Help needed with stiff Pratt Burnerd 3 jaw chuck.

..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..?

Exactly, where I worked anything new was physically checked for compliance with the purchase spec by goods inwards quality. They would have found the jaws too stiff and bounced the chuck back to the supplier in the same condition that it came in. It goes on back a different shelf at the supplier till Fred who put it there retires and the stock gets computerised etc etc

By way of update:

First, thank you to all who responded to my query. That so many people felt able to give me the benefit of their knowledge/experience/perspective demonstrates both the strengths of the membership and the basic ethos of this forum.

Second, I have now made some progress, with at least the inside jaws. Further cleaning with different agents did not help but using marker ink suggested some areas of unwanted contact.

I proceeded with very gentle and incremental stoning, mainly of the sharp corners, and there began a slow but progressive improvement. I can now slide both numbers 1 & 3 jaws in and out of their slots cleanly by hand. They are not yet quite as smooth sliding as the number 2 jaw is in its own slot but I don’t want to go too far and I see the logic of DC31k’s point on future wear.

Next job is to properly clean the unused outside jaws and then retry them. Hopefully I will have the chuck built up again next week.

JR

I'd be grateful to hear how this turns out. Fingers crossed the chuck will be fine, but so far the root cause hasn't been identified and it might be sinister.

I've never heard of a new Pratt Burnerd chuck having a serious problem before but even the best manufacturers drop the ball occasionally. In the past items like this were made in batches, where typically 60% were acceptable, 20% were excellent, and 20% unacceptable. Duds were weeded out by rigorous inspection, but there are many ways this can go wrong. New, sick, tired or p*ssed-off staff; packing mistakes etc. Dishonesty is always a possibility - someone stealing rejects and selling them privately. Getting a productivity bonus by sneaking rejects through the system. You name it, some crook tried it!

Another possibility is damage in transit or when the package was opened; dropping a chuck could bend the body, causing the jaws to bind, twist the grip out of parallel, and have poor run-out.

It's a detective story with a missing last chapter! I want a happy ending...

Dave

You mean a left handed jaw? There's a thought!

I'll leave now.

Edited By Peter Greene 🇨🇦 on 06/11/2022 22:47:22

Dear All

Apologies for the delay in feedback; I was unwell last week.

The chuck went back together nicely today and the scroll/jaw action felt excellent. Unfortunately, however, the concerns expressed by Dave [SOD] have proved to be somewhat prescient.

There is the merest flicker of run out at the spindle nose, as seen with a 0.0005” dti. Measured at the rear of the chuck runout is at most 0.001” but this rises to 0.004-5” just behind the chuck jaws. With a 1” ground steel test bar, runout close to the jaws is 0.006”, rising to 0.012” at 3 inches from the jaws.

These are the very best figures I could manage, after thorough cleaning of the spindle nose and chuck recess, careful placement of the bar and trying the 3 different possible chuck mountings. In the ‘wrong’ mounting positions, run out on the bar is more than doubled. The old Bison chuck gives much better readings.

Any further advice would be welcome, even if it is “best start saving for a ‘new new stock’ replacement”!

JR

What about the inbuilt D1-3 mounting? As the back of the chuck is Ok but the chuck body gets progressively worse as you measure further away from the mount says to me that the chuck may not be seating properly on the nose.

The usual state of affairs is that as long as a 3 jaw self centring chuck holds securely then run out does not matter since you would never take something out and expect it to go back into the chuck in exactly the same way. You do all that you can without removing the part from the chuck. After that if you need to turn more features use a 4 jaw independent (typically) and dial the part in to get concentricity. I have no idea what the runout on my 3 jaw chucks is. I have never measured it and never will. If I want good concentricity in a 3 jaw to a machined surface I use soft jaws and clean them up before each use. Don't try to fix what is usually not fixable. You can grind the jaw faces but as soon as you go to a different diameter they will be out again. Think of the 3 jaw as a quick and convenient way of holding a rough piece of stock for first operations.

Martin C

Dave Halford makes a good point about the camlock mounting. That you mention trying the chuck in three different positions makes me think you might not be familiar with the subtleties of the camlock system or how to adjust it.

Your spindle should have a scribed line on it between two of the cams. Each item that attaches to the spindle should have a scribed line on it (you might have to make the line yourself) such that the same pin on the chuck always goes into the same hole in the spindle. That way you can 'tune' each pin to each cam so the cam closes between the two marks on it, 90 degrees apart.

A good write up is here:

https://yarchive.net/metal/camlock_stud_fiddling.html

I think (but would not swear to it) that the proper operation of the camlock cams is shown in the manual for a Colchester Bantam or Chipmaster or possibly Harrison 10AA.

These are the test figures.

Measured at the rear of the chuck runout is at most 0.001” but this rises to 0.004-5” just behind the chuck jaws. With a 1” ground steel test bar, runout close to the jaws is 0.006”, rising to 0.012” at 3 inches from the jaws.

The jaws look to be 1 thou worse than the chuck body, which is pretty good for a three jaw. BUT it gets progressively worse at a rate of roughly 2 thou per inch. So a max length10" long bar following the 'leave on the stock' approach means you need to remove 20thou before a fixed steady will work properly. This can put you into buying the next size up bar.

I wonder if the back of the chuck isn't quite reaching the flat front of the register all over the circumference, and whilst appearing home, is actually cockled slightly.
Have you tried lightly stoning the back of the chuck to remove any dings or burrs?
When I received my , new to me, Warco 1330, the chucks did show a little more runout than I'd hoped.
Some Micrometer blue, and careful use of an internal bearing scraper on the inside of the chuck's short tapers.
One thing I tried was a narrow slip of aluminium foil in each of the three gaps between the pins. The subsequent different bruising on each of the foils showed that the chuck was sitting slightly unevenly.
I'd already blued the taper on the spindle register, to make sure there were no protrusions on that; there was one or two, which I lightly stoned down.
This allowed each chuck to pull fully home against the front flat part of the taper.
Also, I did the same as you and picked the best of the 3 pin locations and marked each chuck so it would be replaced in the same orientation each time.
When fitting a chuck even now, I get best results by partially tightening pin1, almost fully pin2 and fully tightening pin3; I then go around again to fully tighten them all.
Each chuck is actually quite a tight fit on the short taper, such that it needs a sharp tap with a nylon mallet to remove it.

Bill

Come on you engineers. Having freed the jaws we are now discussing "run out" of the chuck. Really? We all know that a three jaw chuck is simply a holding device and is not accurate in itself to worry about the run out amount that has been described. What matters is simply what happens to a piece of bar when you turn it. So put a piece of 1 inch bar ( the size is not important) in the chuck and have it projecting about 3 or 4 inches (75mm to 100mm for the Napoleonic fans). Reduce the diameter by about .020 to .030 so that the bar has been cleaned up all round. Then use the DTI to determine if the bar is now running out. The purpose of the lathe is to produce a finished surface which is true to its centre. It is the finished product that is important. If it is OK forget about the run out that has been described.

Nup. If a Pratt Burnerd three jaw chuck is holding a ground bar with more than 2 thou runout, something is wrong and needs investigating. Six thou is way beyond acceptable even on a cheap junk three jaw chuck. You do need to be able to hold stock in it and have it run reasonably true for easy general useage. Even the cheap Chinese chucks will usually hold 3 thou or less.

You need to make sure that the chuck is seating firmly on the taper and back face all the way around, using a very thin smear of bearing blue.

You could check the taper in the chuck body has been machined true to the chuck body by mounting the chuck on the faceplate and set the body to run true, then run a dial indicator on the taper and see what is happening. In a pinch you might have to regrind the taper/face using a Dremel etc in the toolpost.

Are you 100% sure the D1-3 mounting is inbuilt and not a removeable backplate? Seems most PBs use a backplate even for camlock. Backplate can then be machined in situ to run true.

Edited By Hopper on 14/11/2022 04:05:28

Edited By Hopper on 14/11/2022 04:11:07

My similar PB chuck has the camlock features integral with the back plate.

I believe (without any actual maker’s information) that with a Camlock interface, the taper and the face should be in hard contact when done up. You can check that a fine feeler or cigarette paper is nipped by lightly tightening the cams. However, I did once need to lap a new faceplate by about 0.03mm to achieve this. The aim is that after undoing the three fully tightened cams, you should be able to dislodge it from the spindle by a light blow from your fist. This ensures that it gives very repeatable fitting.

I would still suspect that the chuck has been dropped on to a jaw, and the front face bent in slightly. This would cause the run out observed. Check with a good straight edge for a slight depression. Since the affected jaws would be moving over a very slight curve, the run out pattern would presumably vary as a function of the diameter of bar being held. The same pattern could also result from warping of the casting.

Thank you again to all who have responded so far. I have some further feedback, which I’m afraid may need to spread over more than one post:

First, using a good straight edge and a bright light source, I couldn’t see any hint of distortion in the front surface of the chuck.

Thereafter, things get more complicated:

I have tried bluing the cam lock mount, both the short taper and the vertical faces. I then tightened the cams to draw on the chuck and seat it. The result was a good even transfer of blueing onto the taper section of the integral chuck backplate, but none at all onto the flat back of the chuck between the pins or, perhaps less surprisingly, onto the flat bottom of the taper recess.

TBC

CONT

However, I got exactly the same result when I repeated the exercise with the older Bison 3-jaw and the used PB 4-jaw that I bought from eBay many years ago and have never actually used.

I do have a genuine 254+ faceplate, again from eBay but in excellent, unmarked and likely unused condition. Mounting that on the spindle after cleaning etc I did manage to transfer some micrometer blue to the flats between the pins but it was only a partial take, which I could not see as being down to any burring or the like in the unblued areas.

As to the camlock mechanism, and responding particularly to DC31k, the ‘new’ chuck did not come with an alignment mark, thus my trying each pin in turn closest to the line on the spindle. All 3 pins were fitted in the chuck by turning hand tight and then loosened so that the circumferential line inscribed on each pin lined up with the chuck body and the relief for the locking screw lined up properly. All 3 appear to be inserted to the same depth in the back of the chuck, and indeed the projection of each pin from the chuck body appears to be the same as that displayed by the other chucks and faceplate.

All 3 cams close smoothly on the pins clockwise within the 90 degrees ‘window’ marked on the spindle with > <; this is reproducible every time, regardless of which chuck pin is inserted nearest the spindle mark. As far as I can see then, the locking mechanism per se is working satisfactorily, or am I missing something more subtle?

All that said, if I release the cams and tap the chuck to free it with the test bar still in place before immediately remounting the chuck in the same orientation, I find the run out on the bar changes significantly. I have only done this once so far but the run out close to chuck changed from 5.5 thou to 12-13 thou.

I note the comments that run out doesn’t matter and I see the rationale behind them. However, the instructions that came with the NOS PB 3-jaw claim that it is a precision instrument so I do feel there is something wrong here.

Any further advice/suggestions would be very welcome.

Thanks

JR

Edited By Jonathan Richards on 18/11/2022 17:40:41

Edited By Jonathan Richards on 18/11/2022 17:43:22

I would try measuring the gap between the flat faces in between the pins while the camlocks are done up. See if you have an uneven gap around the circumfrence. A feeler gauge might fit in there, unless the gap is under a thou or so. (Or you could use Plastigage that is used to measure bearing clearances in engines.)

Then I would try cutting out a piece of good shimstock that is just a bit (thou or two?) thicker than the measured gap to form a ring with holes in it to clear the camlock pins. Tap all the sharp edges of the shim down with a hammer on a flat piece of steel plate so no burrs are standing proud. Then reassemble chuck to spindle with the shim in place to fill any gap there.

Then measure your chuck runout again and see if there is any improvement. If there is marked improvement, you have narrowed the cause down to that fit between the two flat faces.

A purist would regrind the taper in the chuck until the faces just met under camlock presssure. But a pragmatist would simply continue to use the shim when mounting the chuck in future. Perhaps even super gluing it into position very carefully. (I tend to fall in the latter camp but each to his own.)

Edited By Hopper on 19/11/2022 04:12:25

This Camlock problem would seem to be separate from the issue of the stiff jaws.

A correctly sized Camloc interface certainly works very well for accurate concentricity. The configuration uses a taper for location in combination with face to face contact. The taper interface is really just a location press fit, but with the benefit that disengagement occurs over a very short travel. Unfortunately, the taper makes measurement extremely difficult, the total difference in diameters between free and press fit being probably about a thou. The manufacturer presumably has precision go-nogo gauges.

I faced the easily tackled defect that the taper was not tight when the faces were in contact. The opposite defect would need metal removed from the taper, quite a different matter. I think the suggestion by Hopper to try a shim is good. A quick way is to use paper, as thin as possible. This may still be too thick for the taper to grab, but if the result is good concentricity then the problem is identified.

Another approach may be to remove the Camlock pins, and explore the “feel” as the two parts are brought together.

Regarding the fit of the taper, you might pick up some useful info from this video by Mr. Crispin, in which he regrinds a lathe taper.

Rob

Links to https://www.youtube.com/watch?v=ii0Xbzl1kIw