Unintended asymetric turning

I am turning a mild steel doughnut: OD= 4.5", ID 2.125", thickness 0.1875". The purpose is to clean it up and make the two surfaces
parallel.
The process is always the same: Hold the piece by expanding 3-jaw, face off, turn the edge, reverse, apply a spacer (a smaller doughnut held on by superglue), face off the second side, deburr edges. Remove from the 3-jaw. Clamp in 4 jaw on the outside, indicate the jaws to within 0.001". Bore the hole till it "looks right". Deburr the inside edges.
Today, when I got to the 4-jaw stage, I found that the orthogonal jaws could not be set to the same number. For all intents and purposes the doughnut now seemed to be an ellipse with one axis 0.014" longer than the other.
Never mind, I thought, I centered both axes and proceeded to bore. I did not encounter any problems but when I re-clamped on the 3-jaw it was clear that the piece was not rotating concentrically. When I measured it there was a variation of 0.024" in the width of the doughnut (average width=0.960"), i.e. the hole is eccentric to the perimeter.
Not that it matters with this piece but what are the possible causes?
My thoughts:
1) I have not indicated the 4-jaw properly. Possible, but has not happened before. 2) Crummy 4-jaw chuck that came with the 9x20. It is of the "old" style and a pain to use. 3) Should not indicate the jaws but the piece itself (difficult to adjust the jaws then, though). 4) After facing both sides the piece was only 0.154" thick and as such thin enough to flex in the chuck jaws.
Thanks,
Michael Koblic, Campbell River, BC
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snipped-for-privacy@gmail.com wrote in news:94g3g65m52r78e46mvfiknjsrntsul4khv@ 4ax.com:

4) is a possibility. Another is that the material had internal stresses which were released by the machining, resulting in distortion. I've seen this happen big time with plastics like Delrin, but not so much with steel.
If it was me, I'd turn the OD, and then use a "Step collet" for the rest. They can machined in place to fit the part precisely, so are guaranteed concentric. They also apply very even force to the OD of the part, unlike the high point pressure of a bunch of chuck jaws.
http://travers.com/skulist.asp?RequestDataÊ_Search&q=block%20id% 20114899
Doug White
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Assuming stress isn't the cause of the distortion, another approach would be to use a longer piece of material. Face, turn ID and OD, then turn around and machine off the excess. More work removing the excess, but -should- result in concentric ID/OD leaving only parallelism of the two faces as an issue for the last operation.
Jon
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wrote:
[...]

I have never seen one of those so I had a look around the Google to see how they work.
If I understand it correctly for a work piece like mine one would get a 5" step collet and machine it closely to fit the OD of the work piece. Presumably the clamping range of these collets is the same as ordinary 5C collets - rather small. The recess would have to be deep enough to accommodate the work piece and a spacer to allow boring of the central hole.
Even at $172 it would be a good investment if one were to make more of these pieces. Unfortunately I do not think I can use 5C collets on my lathe.
Michael Koblic, Campbell River, BC
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On 12/09/2010 10:23 PM, snipped-for-privacy@gmail.com wrote:

Could it have been as simple as a .014" thick bit of crud on one of the jaws?
--

Tim Wescott
Wescott Design Services
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wrote:
[...]

It's a thought. I do try to be fairly anal about cleaning everything before clamping.
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    O.K. So far, it sounds like a standard independent jaw 4-jaw.

    Strange. This is not a "universal" (e.g. scroll-back 4-jaw is it?

    Nearly doubling the eccentricity from before.

    Possible.
    That still should not result in eccentricity -- just greater difficulty achieving concentricity.

    You indicate from the *jaws*? With no certainty that they are equal sized? This is likely the problem.

    0.154" thick -- but still about an inch in radial dimension so I don't think so.
    Good Luck,         DoN.
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wrote:
[...]

No, it's an independent 4-jaw. Looks like this: http://www.busybeetools.com/products/CHUCK-6IN.-4-JAW-FOR-WOOD-LATHE.html
I bought a similar one for my wood lathe. I refused to believe that it was a metalworking chuck as some maintained elsewhere until I got one with the 9x20.
[...]

You'd better believe it...

I measured the jaws when I got the chuck. I have also used the same procedure on similar work pieces with nothing like this happening before.

Thinking about it I should be able to tell by indicating the face and tightening the jaws.
Michael Koblic, Campbell River, BC
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    O.K. This does not show the back side. Is this, by any chance, the chuck with studs sticking through the backplate, and a nut and washer holding it in, and tightened after you get it adjusted?
    I have read descriptions of these with people who got them on metalworking lathes saying that they are only good for woodworking. :-)
    I see a hint at the bottom of the page that if you are interested in this chuck -- you should also be interested in this chuck (again) at the bottom of the page. :-)

    Sounds like the type with the nuts on the back holding the jaws, then.:-)

    O.K. But *never* trust the jaws to be right -- measure the workpiece, even if it is more difficult.

    O.K.
    Good Luck,         DoN.
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wrote:

Barely...There are much better chucks for woodworking.

They have two versions - one which needs inserts and one which is threaded 1"-8.

Oddly enough I do it because of advice I received on this very group from someone about two years ago :-)
Michael Koblic, Campbell River, BC
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This type of chuck isn't ideally suited for most large lathes, but it's sufficiently adequate for use on a generic 9x20 model. I don't know if the chuck is a good choice for thin rings, though, as jaw pressure could easily distort the ring (much like thin tubing).. the flat faceplate may be a better choice (with some fittings added).
I've used the same type 4-jaw on my own 9x20, and it isn't substantially any more difficult to use than a heavy duty 4-jaw chuck. Like many pieces of tooling from China/elsewhere, it was kinda rough as supplied. After deburring all the parts and edges, and smoothing the sliding surfaces, I replaced the soft flat washers under the nuts with hardened, ground flat washers. That may not seem like a major issue, but those original soft washers don't give a good feel of when a fastener is tight.
Backing off the jaw nuts just enough to allow for minor adjustments may be the key to ease of use with these chucks.
Hardened/ground washers offer a significant improvement in feedback of tight/just-snug within a short wrench swing.. I generally install them where repeated use is common. When a fastener has 90+ degrees of swing from tight-to-snug (backing off), something is wrong.. the bolt isn't stretching, and the cast iron isn't compressing, so it's the soft flat washer. Another cause of wrench "overswing" could be that the threaded hole in the nut isn't perpendicular with the flat sides, which is a fairly frequent occurrence with Chinese hardware.
A HD 4-jaw will have other issues, such as the correct spindle thread and matching register seat for the 9x20 models with metric thread spindles. Another issue with a HD 4-jaw is the workpieces will always be supported further away from the spindle nose bearing, which will most likely result in more tool chatter when not using tailstock center support.
--
WB
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On Sun, 12 Dec 2010 10:06:02 -0500, "Wild_Bill"

Agreed.
Sounds like a cheaper solution to try before getting a whole new chuck.

I had no problem with the spindle thread, unlike the face plate that came with the 9x20. I could not get it on the spindle. The vendor sent a replacement which they swore fitted OK on one of their spindles. Same problem. I had to re-thread both plates and it was not a subtle process of a few thou. I had to take a fair bit off.
Michael Koblic, Campbell River, BC
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On Dec 10, 1:23 am, snipped-for-privacy@gmail.com wrote:

Instead of reading runout on the indicator you could feed it in to zero and read the leadscrew dial. Then you could move the carriage in and out of position to indicate the work beside the jaws.
This job may justify a spacer plate with slots for the jaws to keep it in place and standoffs to position the dial plate out past the jaw ends.
jsw
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On Fri, 10 Dec 2010 18:02:41 -0800 (PST), Jim Wilkins

==========One way around this from the old machining books is to use what is called a bridge or pump staff.
You can make one out of a piece of stiff wire that you can clamp in the tail stock chuck or make a bushing. To use, you bend the free end so that the tip of the bridge rides on what you are trying to get concentric and you can use a drop indicator on the bridge to measure the runout. The actual measurements won't be correct, but generally you are trying for zero runout with no or minimal needle movement.
You can also fabricate a bridge out of a used hacksaw blade that you can clamp in either the tail stock or tool post. Works great for indicating square/hex stock or rough cast preforms or rough cast bores.
If it would be helpful I can post pictures of the ones I made on either the drop box or my web site.
Another possibility if you have several of these to do is to fabricate a holder/fixture that will clamp the donut that you can bolt to the faceplate.
Let the group know how you make out and what your solution was.
-- Unka George (George McDuffee) .............................. The past is a foreign country; they do things differently there. L. P. Hartley (1895-1972), British author. The Go-Between, Prologue (1953).
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On Fri, 10 Dec 2010 21:27:43 -0600, F. George McDuffee
[...]

Yes please. I am having a hard time visualizing it: How do you get around the jaws with the bridge?

Right. That's where that 5C step collet would have been so nice, too. Up to now the pieces have been all different as I am experimenting. At some stage I would like to develop a method where the edge, one face and the hole in the middle can be all done in one clamping.

Next time I do a similar piece I shall take notes and do a few extra measurements.
It is not impossible that Santa will drop a new 4-jaw down the chimney, too.
Michael Koblic, Campbell River, BC
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On Fri, 10 Dec 2010 18:02:41 -0800 (PST), Jim Wilkins

I am being slow. Cannot visualize this.

What I have is a piece that does both. It is smaller than the work piece and I center it and attach it to the work piece with superglue.
Michael Koblic, Campbell River, BC
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On Dec 10, 10:53 pm, snipped-for-privacy@gmail.com wrote:

And I need to practice writing clear descriptions.
When centering work in a 4-jaw you normally turn the chuck while watching the indicator dial, right?
You can't do this when the jaws grab the OD, and moving the carriage out of the way to turn past a jaw means you have to slide the indicator point back onto the work afterwards without shifting it.
I suggest initially advancing the crossfeed until the indicator reads zero (close to a jaw) and then perhaps zeroing the crossfeed dial. Then you can back it out and slide the carriage to the right so the jaws clear the indicator. Rotate the work half a turn, return the carriage and advance the crossfeed to a zero indicator reading again. Adjust the jaws to remove half of the difference between the new and previous crossfeed readings. If you zeroed it and the new one is 30 then move the jaws to make it 15. At this point I reset the zero.
This works with a lever test indicator with a cosine error because you only read it at one point, its zero.
Rotate the chuck a quarter turn and repeat. The work should now be nearly centered.
HTH JSW
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On Sat, 11 Dec 2010 15:16:53 -0800 (PST), Jim Wilkins
[...]

OK, now I get it, thanks. What you are doing is attaching the indicator in such a manner that it need not be touched by human hand while doing the series of measurements except to re-set the zero. You are also indicating next to the jaws.
If I thought about it I should have done that or something similar then. I shall be aware of it with the next piece.
I am more and more convinced that the issue is the flexing of the work piece: All the previous ones I had done I only faced off one side. Also, the recent batch I bought were definitely thinner than the ones I bought in the past. I said so much in the shop at the time.
Recession is hitting everywhere. Even baby bum-wipes are smaller and thinner these days...
Michael Koblic, Campbell River, BC
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On Dec 11, 8:28 pm, snipped-for-privacy@gmail.com wrote:

Do you standardize the size of these dials? If so, maybe you could find a chuck with removeable top jaws and make your own pie jaws, with short protrusions on both ends to clamp either the ID or OD and an undercut so you can cut across the unclamped edge.
/'''|__---------------__|'''\
jsw
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On Sat, 11 Dec 2010 17:50:18 -0800 (PST), Jim Wilkins

This sort of thing?
http://www.busybeetools.com/products/JUMBO-JAWS-IN-4-3%7B47%7D4IN.-12-EX-2-3%7B47%7D4IN.-11.0IN..html
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