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

Bob

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.

Clive

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

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.

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

Have you tried contacting PB?

I 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

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

I finished grinding the inside jaws of the 160mm chuck, both the inside and the 2 outside steps and stripped and cleaned the chuck. The outside jaws will have to wait, I haven't checked their running yet, and Mike needed the lathe, anyway.