I may be stupid but

Exactly.

Even new chucks are imperfect mechanically and the fit gets worse with wear. Although the scroll mostly rotates, it also moves slightly sideways and perhaps tilts as well. Although the unwanted movement is tiny, it's enough to stop the jaws from fully tightening. Turning the adjusters in sequence tends to release jambing due to non-rotary movement, and allows the jaws to tighten a little more. Three is enough.

Worth doing because the purpose of a chuck is to grip work and it needs all the help it can get from the operator! Finking it's not necessary is a poor substitute for not just doing it - only takes a moment.

Quite a good example of a workshop subtlety that a maker of internet videos might inadvertently spread as a bad practice. Very easy for newcomers to copy the well-presented misunderstandings of an inexperienced machinist who enjoys making videos. Experienced machinists often develop inadvertent bad-habits too. The problem is usually failing to have work peer reviewed by a few qualified critics before publication. Write 1000 times before publishing anything: "Individuals, especially me, are really, really bad at spotting their own mistakes.'

Dave

Yes, all true, but - maddeningly - sellers only issue one key, usually only one or at most two key pinions are accessible from any single position, and I've never met anybody with enough arms...

Certainly if I'm trying for concentricity with the gripped diameter, I've found I get the best results (about 2 or 3 tenths TIR) by tightening all 3 pinions - but it's not common to need that, so in most cases I won't bother.

But even if I have to play about with a few attempts, clocking the concentricity as I go, it's generally still a helluva lot quicker than swapping to a 4-jaw independent chuck. I generally use those for eccentrics or components unsuitable for 3-jaw grip. Because 3-jaws seem so much better today than they were when I started turning, I find myself unwilling to disturb it without very good reason.

No old iron i was meaning the main lathe chuck , not a tailstock drill chuck.

Steve.

I also was taught to use all three positions when tightening a drill chuck and have carried that over to my lathe chucks.

I find it does hold better than just tightening with the one point.

Have you ever noticed that when tightening in a single hole that when undoing it you will find that one will always undo easier than the other two positions.

No Bountyboy i was refering to the main lathe chuck which is a Bernard griptru, that came with the lathe.

Well from all the feedback here, it would seem a good idea to give each key hole a nip.

Thanks for all the replies to my question.

Steve.

Many wood turning scroll chucks only have two holes. Some use a pinion like an engineering chuck but my two of differing makes don’t have them, the key drives the scroll directly.

I've not seen any formal manufacturers instructions on conventional 3 jaw chucks, other than for Griptru or similar designs.
These are effectively just a high quality cuck on a separate, but integrated mount for adjusting concentricity.

The Pratt Burnerd instructions give directions for adjusting the chuck body, and then say that any pinion may be used, "when it is not essential to maintain concentricity within the very fine limits mentioned."
https://prattburnerd.com/wp-content/uploads/2022/01/PBI-GRIPTRU-MANUAL.pdf

I read that as, use any pinion, but if you set something up using one, then continue to use that one, if you want repeatability for a given diameter.
I always use the same pinion on each chuck I have, and seem to get the best concentricity.

A drill chuck however seems to benefit in holding power, by using each key hole, but that seems to be about internal friction and holding power, rather than concentricity.

Bill.

As has been said tightening all 3 for Jacobs chucks is the correct proceedure and will result in a lot less ruined drills which have slipped in the chuck. For 3 Jaw lathe chucks I only do this for tapping larger holes especially with BSW threads and even then there is a limit. 3 Jaw lathe chucks have quite a limited holding power.

regards Martin

I tighten all the way round. But I don’t yank up the first position absolutely tight. I look at from the point of view that less force on each point is better than forcing on a single point on the scroll.

Owing to the movement of the scroll as mentioned by SOD above you should first do some tests on different diameters to see which jaw to use as teh last one that gives the best concentricity at each diameter and use that when it matters.

If the chuck was previously used by a brainless gorilla then the scroll will be distorted at common diameters like 1", 1/2" etc.

Hi, this argument has been going on for many years, even in industry, with both drill and lathe chucks. My father (who worked for a well known bearing manufacturer) taught me the way he was taught in his apprenticeship, that the correct way was to slightly tighten each of the three holes and then go round again to give each hole a final tighten. When others that I have worked with, just use one hole on a pistol drill chuck and they get a hole that the drill snags in (which was quite often) the drill would stop and the chuck would spin, eventually resulting with chewed drill shanks, I never got that happening, unless I was forced to use a drill that was already chewed up. I have always adopted tightening the chucks the way my father taught at work and at home.

Regards Nick.

Most times that I use the three jaw chuck I only tighten it in one place and it is accurate, however, if I have particularly heavy machining to do I have found that if I use all tree key holes the job is held more firmly. Can this be due to three times the friction at the scroll gears ? Colin

So, tighten all 6 holes then.

I've got two 5" lightweight chucks with only a single pinion on each.

I was told that it was a balance thing to have equal weight and not to have one pinion throwing it out of balance. I have always used one pinion to tighten the chuck whenever I went it to run true as that one marked O is the one which was used when the chuck was set up. That was what we were instructed to when I was an apprentice and carried on doing so.

David

Like Dave Halford, I too have a 5" chuck with a single pinion. It must have come with a job lot, because I only spotted it 3 weeks ago when I decided to clean my chucks, I was very much surprised at this! No name on the chuck and it was in a light blue box with a German language print saying 3 jaw chuck. Looks unused to me, still in makers greased condition.

Anyone know the origin of these oddball chucks?

Regards,

Andrew

In a recent thread about manufacturer's chuck test certificates (long vee and short vee), I suggested people seek out Indian Standard IS2876-1999, which is a parallel of the ISO standard for scroll chucks.

In that Standard, it states that the tests on which the certificates are based should be carried out using the master pinion.

----

There seems to be some confusion on the three-pinion issue. Everyone is talking about tightening the chuck. My chucks work in a bi-directional manner, loosening as well as tightening. Not having to spin the chuck around is a big advantage when removing the work.

Because of the unavoidable clearances required for the mechanism to work, there is likely to be a slight difference in the workholding accuracy dependent on which pinion is used to tighten the jaws. Only by testing using accurate test pieces can the master pinion be found. That is the least inaccurate one. I use the test bar which is about 3/4" diameter and a Morse taper socket with a 2" diameter ground cylindrical outside. Simply tightening the jaws on a workpiece is not good enough, you should move it in a circular motion as the jaws come in contact until tight. Tests should be made on each pinion and the best results recorded. As many different diameter test pieces as possible will check the accuracy of the scroll.

If setting up to grind the jaws in situ, then one of the pinions should be chosen and that will be the master. Unless the chuck is well worn, the differences between pinions will be minimal and having a master jaw would only be needed for critical work. For instance, turning and boring and then parting off without disturbing the workpiece in the chuck will be exactly the same with any chuck, good or bad.

..TOS come in a blue box with German language labels.. ..some of the smaller Toolmex also had single pinions, not lucky enough to know what colour boxes they come in though.. ..both are good chucks..

FWIW I was taught to tighten on the master and use any to release.. ..also not to wring the life out of a 3 jaw..

Edited By DiogenesII on 24/06/2022 20:50:14