# What is acceptable runout on a 3 jaw Chuck



## tmuir (Sep 9, 2009)

Yes I know someone is going to say zero but that's not going to happen.
I noticed my 3 jaw chuck last night was holding bras quite a bit off centre.
I've just finished removing it from my lathe and cleaning it which seams to of improved it a bit but on measuring the runout I'm still getting 0.12mm between max and min readings at 100mm from the chuck, or in the old money just under 5 thousands at 4inches.

This seems like a lot to me.
Should I expect better from a Chinese 3 jaw chuck or is this about par for the course?


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## black85vette (Sep 9, 2009)

What is the run out up close to the chuck? At 4 inches out I just put a dial indicator on it, chuck it firm but not tight and then tap it with a wood dowel until it is under a thou and then tighten it up. My 3 jaw is Chinese also.


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## tmuir (Sep 9, 2009)

At about 40mm its around 0.08mm, haven't checked right at the chuck, will go back outside and have another play.


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## rake60 (Sep 9, 2009)

I believe black85vette may have the answer here.

I went down and quickly chucked up a piece of brass stock that I
know to be straight. It ran out .004 at the chuck and about .007"
at the end. I got lucky indicating it in. One bump with a plastic
hammer brought the end in to 0 run out. I moved back to check it at
the chuck again and it was running 0 there as well. 

[ame]http://www.youtube.com/watch?v=jzVBrJj7RHU[/ame] 

I wouldn't expect to get it with just one bump every time.
That was a real fluke.

Rick


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## websterz (Sep 9, 2009)

rake60  said:
			
		

> I believe black85vette may have the answer here.
> 
> I went down and quickly chucked up a piece of brass stock that I
> know to be straight. It ran out .004 at the chuck and about .007"
> ...



Time to go buy a lottery ticket. :big:


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## scoop (Sep 9, 2009)

Are the 3 jaws in the same chuck slots that they were in when new?Each of the three jaws has a number on it to let you know which order to insert them into the scroll,but some chucks do not have an indication as to which slot the number one jaw should go in.Try each of the three slots with jaw number one and see which one gives the best run out.It makes a big difference on my chuck maybe yours tonly takes about 10 mins to try all three positions.

 regards Steve C.


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## black85vette (Sep 9, 2009)

scoop  said:
			
		

> Are the 3 jaws in the same chuck slots that they were in when new?Each of the three jaws has a number on it to let you know which order to insert them into the scroll



When I first got my chuck I checked the run out with the jaws in each of the slots (after noting the original location) and picked the position with the best run out.  Then I used a center punch and made 1, 2, and 3 punch marks next to the slot for each so I could put them back the same way every time.


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## tmuir (Sep 9, 2009)

scoop  said:
			
		

> Are the 3 jaws in the same chuck slots that they were in when new?Each of the three jaws has a number on it to let you know which order to insert them into the scroll,but some chucks do not have an indication as to which slot the number one jaw should go in.Try each of the three slots with jaw number one and see which one gives the best run out.It makes a big difference on my chuck maybe yours tonly takes about 10 mins to try all three positions.
> 
> regards Steve C.



Yes mine are numbered and in the correct spot.

Its not usually an issue as I am usually parting of the parent bar but in this instance I wasn't.
Too late tonight but will try the tap trick tomorrow just to see.


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## Stan (Sep 9, 2009)

Any time you get zero runout on a used three jaw it is likely the only diameter that will be even close to zero. There are just too many variables to expect that accuracy with any cheap or moderately priced chuck. Tightening just one pinion, or tightening all three but in a different order will change the runout.


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## rake60 (Sep 9, 2009)

Very good point Stan. The 3 Jaw chuck on my Grizzly 9 X 19 is the one that came with it.
It is Grizzly P/N P4000126 and sells for $117.00USD. That puts it in the economy range.
There are no pinions, it is a simple scroll chuck. The ID of those jaws were dusted true
one time, about 2 years ago. I don't remember what diameter they were at when I did that.
That brass stock in 1". That may have been exactly where it was when I bored them.
Different areas in the scroll are bound to give different answers on the accuracy at the 
jaws. 

Rick


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## Stan (Sep 9, 2009)

Rick: Maybe I used the wrong terminology. Every scroll chuck that I have taken apart had at least one pinion gear that rotated the scroll. Most have three pinions but some of the chucks that SouthBend used had only one. Is your Grizzly chuck different or is just the word I used?


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## rake60 (Sep 9, 2009)

Not at all. That was my error. I was thinking more of an independent 
chuck when I posted that. Keep me honest here! 
I've been a machinist for far too many years to even pretend that
I never make a mistake. I've got several of them out there all carved
in steel.

Yes the scroll is driven by any of three pinions. The driving pinion 
would depend on which socket you chose to stick the chuck key 
in. It shouldn't matter where the pressure is applied. The accuracy
will be no better than the grind of the scroll as it fits the teeth of
the jaws. None of those things will be perfect all of the time.

Rick


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