Author Topic: Chuck runout question...  (Read 6431 times)

Offline AdeV

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Chuck runout question...
« on: February 25, 2011, 08:34:26 AM »
With a typical 3-jaw chuck, what would you consider to be the maximum acceptable "repeatability"? i.e. if you put a piece of straight & true round-bar in your chuck, how many thou of runout would you consider "normal"?

I ask because all my 3-jaw chucks (3 of them now) seem to suffer from major "non-repeatability". e.g. I can take a piece of bar, mount it in the chuck, and get it running dead true (because the 3-jaw is mounted in a 4-jaw...). Undo the 3-jaw & take the bar out. Now put it back in & tighten, and I may have as much as 0.010" total indicated runout. In my mind, this is totally unacceptable.

I'd stick with the 4-jaw, but that's got wear issues as well - and unless I'm really careful - and lucky - it clamps up at a slight angle, so I end up cutting tapers...

Cheers!
Ade.
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Offline Bluechip

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Re: Chuck runout question...
« Reply #1 on: February 25, 2011, 08:45:02 AM »
Hi Adev

Last time I checked about a year ago .... IIRC

My newish TOS 4" SC3-jaw is about .002 TIR 1" out from jaws.

New TOS 4" SC4-jaw a bit better, about .0015"

Ancient Pratt-Burnerd was some .008 on a good day ..  :(

What are you using as a test bar.

Some 'round' bar ain't all that straight.

I used a 1/2" new dormer drill bit with a long shank, gripped reversed with most of the jaws on the parallel part.. All over the shop with commercial MS round.

Try a bit you've turned yourself, so you know its circular, and straight.

BC
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Offline Ned Ludd

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Re: Chuck runout question...
« Reply #2 on: February 25, 2011, 08:57:08 AM »
Hi AdeV,
Have you tried the repeatability test using the same chuck key hole each time. I have mentioned this before elsewhere that there is only one key hole that is accurate. On the outside circumference of the chuck there will be a "O" stamped, if not use the hole in-line with the label on the face of the chuck. I know some will poo-poo the idea but it is true and does work, well it will till the chuck gets worn out. The theory is that the chuck was set up with the labelled hole to allow for play in the scroll.
Ned
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Offline Bogstandard

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Re: Chuck runout question...
« Reply #3 on: February 25, 2011, 09:17:12 AM »
Ade,

Anywhere between 0.001" to 0.003" is about average on a new reasonably priced chuck.

Once the scroll and jaws become worn, as yours surely are, then the scrapman cometh. Unless you are forever going to be doing roughie jobs, and always truing up the piece first using larger material.

As Ned has stated, I broached the idea a few years back and people didn't believe me, until they tried it for themselves and couldn't fathom out why it worked, but it does. In fact the first thing I do when fitting a new chuck is try it in all bolt positions first to get the least runout, then try to find which keyhole gives the best results then mark that up. Invariably, if the hole has a zero on it, that usually is the best one. I think it is something they do in the factory during manufacture, they always use the same keyhole as they set the parts up for machining and assembly.

In your case, I think new chucks might be in order, they have had their day.


John
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Offline AdeV

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Re: Chuck runout question...
« Reply #4 on: February 25, 2011, 09:20:35 AM »
Thanks everyone for your replies. Looks like I have duff chucks....


In your case, I think new chucks might be in order, they have had their day.


I think you're probably right.... I keep trying to persuade myself that I can pick up a bargain chuck and "it'll be fiiiiiiine"; but I guess not. I know both my 4-jaws hold at an angle, even the one we tried to fix; the 3-jaws are just all over the place. I'll look for a stamped "O" on my latest acquisition, but it would seem that someone's taken the rust off it with an angle grinder, so who knows if any markings have survived...
Cheers!
Ade.
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Location: Wallasey, Merseyside. A long way from anywhere.
Occasionally: Zhengzhou, China. An even longer way from anywhere...

Offline picclock

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Re: Chuck runout question...
« Reply #5 on: February 25, 2011, 11:07:46 AM »
If there is enough meat left on the jaws of the 4 jaw, its not too hard to get them true if you have the time. Mine was never great from the start, but I guess it depends on what accuracy you are looking for.

I experimented with the toolpost grinder way of doing the jaws but although things changed it was not an improvement. In the end I reasoned that for the jaw to be correct, in the bottom position it must be parallel to the ways. I checked this using a dti attached to the crosslide, and moving the carriage back and forth along the jaw soon showed where the high and low spots were. These were ground as shown in the pictures.

http://s917.photobucket.com/albums/ad19/picclock/chuck%20jaw%20grinding/ 

When done the surface was like a mirror and the runout at 9" was 1/2 thou. Once you have one flat its probably a good idea to check it with the chuck rotated by 90 degrees to ensure that the spindle is true to the ways. If not, that's a whole new can of worms which you will have to sort out first. :doh:

Obviously, once ground each jaw is only good for the jaw position it was ground in.  Best to mark with paint or similar if they are not numbered..

This method certainly worked for me. I would have bought a new chuck if I could be certain that the jaws were truly accurate.

Good luck

picclock
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Offline AdeV

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Re: Chuck runout question...
« Reply #6 on: February 25, 2011, 11:58:09 AM »
If there is enough meat left on the jaws of the 4 jaw, its not too hard to get them true if you have the time. Mine was never great from the start, but I guess it depends on what accuracy you are looking for.

I took (one of) my 4-jaws to John, who had kindly offered to true them up on the surface grinder; which we (he, really) duly did. However, the runout was no better when I returned them to the chuck (the jaws were numbered, so no problems there); we can only think that the wear is in the "ways" of the chuck (and maybe the jaws), so that as one tightens up, the jaw is actually kicked outwards slightly by the pressure = taper.

No, I think I'm just going to have to bite the bullet & risk buying a brand new chuck... Trouble is, I really need at least a 6" one, otherwise it won't fit over the spindle nose... and they get expensive quickly when they get big...

Edited to add: I've just looked at RDG's site (the only co. I can find who do larger than 160mm), and actually a 200mm chuck is quite reasonable at 132quid, and 30 extra quid gets me a set of soft jaws...

Very tempting..., but I guess I need a backplate as well?

Any made-for-idiots tutorials on how to fit & true up a backplate? The spindle on my lathe is a simple flange, with 3 bolt holes; obviously the idea is the chuck backplate contains captive bolts, which go through the holes; the chuck registers on the outer edge of the flange, & one tightens the nuts from behind the flange (as the bishop said to the actress...).
« Last Edit: February 25, 2011, 12:09:53 PM by AdeV »
Cheers!
Ade.
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Location: Wallasey, Merseyside. A long way from anywhere.
Occasionally: Zhengzhou, China. An even longer way from anywhere...

Offline picclock

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Re: Chuck runout question...
« Reply #7 on: February 26, 2011, 04:00:38 AM »
I suspect your right about the wear in the chuck, but using the method I outlined,  this is mostly taken care of. I think this is why this method worked on mine so well.

But, its a lot easier to get a new chuck than to muck about if you can get one good enough for your requirements.

The only other thing that may cause your error is if the face that the chuck is fixed to is not at right angles to the spindle, but I'm sure you would have checked this.

Hope you get it sorted Ok

Best Regards

picclock

Engaged in the art of turning large pieces of useful material into ever smaller pieces of (s)crap. (Ferndown, Dorset)