Help needed with stiff Pratt Burnerd 3 jaw chuck.

Check to that you have a chamfer around the edge where the tapered hole in the chuck meets the flat back face of the chuck. If your spindle does not have a clearance groove to match that location, the chuck could be hanging up on that edge and preventing the full seating of the taper.

If the gap between the flat surfaces on the chuck and spindle is very small, which it sounds like it is, you might get contact by removing any small burrs from the taper surface or by a very light and even dressing of the taper in the chuck by rubbing some fine emery paper over it by hand. It would only take a tiny amount of metal, removal from the taper to give you a thou or two of lateral movement between the flat faces.

Thanks to all for the further suggestions and advice. I will have to split this response again, I'm afraid.

I don’t have any Plastigauge [which I’d not come across before] and I could only find a metric feeler gauge and single piece of shim material which measures up at about 2 thou.

With the PB chuck in place I can get a 0.01mm feeler to go in into two of the interpin spaces and a 0.005mm feeler into the third space, the latter with some ease. This remained the case in all 3 possible pin positions, with some minor changes in feel.

With the old Bison chuck in place I can get the 2 thou shim into one of the 3 inter pin positions but not the other two.

The faceplate, 4-jaw chuck and a backplate I bought to fit to an ER32 Collet chuck all now fit snugly and none will admit the 2 thou shim at any of the 3 inter pin positions. That finding, and the helpful Mr Crispin video Rob pointed to, have assuaged my previously growing concern that there might be a problem with the spindle nose. The nose actually looks very good, with no sign of burring or swarf damage, despite being an ex-school machine.

TBC

Edited By Jonathan Richards on 22/11/2022 17:43:53

CONT

I will need to get some plastigauge and some more suitable shim materials before experimenting further, so there will be a bit of a hiatus in the thread, I’m afraid.

However, if I do find that shimming with, say, 5 thou shim improves the run out and therefore want to glue a proper ring of shim to the chuck back as Hopper suggests, would I be better off with brass or steel shim for that modification?

Thanks

JR

Brass shim is easier to work with and should work ok.

Those are some very thin feeler gauges you quote. 0.005mm is only two tenths of a thou. And 0.01 is about four tenths of a thou. I don't think you will get shim that thin. And the gap is so small it really should not affect runout that much. The taper should locate the chuck, in theory - but there are varying opinions on that. If you go more than about half-thou shim, the taper will lose contact. Maybe one thou shim with a bit of "nip".

For that small amount of lateral gap, you should be able to take about a tenth of a thou or less off the taper in the chuck by hand with emery paper and close it up. It takes very little metal removal on a taper to get a large lateral movement so be careful.

Are you going around and tightening the camlocks up half-tight all the way around before final tightening so it is tightened down evenly? Have you tried easing off one of the camlocks on the "high" side to see if it lets the chuck back into alignment?

Something else you could try is set up the four jaw on the faceplate and get the body running dead true then put a dial indicator on the chuck taper and see if it is running true to the chuck body.

And then you could hold a piece of round bar in another chuck in the lathe and finish turn it so the OD is running dead true. Clamp the errant chuck onto this true spigot by its jaws and then clock the taper and see if it is running true to the jaws.

Edited By Hopper on 22/11/2022 22:04:22

Edited By Hopper on 22/11/2022 22:06:27

I suggest you verify things simply, with ordinary copier paper which is about 0.1mm for 80gm weight. Other weights of paper are roughly pro rata thickness. If that fixes the run out then it is worth making a proper shim.

The back (Camlock) face of my PB chuck is flat right across, so a shim made with a larger outside diameter could be held in place with a ring of thicker material fastened with small screws tapped into the back plate. The holes for the screws could go right through, provided the screws do not project inside. Obviously the ring ID needs to clear the spindle OD. This scheme would be easy to do, and give good protection, even if the shim was also bonded.

This diagram is a very quick concept and not correct in detail.

Edited By Macolm on 22/11/2022 22:12:06

Apologies, particularly to Hopper.

There is a decimal place error in my last post. I meant 0.1mm and 0.05mm in relation to the feeler gauge. I think that makes Macolm's paper trial suggestion even more attractive.

Also for Hopper, in your penultimate paragraph, did you mean 'four jaw OR the faceplate' rather than "on the faceplate"? If not, I'm struggling to follow this step, I'm afraid.

I'm told that domestic maintenance tasks are required for the rest of the week but I will be raiding the stationary stores at the weekend and trying the cam lock tweeks Hopper suggests, before if need be tackling the further concentricity tests [assuming I understand them correctly].

Thanks for keeping on with this thread. I do appreciate the help.

JR

Sorry, I meant to clamp the errant PB chuck, minus jaws, on the faceplate and set it to run dead true, with the camlock taper side facing outwards so you can put dial indicator on it. That way you can check how much runout there is between the PB chuck body and the taper. Also between the front face of the chuck that is clamped to the faceplate and the flat camlock surface surroounding the taper. (Dunno where I got "four jaw" from in the other post. Sorry about that. Makes no sense at all!)

But of course, chuck bodies often run a thou or two out of true. But it is nice to know where you are at when troubleshooting. In the end, it is the jaws relative to the taper and to the flat camlock surface that is important.So to check that, hold a piece of 1" or so bar in a chuck on the lathe and take a fine finishing cut over it to give a nice true, smooth surface. Then clamp the errant PB chuck onto that true cylindrical surface by tightening its jaws onto the surface. Then you can put a dial indicator on the taper and the flat camlock surface of the PB chuck and see if it is running out, relative to the jaws gripping the known true cylindrical surface you have just machined.

Once you have done these tests, you will have a better idea of whether the body and the jaws of the chuck are running true to the taper. If they are, then the problem must be in the chuck sitting cockeyed on the spindle.

Which brings us back to the shimming. As far as shimming goes, 0.1mm to 0.05mm makes more sense. Plenty of shim available in that sort of thickness range. (2 to 4 thou) Paper might work but I don't know how compressible it might be compared with shim. Quick and easy to try though.

I only suggested paper as a one off check to see if getting the faces parallel fixes the problem. Then it would need the correct thickness of steel shim as a working fix. Unfortunately, thin shim stock is not at all robust, and in my experience, bonding thin items does not have a good long term success rate unless additionally protected. Hence the suggestion for a way to provide that. I suspect the shim would indeed need to be bonded as well to prevent ingress of swarf.

I think the Camlock taper is 7 in 24, so a 0.04mm shim would change the diametric fit by only 0.01mm! The configuration is extremely rigid with neither part likely to flex on the diameter.

On checking, the Camlock taper is in fact 1 in 8 on the radius, so 1 in 4 on the diameter.

It occurs to me that, provided the gap when the cams are correctly done up is indeed only 2 thou (0.05mm), then only a bit more than half at thou would need to be removed from the taper diameter. I think this would be within the scope of lapping, and suggest a tool roughly as below to carry probably wet and dry paper, which is thin and uniform. This would need to be cut as an arc so as to fit the conical surface. It could be retained in place with pegs in slotted holes as shown.

This would sidestep the near impossibility (except in a tool room) to get the taper true enough to the precision necessary for grinding. It would not maintain the exactness of the original machining of the taper, but this could only be wrong by less than the amount removed. The taper provides a location, the faces in contact ensure the axis is correct.

Just re-reading my earlier post and it looks like a few words fell off whilst I was editing it.
I mentioned "Micrometer" blue and a bearing scraper, but didn't make it clear that I gently scraped the inner taper of the chuck(s) to suit the lathe spindle on my D1-4 GH1330.
It worked fine, I opened the taper out a touch and that allowed the flat faces to contact each other.

Bill

Thanks to all for the continuing input and advice. This is just a holding update because I haven’t had chance to progress matters as I had planned.

I have, however, established that a ‘shim’ cut from ordinary typing paper reduces the run out to 4 thou at 4 inches from the chuck, which is very much better than previously.

I need to try Hopper’s jaws/taper concentricity test next. Unless that throws up a major anomaly, perhaps unlikely given the paper shim result, I am going to try easing the taper slightly as suggested.

I don’t think I’m adept enough with a bearing scraper to risk that approach so I’m going to see if I can make a lap, as Macolm advises. I’ve never yet cut a taper on the lathe [at least not intentionally] so that should be a good learning exercise.

I’ll post again once I have some progress to report.

JR