C3 bearings for lathe spindle

Hopefully just a quick question, I have an old union graduate wood lathe that I've been coming back to again and again over the years, chasing a chattering gremlin.

I machined a new spindle for it when I got it and have done so again all these years later with my machining skills having improved, cleaner threads and closer tolerance on journals etc but to no avail.

Am now wondering if it could be as simple as having used C3 bearings when I should've used..? C1? Or anything else?

Engaging a tool outboard of say 7-8" from the spindle axis causes the whole machine to groan at a low pitch, and I get marks/chatter patterns in the cut as a result of this.

I'm reasonably certain it is not technique, inboard further I get nice clean shavings and a nice cut, but even a newly sharpened chisel presented in any manner to the outter rim causes this groaning.

Is it worth me tearing down again and finding tighter bearings?

Thanks in advance and merry Xmas/hny!

You should find out where the play is before replacing bearings. If you put radial force on the spindle (push/pull it at right angles to rotation), what sort of movement do you get relative to the housing? Is there axial movement if you push in/out?

C3 is a rating for increased internal clearance.....which wont do anything for loose bearings ........anyhoo as a walkin customer ,you wont get anything but standard from a bearing supplier......cheapest way to eliminate clearance in ball bearings is to end load them slightly .....this in effect makes them angular contact.............to go a lot more costly,angular contacts in the same size for like 10x the cost.

You need to compare the speed rating limit and size of bearing to the speed you run. Generally a spindle bearing is CN or C2 unless running near the top end of the limits. If you have C3 and your bearings run cool, you can go with tighter clearance. Whether it improves your situation is a tough call. It seems like there is something else going on. The clearance difference between CN and C3 is more relevant in metalworking where .0005 is a big number. DAve

I was looking at a tolerance chart for ~30-50mm bores I think it was, and C3 mentioned max clearance between races can be .033mm. I've missed marks by half that and felt how loose/wobbly that fit is so wondered if it might just be as simple as too much play in the bearing.

I'm away for the break at the moment but I will get a DI on it and some proper data on what's moving when I'm back, and maybe a short vid of what's going on. I've been wrestling with this thing for years now I do need some help with it!

It appears the problem is the chuck. I got a blank to the point where I could make it groan with a cut when chucked and then screwed the biggest faceplate I have into the foot, and resumed the cut. Seems I couldn't make it groan whatever I tried. I thought I'd tried this years back but obviously not.

I wonder if I'm just under chucked or if there is something wrong with it. It lands on the spindle shoulder with a nice clean clunk, and I did dial it while I was taking measurements, it didn't have any more radial play than the spindle itself.

The spindle wouldn't move the needle at all by wiggling it with fingers. I got maybe .01mm radial out of it with a bar inside taper and some force. And .02mm axially pushing with max hand force.

I'll get a bigger chuck anyway and report back.