Further testing needed ...

Further testing having been completed the chuck has runout with all 3 sets of jaws of around .007" . Chuck body is more like .002-.003 . A known straight 1/2" rod is not pointing straight with the ways ... looks like it's time to set up the toolpost grinder . So it looks like maybe a cheap chuck , though well made . Fit and finish are good , everything fits as it should and the scroll moves smoothly with all 3 pinions . Which brings me to a question - since there's more than one pinion is it going to make any difference which one I use ? Do I need to give 'em all 3 a twist when I mount a piece of stock ?

Does a known straight rod point straight and parallel if is directly in a collet that fits in the native spindle bore? If not then level (untwist) your lathe before doing anything else.

Which pinion to use? I have no idea. Whenever I use a scroll chuck I try to do all operations that are important to be concentric in a single setup so runout is not a big deal. If I can't and close is good enough sometimes I wing it or carefully clock the part, but if it must be dead nuts I either dial in the set tru for that size or use the 4 jaw.

I expect when I get the turret/collet lathe finished I'll do most of the close enough repetitive work in it.

Does it have a pinion with a 0 next it? All my 3 jaw self centring chucks have one and is supposed to be the pinion which result in the minimum runout.

Further testing having been completed the chuck has runout with all 3 sets of jaws of around .007" . Chuck body is more like .002-.003 . A known straight 1/2" rod is not pointing straight with the ways ... looks like it's time to set up the toolpost grinder . So it looks like maybe a cheap chuck , though well made . Fit and finish are good , everything fits as it should and the scroll moves smoothly with all 3 pinions . Which brings me to a question - since there's more than one pinion is it going to make any difference which one I use ? Do I need to give 'em all 3 a twist when I mount a piece of stock ?

The lathe is level and no twist as measured with a .005"/foot precision level . I have no collets to test with , but with the 5" 3 jaw I get no measurable taper .

I'll have to look and see . I haven't noticed any markings next to one of the pinions .

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In the new Mongolian 6-jaw I cleaned up the scroll plate fit quite closely on its pivot in the body, too tight until it was cleaned. I don't think it could shift away from whichever pinion I tightened. Perhaps it might in a worn chuck.

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I don't think this chuck has much wear . All the jaws look "new" but that doesn't always mean much . I was told the original owner was into woodworking , which doesn't necessarily require the precision needed for metal working . By tightening all 3 pinions I have gotten it down to average .004" , which is pretty close to my original equipment 5" chuck . I can live with that ...

I don't think this chuck has much wear . All the jaws look "new" but that doesn't always mean much . I was told the original owner was into woodworking , which doesn't necessarily require the precision needed for metal working . By tightening all 3 pinions I have gotten it down to average .004" , which is pretty close to my original equipment 5" chuck . I can live with that ...

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I've considered using a bearing press disk set to measure the jaw inner face runout at several diameters, to see if it varied due to wear or errors in the scroll or was a constant error of the jaw faces, which might be corrected on a surface grinder. A 3-jaw will grip evenly if the disk isn't quite round.

For a 6-jaw that won't a stepped cone of plastic or a trued lawnmower blade balancer could have enough flex to allow all jaws to clamp tightly, then measure runout on the outer jaw surfaces and subtract their thicknesses, a job for a spreadsheet.

I haven't tried this because I have 5C collets for precise work, a Set-Tru, and lower cost 3 and 6 jaw chucks on 5C mounts that can be moved between lathe and mill without unclamping the workpiece. Since I design what I make some surface can usually be sized to fit a collet or mandrel for second operations or later mods.

It appears that tubing thin and flexible enough to need a 6-jaw is also flexible enough to fit a shaft despite jaw runout, for instance bearing spacer rings. I haven't yet had a job that would require fitted pie jaws or a pot collet. jsw