Which position to tighten the three jaw chuck?

[rodm1]

I tighten at jaw 3 that is the one I use it indicts the best on my chuck.

 

[woodchucker]

A three jaw chuck scroll is worn the three jaw is not very accurate.
The scroll bearing needs to be under 0.000,1" clearest to a good chuck.

Machinist on a used chuck will use brass hammer for more accuracy.

Dave

Dave, I have no clue what you said.

 

[CJ5Dave]

My chuck is missing 2 pinions. Don’t have to think that hard.

 

[darkzero]

Hmm, my 4 jaw has 4 pinions, which one should I use?

And my.... this chuck thing has only 2 pinions, which one I should I use?

(Sorry, I couldn't resist)

 

[Rex Walters]

Anyone ever do an actual test to see if there is a measurable difference in run out depending on which pinion was tightened and 1 vs all 3 tightened?

I did a long while back. I don't remember the exact numbers but there were minute (but perceptible) differences. The difficulty is that the diameter of the work (position on the scroll) and the torque you put on the chuck key also have significant effects. Manufacturing quality and the amount of wear in each pinion likely has a pronounced effect as well (my most clapped out chuck only has a single pinion, so I can't compare between my two 3-jaw chucks).

Whenever I'm doing anything finicky, I've gotten in the habit of always using the same pinion for the initial tightening, then cinching up the other ones (which invariably have a slight but noticeable amount of slack), and I always try to apply the same amount of pressure on the chuck key in each position. It's probably superstition, but I figure the fewer variables the better. Of course this probably also means that I'm causing the most wear on a single pinion!

FWIW, I always start with the pinion closest to the branding badge on my 3-jaw chuck. I think this is usually the jaw marked "1".

@Net30 ’s question: jaw location with regard to the stock, nothing about which pinion to use or should all pinions be tightened.

I've also learned to listen closely to @benmychree. Experienced pros like John know what they are talking about, and I like to hear their thoughts, even when I initially think their answer might not be related to the initial question.

The initial question was which "position" to tighten the jaws. There are two ways to interpret that question:

1. Which pinion to use when tightening.

2. Which way to orient the jaws when loading stock.

To me, the second interpretation seems a little silly, but there are NO stupid questions. I can't imagine it making a lick of difference as long as the stock ends up in the middle of all three jaws after tightening — positive mechanical stops are positive mechanical stops. What matters is where those stops are after tightening.

Changing which pinion you use might have an effect on runout if one pinion has more wear than another. The amount you tighten might have an effect as you change the position on the scroll slightly, but I can't think of any reason for the jaws you rest the stock on before tightening to have any effect on accuracy, stability or anything else.

With heavy, large diameter stock, resting the stock on two jaws underneath before tightening is a little easier, but if that isn't intuitive then this might not be the right hobby.

If 2 had any effect it would be negated before the chuck completed its first revolution!

 

[Winegrower]

I recall a mechanical engineering professor telling me that the universe is actually made of jello, meaning that there is an elastic modulus for about anything. Therefore, I generally tighten each position much as I would treat wheel lugs, keeping the scroll from differential stretching. Does it matter much? I think it might, especially when yarding down on a critical piece that can't come out.

 

[Smithdoor]

Dave, I have no clue what you said.

To very accurate scroll bearing needs to be under 0.000,1" clearest to chuck boady.
If ever check this on old worn chucks you find 0.002 or 0.003 gap.
So on a worn chuck each time re-chuck a part it off by 0.002 or 0.003"

On good new chuck it scroll to body is under 0.000,1 it re-chuck within 0.000,1"

On a air/hydraulic and 4 jaw independent chucks there is no scroll they can re-chuck within 0.000,1" or better.

Dave

 

[Smithdoor]

Dave, I have no clue what you said.

FYI
This why some will say {just use same position to tighten the three jaw chuck}
It doest not work as the part could move as you machine 0.002 or 0.003"

Best just get good 3 jaw chuck or use a 4 jaw independent chucks so parts come out accurate.

Dave

 

[Rex Walters]

This why some will say {just use same position to tighten the three jaw chuck}
It doest not work as the part could move as you machine 0.002 or 0.003"

I'm curious about this even though I wouldn't re-chuck anything in a 3-jaw scroll chuck if I needed better precision than ~0.003 TIR accuracy anyway.

If I understand you correctly, I think you're saying that a worn scroll could have ~0.002" of play (basically backlash between the pinions and scroll and play between the scroll and the ID of the chuck body).

I think you're saying that if you only tighten one of the pinions, then there will still potentially be some backlash in the other pinions and the scroll could still move slightly radially. This jibes with my experience as I said because there is invariably a slight but perceptible amount of adjustment remaining in the other pinions after tightening the first one. Tightening all the pinions removes any backlash and ensures the scroll can't move radially.

Still, this is exactly why I always tighten the same pinion first, and then cinch up the remaining ones: to ensure that the scroll always shifts in the same direction direction each time. If I used different pinions to tighten first, then each time I chuck up a part it could move in a different direction each time before everything becomes fully tightened, reducing accuracy.

Do I understand you correctly?

Of course, it's possible to do precision work on even the most clapped out 3-jaw scroll chuck: just leave some stock to hold onto and don't remove the part until all features are finished and the work is parted off. I rarely hold anything but rough stock in a 3-jaw chuck. Once I have machined surfaces and need accuracy I use collets, dial in a 4-jaw independent chuck, or even turn between centers.