Which position to tighten the three jaw chuck?

f350ca said:
As a side note.
On a quality chuck, one of the pinions will be marked, usually with a 0. That is the one they used when grinding the jaws. If you use it you will get the least runout.
Unless the chuck is totally worn out.

Greg
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You most used chuck are worn because most machinist do not lub chucks. If lube the chuck it may spit oil or grease. They did not buy the chuck and do not care about lubrication. They do to any machine tool oil free.

Dave
 
My Buck Adjust Tru chuck has only one pinion; when the pinion is on top, one jaw is clocked horizontally pointing towards the operator, in my opinion, there is no need to tighten all the pinions as I have seen all too many videos portray, the best advice, I think, is indeed mark one pinion and use it only.
 
If you read the original post in its entirety, you will see that only @GrifterGuru , @jwmay & @55fairlane addressed @Net30 ’s question: jaw location with regard to the stock, nothing about which pinion to use or should all pinions be tightened.

Point being, all too frequently we (and I include myself) don't read an entire thread before posting a response, and many times don't use "Reply" to target our post to another specific post so that our reposes is clear. I plan to be more thorough in the future, so that when someone poses a question, asks advice or requests help with something that my response is directly related to the original issue.

That said, providing other, related information can be useful, but should be noted as such; for example, "I never really pay attention to which jaw or jaws are under stock in a 3-jaw chuck when I am tightening the chuck, but I do make a practice of always using the pinion between jaws 2 & 3 to tighten the chuck." Or, "Regardless of how you locate the stock inside of the jaws, I was always taught that after initially tightening the jaws, you should check that all three pinions are tight, just like with a Jacobs-style drill chuck."

Mini-Rant concluded.
 
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ChazzC said:
you should check that all three pinions are tight, just like with a Jacobs-style drill chuck."

Mini-Rant concluded.
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Ive never understood this line of reasoning, especially with a jacobs chuck.

On a jacobs or similar, your turning a sleeve with the key. Where you turn it from is irrelevant as it will always be a sleeve.

As mentioned above, if BUCK thought one key was good enough (even if its sometimes inconvenient) then tightening one key is good enough for me.
 
Whether a scroll chuck is sensitive to the pinion gear being used to tighten it will depend on the clearance between the the scroll and the radial bearing surface. If there is excessive clearance, the pinion will create a tangential force which will tend to displace the scroll in that direction which can create unequal movement of the jaws resulting in runout. Using a different pinion will cause the displacement to be rotated 120º.

I would expect that quality name brand chucks would control that clearance more tightly and thus be less susceptible to pinion related runout. I would also expect that chuck wear would have an influence.
 
One thing I often have to consider is this...

What happens if/when/where the stock is larger than the chuck bore? It is a lot easier to rest the stock on a jaw to lessen weight in the hand whilst tightening the jaws.
 
I have no choice but to tighten the 3J with one jaw down. The pinions are located centered between the jaws, The pinion must be straight up in order to be able to have the handle of the chuck key clear the head stock. Thus I never really gave it much thought. I am of the school that jiggles the stock as the jaws are tightening to help settle everything.

Resting a heavy piece of stock on the bottom jaw is not an issue, ultimately all 3 jaws will be exerting the exact same pressure on the part and on the scroll. If the pressure is different on one jaw then the chuck is not yet tight. If you think about it one jaw will always make contact with the part first and that jaw will move the part in the general direction toward center until contact is made with the second jaw, now those 2 will continue to move the part toward center until all 3 are touching at winch point the part is centered and tightening begins.
 
Flyinfool said:
I have no choice but to tighten the 3J with one jaw down. The pinions are located centered between the jaws, The pinion must be straight up in order to be able to have the handle of the chuck key clear the head stock. Thus I never really gave it much thought. I am of the school that jiggles the stock as the jaws are tightening to help settle everything.
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I had that problem with my four jaw chuck key, I cut about 1.8" off each side of the bar and my chuck key clears the headstock. Some touch up paint erased the unsightly gouges.
 
I spy a Bison chuck in the OP's pics. Does it not have a 0 stamp near one of the pinions? Both of my Bison chucks do & most other Bisons I have seen. From what I was told the marked pinion is the one the manufacturer used for final grinding of the jaws during production & in theory should give you the best repeatability. I always use the marked pinion for tightening. I really only use the other pinions for loosening.

Some people like to tighten all 3 pinions (when applicable) evenly. I never do that with my own chucks, I don't have a need to. I've only done that with chucks that are worn and/or don't grip well by just tightening one pinion.

Most economy chucks don't have a marked pinion or even the jaws numbered or serialized with the chuck. I have a cheap China chuck for certain uses that didn't have any pinions marked. I choose one that I thought was best, marked it & I consisteny use that one for tightening.
 
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?
 
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