Understanding 3 jaw chuck accuracy

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Ramv
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Understanding 3 jaw chuck accuracy

Post by Ramv »

Hi folks,
I recently purchased a new 5" 3 jaw chuck . The chuck that came with what looks like an inspection report.
IMG_4003.JPG
All dimensions in mm

d1, d2, d3, d4, d5 are 25,30,50,100, 62

a b c are 0.03, 0.035, 0.020 according to the back of the sheet.

repeatability is claimed to be 0.015

I am a little confused by what the numbers represent.
For instance, does 'a' represent TIR at d1? If I chuck in a "perfect" ground drill rod measuring 25mm, I will see at the most a 0.03mm deviation in measurement?

What happens when I rechuck the part? Should I expect it to have a TIR of at most 0.03 + 0.015 = 0.045? That doesnt really make sense because TIR just went up for no reason.

Curiously,
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Harold_V
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Re: Understanding 3 jaw chuck accuracy

Post by Harold_V »

Ramv wrote:For instance, does 'a' represent TIR at d1? If I chuck in a "perfect" ground drill rod measuring 25mm, I will see at the most a 0.03mm deviation in measurement?
I interpret the reading as you suggest, but keep in mind, a three jaw chuck tends to be diameter specific. It may be within a particular region of error at one diameter, yet better or worse at another diameter. That's due to the relationship between the scroll helix and dead center, which may or may not be true. To be perfectly clear, it is most unusual for ANY three jaw chuck to provide identical readings in regards to runout when the chuck is used for different diameters.
What happens when I rechuck the part? Should I expect it to have a TIR of at most 0.03 + 0.015 = 0.045? That doesnt really make sense because TIR just went up for no reason.
There's more to it than you might think. For example, if your chuck has been sprung (a crash, or by over tightening with only the tips of the jaws engaged, the piece being chucked may not be gripped dead parallel with the spindle centerline. In such a case, you can expect strange readings, often not repeatable, and they'll get better (or worse) as you traverse the shaft in either direction. And, as suggested, above, you can expect different readings as you change sizes in regards to the pieces being chucked.

If you hope to grip pieces with great precision, you have a few options. One of them is to use collets (questionable quality may result), or to use a four jaw chuck and dial the part to the level of precision you desire, and, thirdly, soft jaws. If you understand how they're properly applied, you can generally achieve a level of repeatability of less than a half thou in concentricity, and if machined according to the given application, virtually dead perpendicularity. All with the added bonus of the ability to chuck parts without jaw damage.

It is VERY important that you understand proper use of soft jaws. If you do not, none of the above will be true.

Harold
Wise people talk because they have something to say. Fools talk because they have to say something.
Ramv
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Re: Understanding 3 jaw chuck accuracy

Post by Ramv »

Thanks Harold for your reply. I am familiar with the use of soft jaws mainly because of your excellent and detailed post from years ago. I made it a point to get two piece jaws and have both 1018 and aluminum soft jaws for this chuck. I understand why one can get very good repeatability with soft jaws.

My question was primarily academic and theoretical. I was interested in learning more about how the manufacturers specify their repeatability and TIR numbers.
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liveaboard
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Re: Understanding 3 jaw chuck accuracy

Post by liveaboard »

3-jaw chucks have proved quite disappointing for me. Quick and convenient, but not accurate.

I've found that by marking the workpiece position in the chuck, I can remove it from the 3-jaw and put it back in very close to the same position. For test fitting and so on.
It's not a perfect system, but often lets me get away with it when I run into that problem.
ccfl
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Re: Understanding 3 jaw chuck accuracy

Post by ccfl »

Make an adjustable backplate and stop worrying about the scroll chuck's lack of accuracy. Repeatability (removing/reclamping the same workpiece) is/can be still an issue, but most chucks have pretty good repeatability even if the accuracy and variation across different diameters is not very good. I have a 4-jaw but rarely use it except for odd shaped pieces when there's no alternative.
adjustable_backplates.jpg
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Ramv
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Re: Understanding 3 jaw chuck accuracy

Post by Ramv »

Yes, it is a set-tru chuck which allows me to get the chuck near perfect repeatable with the right backplate. Also I will be using soft jaws(thanks Harold) almost exclusively.
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tornitore45
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Re: Understanding 3 jaw chuck accuracy

Post by tornitore45 »

Due to the fact that the 3 jaws that came with the lathe was not accurate I had to quickly learn how to set center work in the 4 jaws. I got it down to a point where I rarely put back the 3 jaws.
If I have to make a dozen parts with low accuracy demands that need multiple rechucking then is worth the hassle to change the chuck.
In other words, develop the skill to center the 4 jaws and go with that preferentially.
Mauro Gaetano
in Austin TX
SteveM
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Re: Understanding 3 jaw chuck accuracy

Post by SteveM »

I have a dedicated soft-jaw chuck, and I find that even when I chuck up to a different diameter than the last time I used it, it's pretty well centered.

Nice thing is that because the jaws were bored straight thru, they grip all the way front to back.

Steve
stevec
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Re: Understanding 3 jaw chuck accuracy

Post by stevec »

SteveM wrote: Nice thing is that because the jaws were bored straight thru, they grip all the way front to back.

Steve
I may have missed your post as to how you did that! :?
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Harold_V
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Re: Understanding 3 jaw chuck accuracy

Post by Harold_V »

stevec wrote:
SteveM wrote: Nice thing is that because the jaws were bored straight thru, they grip all the way front to back.

Steve
I may have missed your post as to how you did that! :?

Can't speak for Steve in this instance, but I can address that issue in general.

Soft jaws are generally larger than the master jaw, so by placing a spider (or a short piece of stock) behind the soft jaws, where it can be gripped by the master jaw, the soft jaws can be bored through. The results are, by far, superior to the use of the hardened jaws that typically come with a chuck. I keep such a set of jaws on hand and use them almost exclusively due to their superior performance as compared to the hard jaws.

Harold
Wise people talk because they have something to say. Fools talk because they have to say something.
SteveM
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Re: Understanding 3 jaw chuck accuracy

Post by SteveM »

Harold_V wrote:Can't speak for Steve in this instance, but I can address that issue in general.
Harold has my point correct.

Here's what my jaws look like:
Image

They are about an inch deep and since they were machines in place, they are straight and parallel with the bore, so no wobble.

When they are bored, they are centered with the bore AT THE DIAMETER AT WHICH THEY WERE MACHINED, however, I have found them to be pretty good at other diameters.

Steve
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