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Master key - precision chuck.

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mgj03/06/2021 13:01:42
1017 forum posts
14 photos

I have a Burnerd super precision chuck (not a Grip - Tru). It is just a precision 3 jaw.

It came new with a bit of paper giving the master sockets, one for internal and one for external tightening, and for the 2 different sets of jaws. (and all the run-out for each).

The problem is, which socket is which. The jaws are numbered, but looking at the front of the chuck from the tailstock, is the master socket to the left or the right of that jaw? Because the sockets are not numbered.

Anyone know the convention?

JasonB03/06/2021 13:10:18
22578 forum posts
2637 photos
1 articles

Are the sockets not marked like this one?

mgj03/06/2021 14:11:40
1017 forum posts
14 photos

Jason - yes. The jaws are marked, like that, but which socket, for the chuck key, refers to which jaw? So is no 3 socket left or right of no 3 jaw?

With all the internal and eternal jaws, there is a lot of fiddling to find out by trial and error!

best. M

JasonB03/06/2021 14:27:03
22578 forum posts
2637 photos
1 articles

I meant the "O" by the visible socket

Zan03/06/2021 14:33:38
308 forum posts
20 photos

So so easy to find out. Just Chuck a piece of very round material. Tighten with each socket in turn noting the dti deflection. Remove bar, rotate it a bit and repeat.
it won’t take you long to find the best socket with the minimum eccentricity which can then be marker with an E for external. Repeat for internal with a suitable blank, and mark the socket I for internal.

if you want a more accurate test, repeat with different dia stock before marking.

 Not much faff, it will only take 5 mins

Edited By Zan on 03/06/2021 14:35:25

Speedy Builder503/06/2021 15:24:37
2592 forum posts
207 photos

Can someone explain why it makes a difference (It obviously does or it wouldn't be mentioned).


Clive Foster03/06/2021 17:13:04
3104 forum posts
107 photos


Has to do with how the various internal clearance needed so things can actually move are taken up when the jaws grip an object. Also the jaws aren't identical the scroll isn't perfectly concentric and the slots aren't exactly at 120°. So when the key is turned and the scroll shuffles a teensy bit to take up the load the amount the workpiece shifts isn't quite the same for all three pinions. Inevitably the amount of shuffle varies between pinions and the actual effect of any such shuffle on concentricity depends on the various errors in other components.

All very small but the effect is real. But pretty much irrelevant in general use. Maybe of importance if you want to minimise induced error when re-chucking work in the three jaw. But expecting concentricity to be held when doing that is maybe optimistic.

Certainly I've never bothered with master socket and similar with my PB Precision 3 jaw. I just wanted the better quality chuck and less cumbersome jaws.


Edited By Clive Foster on 03/06/2021 17:13:23

old mart03/06/2021 18:17:03
3720 forum posts
233 photos

When you look at the tolerances on both types of chuck and find they are the same, it is obvious that the extra price pays for somebody to do those tests which the buyer could do for themselves just as well. I tend to be suspicious of makers who do not put the country of manufacture on their products, all the P, PB and B chucks that I have are marked made in England.

Edited By old mart on 03/06/2021 18:21:57

mgj03/06/2021 18:21:56
1017 forum posts
14 photos

Jason - no. Mine doesn't have that zero on it.

Zan, Thank you - I have a ground test bar.

I suspect a large ball bearing race would be a good external check.

Neil Wyatt03/06/2021 20:16:03
18990 forum posts
734 photos
80 articles

I think it's usual for Jaw 1 to be OPPOSITE socket 1.

But (a) I would not bet my life on it and (b) different manufacturers probably have different conventions.


old mart03/06/2021 21:24:08
3720 forum posts
233 photos

I had never considered the numbering of the scroll sockets. When I check for the best performing socket, I try with bars of different diameters and choose the socket with the least averaged errors. With a scroll chuck there could be three different master sockets in theory, with inside jaws the clamping is only inwards at the centre, but outwards for the steps. With outside jaws, the clamping should be only inwards, the outer part of the jaws is not intended to hold work. This may not be true if the chuck has two piece reversable jaws.

I have found the best socket, got the chuck body running true, and then set up the jaws for grinding. The best PB 5" is very good for a scroll chuck, 0.001" or better at any diameter when its clean. If better is required, the register is loose for a poor mans set true.

Paul Lousick03/06/2021 23:39:34
2013 forum posts
711 photos

My Burnerd 3-jaw chuck has 1 socket which is deeper than the other 2 (which only allows the key to go half way into the chuck). Therefore only use the full depth hole for final tightening.


Edited By Paul Lousick on 03/06/2021 23:43:04

old mart04/06/2021 12:03:56
3720 forum posts
233 photos

I have never noticed any difference in depths of PB key holes, so after cleaning, the pinions get put in any position when re assembled. It seems a strange thing to have shallow holes, as the 3 positions are intended for convenience when loosening the chuck jaws, saving having to turn the spindle. Six jaw chucks are more likely to have only one key pinion, so that there is no choice either way.

Edited By old mart on 04/06/2021 12:09:17

mgj04/06/2021 15:41:08
1017 forum posts
14 photos

Old Mart thank you - that is exactly the faff I was on about. It isn't just about holding a single diameter bar. And indeed, on this chuck, the internal and external "best" sockets are different.

Depending on which measurements one is taking, in addition to a test bar, you need a fairly stout ring that won't deform, and at about mid range. This is a 6" chuck so a ring of wall thickness of around 3/4" and 4" id, turned truly concentric on both ID and OD is what one is on about.

So one needs a chunk of ali or 220M07 a couple of inches long and 6" or near in dia. Then I have to bore a 4" hole in it.

That to me is a bit of a faff, and not quite a simple as some have supposed. Especially as Messrs PB have already done the work!

So, back to the question - does anyone know the convention and my thanks to Neil Wyatt for the suggestion.

JasonB04/06/2021 15:45:48
22578 forum posts
2637 photos
1 articles

Have you tried contacting PB?

old mart04/06/2021 15:57:44
3720 forum posts
233 photos

I am lucky to have some Morse taper sockets for capstan lathes, they are different od's and ground true. Also, I have various sizes of ball race inside and outside elements kept mainly as parallels for milling. With care and testing, even the inexpensive Chinese and Indian chucks can give pretty good results. At the museum, there is a Chinese K11 125mm Huhhot chuck, made in 2003 which is the equal of any of the PB chucks we have. Unfortunately they seem to have gone out of buisiness.

I mark the best socket with a small scratched cross, just in case the sweet spot changes in the future.

Edited By old mart on 04/06/2021 16:00:04

old mart05/06/2021 17:34:35
3720 forum posts
233 photos

_igp2787.jpg_igp2786.jpg_igp2785.jpgI have an update from preparing a 160mm three jaw scroll chuck for jaw grinding. Firstly, I modified the backplate for the lock to make running in reverse safe. After that was done, I skimmed the front face, it had 0.001" tir wobble. The chuck got dismantled and cleaned in the parts washer. The jaws were drilled in two places each, 6mm x 10mm deep with a solid carbide drill. The steel is surface hardened, but still quite hard in the core. The chuck is one I bought cheaply second hand, but it had never been used. I think it is of Indian origin as the jaws seem to be Bison dimensions, not the K11 design.

Before doing anything else, I got the main body running true, and then tried out three sizes of ground barstock. Smallest was a 16mm od test bar, then a 1" od MT capstan lathe socket, and finally a 55mm size. I couldn't find anything bigger, but not too big.

The results have been remarkably consistant. All three sizes using all three sockets gave 0.003" or slightly less runout with the high spot always in the same place. So far, I have only checked the inside of the inside jaws. It should be pretty good after grinding.

Since there was no definite best socket, I will use the one marked with an O.

Edited By old mart on 05/06/2021 17:37:22

Edited By old mart on 05/06/2021 17:39:19

Tony Pratt 105/06/2021 19:28:05
1930 forum posts
12 photos

A 3 jaw is a 3 jaw, never super accurate but good enough. Not the best work holding device for 2nd op machining.


old mart05/06/2021 19:35:35
3720 forum posts
233 photos

I have a serrated jaw chuck for when precision is needed and the 4 jaw independent is not ideal. Of course, the soft jaws have to be machined up every time, and getting the exact size without a three point bore mic is a bother. I have turned up a stepped gauge in the past when boring the jaws when the best fit is needed. With that chuck the master key that I find easy to remember is the one nearest the round label.

Edited By old mart on 05/06/2021 19:38:53

John Reese06/06/2021 23:17:04
1035 forum posts

I doubt that it makes much difference which slot the no. 1 jaw goes in. Just make sure that the jaws are in the right order and that engage the scroll properly. As to the order of tightening the pinions I suggest you find out which pinion gives the best concentricity and always use that pinion when tightening. My 6" Buck and 8" PBA have only one pinion. I don't bother with which pinion to tighten!

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