View Full Version : Pratt Burnerd 4 jaw - jaws are TIGHT

03-19-2007, 08:41 PM
I've got what appears to be a very low use Pratt Burnerd 8" 4 jaw chuck. The body is pristine with only some patina from hand prints. The L00 taper is very nice and clean (unlike many that get chip scars). And the jaws look almost new, though they do have a scar from what appears to be a carbide index tool the put a small diameter groove in the "inside" jaws. So, point being, it's not new, but it's not thrashed by any stretch.

I bought it at an auction for a very good price. But no chuck keys around, so I just looked at it's condition and "wiggled" the jaws. "Man, nice and snug, no wiggle at all, must be all but brand new." I say to my self. Bid went low, and I got it.

Fastforward to home that evening, examining my prizes.

Took my chuck key (didn't get one at the auction, box of keys in all sizes went for twice what the chucks did <shaking head>) and tried it out. MAN those jaws are stiff, but appear clean, what's up? And then it happened, turned the grub for jaw #1 and it turned REAL easy, but the jaw didn't move. Grub was broken at the thrust yoke. (sigh) And after clearing the thrust yoke, that jaw had to be driven out with a brass drift. Not wailing on it, but it sure didn't come easy. PB had a screw in stock and it is in transit, so still well worth the money.

Question is, why are the jaws so freaking tight? The appear to fit the screws just fine, but you can see it's rubbing HARD at the top of the jaw where the shoulders press against the face of the chuck. I blued up several jaws and the story is the same on them all, though #1 is the worst by a considerable margine. I used a stone to debur them and it helped, #4 is now fine, #2 and #3 are passable but slightly stiff over a small range, and #1 is still tight.

Makes me wonder if someone mixed up the jaws? How best to handle this? More stoning followed by Clover lapping compound? Should I be stoning the chuck or the jaws? I did try a tille light stoning on the jaws and the stone barely touched it.

03-19-2007, 08:50 PM
You may want to follow the guideline of modifying the part that's cheapest to replace...which in this case would be the jaws, not the chuck body.

But I think I might first try a bit more to figure out which is the problem; which part is "bad." Assuming that's at all possible, which it may not be.

03-19-2007, 10:16 PM
Measure the slots the jaws fit in with jo blocks or adjustable parellels to see if they are all the same. We had a new chuck at work that was tight like that . I spent about 1/2 day with files and stones working on the chuck body to get the jaws to slide in and out the jaws are easy to mic. so check them also.I bet the body has a slight warp to it but should file and stone out . sit down and get after it .you will have a good chuck for cheep and a little effort. That is what it is all about. fixing things Good luck.

03-19-2007, 11:17 PM
Unfortunately, I have no jo-blocks or adjustable parallels. I will do some measuring tomorrow with telescopics. Oh, or maybe I can use my planar gage... Hmmm...

Anyway, I'm going down town tomorrow to visit my friend with the nice shop. Depending on what measurements say, I think I'll take the chuck with me and see about using his surface grinder as mine is still not fully functional.

03-20-2007, 02:43 AM
Did you try putting all the jaws in the #1 slot, one by one to see what happens? Mix them around and maybe you can find a combination that works.

03-20-2007, 04:45 AM
Just give it a good sandblasting, you'll do fine.


03-20-2007, 08:19 AM
The jaws may have been from another chuck, and somewhere, someone has a PB four jaw with loose jaws. They may have come that way, which would explain the condition of the chuck.

Follow Lane's advice, don't be afraid to do a bit of careful filing on the chuck body, as the jaws are hardened and will be difficult to work with normal shop tools. It will be easiest to remove the yoke and screw when fitting the jaw, and work until you get a nice sliding fit. It is a four jaw, and tenths accuracy is not needed.

03-20-2007, 09:38 AM
I'd second the notion of doing some careful checking to see where the binding exists. I received a chuck yesterday that I bought from a guy over on the PM site. It had never been installed on an import lathe and as such was as-new. It was horribly tight and I thought it was the jaw fit. I don't want it to be *loose*....just to be movable. Tighter should produce lower tolerances through better consistency. As it turned out, upon disassembly, the scroll was nearly packed with some Chinese version of roofing tar that was supposed to pass for grease. I have the parts soaking now. Given that this was internal, I am sure the intent was that it was lubricant, but it looked like that horrible packing grade of Cosmoline that resembles roofing tar.
If, per chance, your drag was a cumulative effect, from 2 or 3 locations, and you focused on only one of them, it would be easy to stone on those until you loosened an important fit without solving the problem.

I have noticed that the chucks I have all have a matching number on the jaws and the chuck (somewhere). Only one of these is an expensive chuck (Rohm)...the rest are chinese and even the cheap chucks have this. Does your chuck have this and do the numbers match?


03-20-2007, 12:40 PM
Evan: No, I have not swapped the jaws around. They are all snug to TIGHT, so with none being loose, seems not much point. And then the jaw numbers would be wrong.

Scatterplot: Err, no... ;)

JCHannum: Yeah, I’ve considered both possibilities. Based on condition, I’m guessing it was just a QA failure at PB. And I agree on the chuck being the focus, but it might be easiest to put the jaws on a surface grinder. My biggest concern with working the chuck is getting the channels out of parallel so that part of the travel is loose and part tight. Then again, I guess I could set the chuck on the grinder and touch up both sides...

pcarpenter: This is an independent 4 jaw, so no scroll or inner workings and no cumulative effect to worry about. It also is clean inside the channel/screw slots. Each jaw is numbered and in the correct slot.

On the “roofing tar”, there is an actual chuck grease that is very similar to what you describe. Purpose being it does not sling out and it’s not as sticky as normal grease so that it will not retain swarf as much. Perhaps that is what you have in your chuck, but they just put too much?

03-20-2007, 01:10 PM
I guess what I was asking about was whether the jaws verifiably go with that chuck and not so much about whether they were in slots with matching numbers. The numbering I have seen was something like A5479 which I thought perhaps to be a model number. The jaws have the same number stamped on them (in addition to their location number) so that as you have a half dozen chucks floating around a shop with perhaps two sets of jaws, each, you can always match the jaws to the chuck they actually go with.

I think Evan's point was that you can have a combination of interferences that make them all tight when in the wrong location and all fit correctly in the correct location. I tend to wonder if good chucks don't have their jaws final fitted using some sort of a lapping compound.

I don't even know that otherwise there is a valid reason to number jaws by location for a 4-jaw. The numbering is certainly not about minimizing concentricity;)


03-20-2007, 05:42 PM
I have a "new" PB three jaw that I got from Dave Sobel. It was new in the can, all covered with cosmoline. once I cleaned it I found that the jaws are quite tight. I have lived with it over the last couple of years, and they seem to be loosening a little with use.
I don't think I'd advise you do anything other than dis-assembly and good cleaning, and then check for burrs.

Just my $.02


03-20-2007, 09:38 PM
They do go with the chuck, I verified the numbers today. And bluing shows only the one point of contact, which scrapes it 100% clean and consistently across the surface.

And that super tight #1 jaw has already broken a screw, so I don't think it's a matter of just waiting for it to wear in.

Didn't have a chance to work on it today, maybe tomorrow...