning and stoning, albeit that I can't see or feel any burring, I will attack it again over the weekend and report back.

Thanks to all. JR

You could use some type of marking fluid, such as an ink marker to high light the offending surface. Then just put the jaw in and give it a tap and remove and look for the place where the ink (or chalk) is removed. Could put some on the jaws also.

Not sure Ultrasonic Cleaning would be my first choice. Paraffin, stiff brush and lollipop stick would be first choice.

Robert Butler

If it is as tight as that, you should be able to see the tight spots. Wipe off all oil on the jaws and slots, then look for polished patches, though could also be a torn surface. As both inside and outside sets stick, this looks like the chuck body is distorted, but try the jaws in the "wrong" slots and note what is free and what sticks. Does the tightness reduce if you slightly slacken the back plate to allow the scroll a few thou extra back and forth clearance? Also check the D1-3 pin threads do not project through so as to rub on the scroll. Beware of removing metal other than the tight spots. It is easy to end up with too much slack.

The OP refers to attempting fitment " and without the scroll in place".

Robert B utler

That’s what comes from scanning the thread to see what has been suggested rather than careful re-reading of the original post. That narrows the possibilities, the most likely being that the casting escaped proper seasoning and subsequently warped, or that the chuck has been dropped.

I have a very similar PB chuck, and looking at it you would be very unlucky to distort it without other evidence. However, if the inside jaws were quite far out, or the outside jaws nearly in, it does look possible to bend the track about half way along with enough force. It would need to be dropped a long way, and onto a wooden floor for there to be no marks.

I suggest checking the front face for distortion (with the jaws removed). This is easier said than done, but a good straight edge and a light may show something up. It would only need to be a couple of thou.

Edited By Macolm on 04/11/2022 15:54:28

I rescued a chuck from stiffness by just lightly stoning/scraping the slots in the body. it had become distorted as a result of having to unscrew it from a Myford Super 7. Sufficient force had been applied to shear the key in the headstock spindle. (In the end I had to reduce the backplate to swarf to get it off!)

So you should be able to locate and tease away the offending distortion and end up with a smoothly-operating chuck. Good luck with it!

Something that came to mind today is to try alternative positions for the jaws to see if the fit improves.

Clearly, you have to maintain the 1, 2, 3 jaw sequence for the scroll to work but you could try jaw 1 in slot 2 and jaw 1 in slot 3.

NOS = returned

First question: What is the fit of the other set of jaws?

Next question, if there are none, do the jaws have the same number?

As Dave says, probably a returned item, not actually new old stock.

The OP stated the outside jaws suffered the same problem

Robert Butler

If both sets of jaws bind in the same place, problem is most likely the slots in the chuck that they fit into. I would look very carefully in there for any burrs or shiny high spots where the jaws have rubbed on them. Then you would have to get in there with a very small rubbing stone, or something like an old magneto points file, and remove the burr or high spots very gently. Even a little bit of fine emery paper wrapped around a suitable piece of flat bar or wood might ease the few tenths of a thou needed to free things up.

Covering the jaws in felt marker pen and then sliding them in and out a few times might show up where the high spots in the slots are. according to where the felt pen ink is rubbed off.

I know when you return an item you are supposed to include all packaging, but for someone to reapply the manufacturer's protective grease (the OP states he has stripped and cleaned the item) before returning it seems to me to be going to the extreme.