True up a three jaw?

Got one that seems to run 0.022" out. I turn a rod, indicate off it and it's great, rotate it in the chuck 120 deg and then it indicates
22 thou out, turn it another 120 deg and the same story, another 120 and it's back spot on again. It's repeatable. Kicker is that with unturned stock chucked it it reads good. I'm confused
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Gerry, Let me see if I got this straight.
You put in a nice rod, like a dowel pin, indicate it, and it's running true, (or good enough).
You put in some stock, like cold roll, and turn it. Then indicate it, running true. Loosen the chuck, rotate the part 120 deg. it's now .022" TIR Repeat, same result. Repeat, now indicates zero runout.
If this is true, AND the dowel pin is a different size, I suspect the scroll has some burrs, or a chip floating around inside the chuck.
Try some different sizes of stock or sockets, just some true diameters to indicate. It may give me some ideas. A new Chinese 3 jaw should be .002" TIR or less (IMHO).
Dave
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Gerry wrote:

All scroll chucks periodically need to be disassembled and cleaned. That is what you should try first. When you get it open, look at the central bore of the scroll. If it is loose and sloppy you can cut a piece of shim stock and wrap it around in the gap to greatly reduce the slop.
Try those two things. If that doesn't work, then you need to grind your jaws or replace the chuck.
Grant
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What I neglected to mention is that I pulled the chuck off and reinstalled. Thinking that it should mot matter the position the chuck goes back I just stabbed it in place. I have since removed the chuck, D1-4 BTW, moved it 120 deg and reindicate on a piece of 1.5" cold roll. Repeating this has allowed me to get the runout down to around 0,010. Chuck and spindle are now marked. Never had a quick change chuck before.
On my old Sheldon, I trued the jaws by clamping a ring internally and shaving the inside of the jaws and never had another problem with them be out of true. Chuck was old and beat up. Don't know if this is SOP for new chucks or not but it seem like this one may need it. Have not checked to see how hard these jaws are yet
The feedback is much appreciated
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Gerry wrote:

OK, a D1 mount is a little different. Generally, there are alignment marks, and you are supposed to always align the marks as that gets the backplate in the same position as it was when it was faced initially. If this is a new lathe to you, or a new chuck, you should clamp only the backplate to the spindle and face it until the face cuts all the way around. Then, remount the chuck and see if it is better. Then, you need to center the chuck body. Put a hardened and ground shaft in the chuck, loosen the bolts that hold the chuck to the backplate and shift the chuck around (you may have to remove and rotate the chuck body one bolt hole at a time) until it centers the best you can. Also check the back surface of the chuck body for dings and debris, stone down burrs if needed.
I got a good deal on a Phase-II copy of the Buck Adjust-tru chuck, which has 4 radial setscrews that center the chuck body by pressing on a boss that sticks out in the center of the backplate. You can get this chuck centered VERY well, better than .001" TIR over the full range of clamping diameters, without recentering the chuck body. It may be possible to make a similar setup on your chuck.
Jon
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Grant sez: " . . .When you get it open, look at the central bore of the scroll. If it is loose and sloppy you can cut a piece of shim stock and wrap it around in the gap to greatly reduce the slop.. . . ."
I suspect one would get dizzy trying to concentrate on that 'central bore' of a scroll.
Bob Swinney
Gerry wrote:

All scroll chucks periodically need to be disassembled and cleaned. That is what you should try first. Try those two things. If that doesn't work, then you need to grind your jaws or replace the chuck.
Grant
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There's a hole in the middle, Bob. If you're dizzy you might check your blood pressure .. :-)
Grant
Robert Swinney wrote:

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Gerry wrote:

Very strange. Take readings from several different distances from the face of the chuck. You may have a warped or worn jaw that causes the rod to be tilted in the chuck.
If so, then : First, remove the chuck and check the mounting for chips. If threaded, clean both the spindle threads and the internal threads on the chuck. If still wobbling, remove chuck from backplate and see if backplate runs true, indicate off the face of the backplate. If the face has a wobble, face it off until you cut the full face, then clean and remount the chuck.
If NOT, then check the looseness of the jaws with nothing clamped in the chuck. If the jaws can wobble side to side in their slots, it is bad news, the chuck probably needs replacing.
I do other checks with hardened and ground shafting out of old printers and such. I make sure it is straight first by rolling on a surface plate, good lathe bed, etc. When clamped in a chuck, this should be held true, without any wobble. Your conundrum of unturned stock running true, but turned stock being off-true is really wierd. If it isn't a permanent wobble in the way work is held, the work may be shifting in the jaws when cutting due to worn jaw tips or loose fit of the chuck on the spindle. If the jaw tips are worn, then the jaws only hold work at the back of the jaws. Once you have determined this is truly the problem and ruled out all other causes, you can grind the jaws to hold work along the full length.
Jon
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Gerry wrote:

What's the lathe in question. I have a lathe with a D1-4 mount and can't imagine how you would get a reduction from 0.022" to 0.010" just by fitting the chuck in a different position unless the chuck was not seating correctly. As others have mentioned there should be an alignment mark so the chuck is always put back in the same place but I think that is more to do with the camlock pins engaging the correct amount as they do need to be adjusted for correct locking , the chuck being centered on the short tapered nose.
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On Sun, 13 Jul 2008 20:23:43 +0100, David Billington

Check that the chuck is pulled fully home to metal to metal contact with the spindle nose mounting face.
If there are burrs or if the chuck taper register is even marginally undersize the flat mounting face no longer locates and the chuck mounts cockeyed.
Check with engineers blue or 1/2 thou long play video tape.
Let us know the result when you finally solve it.
Jim
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What you might check is the threads on the chuck and how it seats on the spindle. I had a small chip of brass in a thread and it would not seat - Dad put the chuck on it after the last move for himself and then I got it. The chuck was full of small chips and one was enough to lift the chuck.
I cleaned it and cleaned the teeth and I was back in work.
Martin Martin H. Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net TSRA, Endowed; NRA LOH & Patron Member, Golden Eagle, Patriot's Medal. NRA Second Amendment Task Force Charter Founder IHMSA and NRA Metallic Silhouette maker & member. http://lufkinced.com /
Gerry wrote:

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