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

What's the provenance?

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.

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.

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.

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

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.

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.

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.

..if it went to a works / trade customer with two of each set of jaws binding in the same slots it probably would have gone straight back home still a virgin..?