Dialing in a 3 Jaw Chuck

[J Tiers]

..........
Set tru type chucks are an extremely expensive work around which the chuck manufacturers would like everyone to spend their money on.

I always leave a little play in the chuck to backplate register. That is the poor mans way of simulating a set tru chuck. In fact, I would rather do the fine adjustment that way and would not want a set tru chuck. .............

Why the hatred of set-tru chucks? You seem very bitter.

I'd rather dial in the chuck in a minute or so, with positive adjustment, than fiddle-fart around tapping this way and that way, in an effort to get the chuck centered, only to have it move off when tightened. Do you also prefer a "wax chuck" like watchmakers had to use 160 years ago?

The set-tru are very useful, not that expensive when bought used. I got a very nice 4" set-tru chuck for $50 or so. Contrary to the usual set of "stern warnings" from the "experts", they DO repeat well on successive workpieces, so long as each workpiece is not cut from a different piece of mystery metal.

I'd call a set-tru chuck a "poor man's collet set", with the advantage that one size really does fit all, up to the max size the chuck will hold. (naturally, you reset for a different size work, but you'd have to change collets also).... And collets for 2" work are not exactly cheap, even if you can get one, nor do they hold like a chuck at that size.

[old mart]

A new Bison 6 jaw 125mm with the fine adjustment costs £1120.67 with a generous discount of 28%. Pratt Bernerd 125mm 3 jaw fine adjustment a snip at £545.70 with 15% discount. Not many people can find one at $50. I can slacken off six screws and dial in a chuck in less than 4 minutes. I agree that if you put the same size of work in a scroll chuck there should not be a noticable deviation.

[Willy]

If you mark the part and jaw with a sharpie, often you can take it out and get it back with pretty decent accuracy. Try different pressure on the tightening chuck key to move it a little until it dials back in.

Thank you lakeside53!
Have tried this in the past with different chucks and achieved mixed results.

Tried it today just for yuks with the 3 jaw that is on there now and I don't think I could have dialed a 4 jaw in any closer after repeatedly taking out a piece of 1.5" bar of 6061 that I'd turned beforehand in order assure concentricity. Try as I may as long as my witness marks were lined up it was good to go. I had dismissed this tactic previously but it is now back on the roster for future use if I need to. I got maybe at most a .0005" variation using a Mitutoyo dial indicator. Good enough for the girls I know.

I didn't get a chance to swap it end for end to see what turns up...maybe tomorrow.

[lakeside53]

Glad it was useful.. Like most things, there are 10 ways to do everything; trick is remembering and choosing one. lol End-to end can work also IF you leave an unmachined portion to grip. Not always possible.

On my chucks I found the scroll pressure to be the biggest factor in getting it "dialed in". Amazing how stuff moves around.

[Willy]

Yes I too have found scroll pressure to a big factor in eliminating runout, along with key position and or sequence.
Next chance I get I'll be trying to swap ends on a piece with a bored thru hole. I know we don't use a dial indicator each time we chuck up a piece as it isn't necessary especially on one setup jobs but it sure is an eye opener.

Maybe I'll go back to using my 4 jaw chucks as my go-to chucks once again. LOL It assures at least that one will use a dial indicator.
At least everything turned out well on this job, but only because I knew in advance to do it as a one setup operation. I can't over emphasize to those new to this sport how important this aspect is in order to achieve success.

[JoeLee]

Buy a new Bison, use the master pinion only, (they are marked with a -0- next to them or a ')forget the other two exist, throw the old chuck away.

All my chucks are Bison, and using that master 0 pinion helps. My small 5" 3 jaw that I use on my grinder isn't a set true. So when I machined the back plate / 5C adapter plate I reduced the register line by a thou and a half so I could tap the chuck into dead true as OldMart mentioned. that's OK for the position the jaws are in at the time of indicating but may change at various jaw positions. If that happens, repeat the process.

JL..............

[JoeLee]

Thank you lakeside53!
Have tried this in the past with different chucks and achieved mixed results.

Tried it today just for yuks with the 3 jaw that is on there now and I don't think I could have dialed a 4 jaw in any closer after repeatedly taking out a piece of 1.5" bar of 6061 that I'd turned beforehand in order assure concentricity. Try as I may as long as my witness marks were lined up it was good to go. I had dismissed this tactic previously but it is now back on the roster for future use if I need to. I got maybe at most a .0005" variation using a Mitutoyo dial indicator. Good enough for the girls I know.

I didn't get a chance to swap it end for end to see what turns up...maybe tomorrow.

I do that all the time if I don't want to set up an indicator.
I always gently and slowly close the jaws up on the part while slowly rotating the part in the jaws. By doing this I can feel any high spot hitting any one of the jaws. I try to find a happy medium and then tighten it down. I then usually turn a clean and true surface and then put that end in the chuck unless there is some reason I'm not able to do that.


JL.................

[lakeside53]

?? But Joe, the clean surface just inherits the "out of true" of the chuck, so it the natural center is 2 thou out , so now will be your worked surface. Sure, if you have a set-tru your way can work (I do that also with my set-tru chucks) ,but that not what Willy has.

[old mart]

If your chuck is not marked for a master pinion, then using a piece of ground stock, I use a Morse taper socket with a ground parallel outside diameter, Check the runout with each pinion in turn and make a small mark on the one with the best results. This could vary with different diameter work, due to the scroll wear, or manufacturing tolerances, so it is not a perfect method, just better than nothing. When I have ground the jaws for scroll chucks, I choose the pinion nearest to the round label which is common on many brands, to tension the jaws before grinding.
I very rarely ever need the work in a scroll chuck to run perfectly, so the fine adjustment is not used often. I am fortunate to have chucks which hold any diameter of work within 0.002" without further adjusting which is good enough for government work.

[JoeLee]

?? But Joe, the clean surface just inherits the "out of true" of the chuck, so it the natural center is 2 thou out , so now will be your worked surface. Sure, if you have a set-tru your way can work (I do that also with my set-tru chucks) ,but that not what Willy has.

Yes, I understand. So if you don't have a set tru you can bump the chuck around a little on the back plate providing the chuck has a back plate mount. Am I missing something ??

JL...................

[lakeside53]

ah. most recently we were discussing not bumping the chuck around at all. i.e. Just putting back work into chuck and keeping register on a prior machined surface.

[old mart]

It is certainly possible to remove and replace work as long as the work is marked, I have done it more than once, but most removal and replacement is to turn the work around and that is a different matter entirely.

[JoeLee]

It is certainly possible to remove and replace work as long as the work is marked, I have done it more than once, but most removal and replacement is to turn the work around and that is a different matter entirely.

That's something I do all the time. I use the 0 mark on the chuck.

JL.............

[old mart]

It is particularly when a bored through hole and the od are machined in one chucking and then turned round to finish the other end that the chuck runout is most noticable. Willy in post #35 mentions this. The first end and bore will be in line with the lathe axis, but after turning the work round you are at the mercy of the chuck and any subsequent external turning may not be concentric with the bore, the bore now having runout. The two external diameters will have the same runout as the bore has to the second od.
Collets are not guaranteed to run perfectly either, the only ones that I have that come close are the ones that I bored with solid carbide while in the spindle. I bored some redundant collets to 6,8,10,12 and 16mm plus 1/4", 5/16" 3/8" 1/2" and 5/8". The Smart & Brown model A only takes collets up to 3/4", the spindle bore is about 15/16".

[mattthemuppet]

I tried the "tap tru" method of dialing in a 3 jaw with an old but decent 4" Bison. Not very impressed. It's really hard to knock it in one direction without messing up its position in a different direction (imagine truing up someting on a faceplate with a mallet) and it can get knocked out of alignment if you have a jam. Next backplate I make will be the usual "hair less than the chuck back ID" method and I'll use the 4 jaw if I need to be fussy or rechuck a piece.