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Any idea why this lathe is chattering? Spindle bearings?

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loply:
Hi folks,

Video showing the problem - shot of the surface finish is at the end -

So I've just finished doing a full refurb on this lathe, scraped it all in, replaced the old gear drive with a direct belt drive  & VFD, serviced all the parts etc.

Put it back together and it's chattering like mad. Anything steel above about 30mm is more or less unturnable, and even on steel around 15mm it only turns nicely if I use a high rake tool (like what you'd use on aluminium). It seems like it just can't handle a heavy load.

I've double checked everything and eliminated all obvious variables - tried different bits of stock, steel, alu, all different tools etc. Checked nothing is wobbling on the saddle/slides/etc. Absolutely everything I've eliminated I think.

The chattering noise is coming from the headstock. The poor surface finish does not improve the closer to the chuck it is. The bearings I adjusted tighter and tighter but that didn't help. I have about 0.007mm of end play in the spindle (if I chuck up a big bar and pull on it).

I'm wondering if the bearings are knackered? They're timken taper rollers and the races look okay to a laymans eye, but I don't know for sure how to test them?

Would appreciate any advice as I'm a bit disappointed at this stage!

Thanks,
Rich

John Swift:
may be not enough pre-load on the spindle bearings
excessive overhang of the tool and play in the gibs will add to the problem
try using a centre to support the end of the bar
if the chuck jaws are slightly bell mouthed it would not help having so long a piece unsupported

   John
PS
having another look at the video the tool length is not the problem

loply:
As I say I tightened the spindle bearings a lot and it didn't really help. The official spec is for no preload on them.

Had the part chucked tight as hell though as I say the problem happens even right next to the chuck so I don't think it's a workholding / rigidity issue.

Arbalist:
What type of tool is that you're using, it's difficult to make out for sure.

loply:
Tried everything - all the same result - two types of carbide insert, HSS, really sharp HSS, and sharp brazed carbide...

Best results came from tools with big back rake - like 35 degrees - but these won't last on steel. Still could only take 3 or 4 thou without chattering.

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