Weird chuck run out anomaly (every 3rd rotation?)

[GunsOfNavarone]

Ok, no matter what chuck I use nor what drill rod, hardened rod for CNC slide etc.... I always have this weird run out. I can use a independent 4 jaw and get it dead nuts...but if I rotate it enough, it will at some pattern, gain/lose .001 to .0015 of run out. I marked the pulleys to see if all of them were aligned every so many rotations, if it could make the perfect storm. Being that the spindle it v belt driven and I even loosened it a bit.....I didn't think so.
What could be at the heart of this? Turn a full rotation, dead on, turn it 3 times, a bit of run out. Now it's a workable amount, but I just gotta know why so I can sleep better. It maybe futile, but I'm sure I'm not alone in the need to understand this thing. If I'm not using an independent chuck, this is now .003 to.0035, not where I want to be.

 

[Firstram]

Sounds like a bearing problem, the caged balls take a few rotations to to make it all the way around the races.

 

[Winegrower]

I believe only bearings could account for this. In the disk drive business, we could tell how many balls were in the bearing by the runout characteristics.

 

[KevinM]

I would suspect a "out of round" change gear that meshes with the spindle. WAG

 

[SLK001]

I would say that you got some "fairy dust" sprinkled all over your lathe. Given as little information as you gave about your lathe, etc, that is as close as I can get to the cause of your problem.

 

[Firstram]

I believe only bearings could account for this. In the disk drive business, we could tell how many balls were in the bearing by the runout characteristics.

WOW!

 

[Janderso]

This has come up before.
Bearings seem to be the remedy in all cases thus far in my experience.
I’m sure you didn’t want to hear it.

 

[mksj]

Per above, bad balls... You give no description of what type/brand of what I assume is a lathe, and if this is something new or has always been a problem. I would remove the chucks and index the spindle TIR directly. But the only thing I can figure would account for somewhat repeating pattern every so many turns would be bad bearing balls. Put in in neutral and see if you can feel any unevenness, also would check the bearings preload.

 

[Nutfarmer]

I thought there was a problem with my lathe spindle. The indicator was the problem. It was sticking and hanging up ,showing a thousand and a half run out sometimes and not all the time. Try checking with a different indicator before tearing the lathe down.

 

[GunsOfNavarone]

Sorry....Griz G0602. Has taper roller bearings (32009) Seems pretty easy to replace them. (easy access at least) I'm not sure why bearings never occurred to me....like to upgrade to Nachi anyway.