CNC lathe: Checks to help diagnose vibration source

We've got a Leadwell LTC-35 that's about 3yrs old. 15" Chuck, 12 station BMT-75 turret.

Mostly we don't get too much trouble with vibration. We can generally rough, finish turn and thread no problem. We do get vibration on some parts(sometimes) when we try to get a decent finish on grooves with a 3mm iscar grooving tip in a 1-1/2" tool holder on 2" and 3" 1045 bar.

Right now i'm working through issues machining 3" induction hardened 1045 chromed bar. We keep breaking our kyocera ceramic tips. These have worked pretty well for us on 65mm & 50mm hardened chrome from another supplier at identical speeds, feeds & DOC. We just measured the hardness to be 68Hrc. The spec from our other material is 55-60. I think the hardness is the biggest problem but we are also seeing some vibration and i think with about 5" of stickout we should be rigid enough.

This machine has been crashed a couple of times. One pretty big crash in x where we had to re-allign the spindle.

What i'm wondering is are there any simple checks i can do on the different machine elements to see if these might be contributing to the vibration we see?

I'm thinking about investing in some CBN tips but want to make sure we don't have a machine issue contributing to vibration that could kill these tips.

Is the machine just sitting on the floor or is it anchored and leveled?

JH

68Rc sounds impossibly hard for carbon steel, maybe you're getting a distorted reading through the hard chrome?

Is the bar being properly supported at the opposite end? If you only chuck on one spot without further support of the bar, you can get harmonic vibration transmitted through to the free end of the bar and make the whole bar resonate. A proper steady will damp that vibration.

HuFlungDung said:

68Rc sounds impossibly hard for carbon steel, maybe you're getting a distorted reading through the hard chrome?

Is the bar being properly supported at the opposite end? If you only chuck on one spot without further support of the bar, you can get harmonic vibration transmitted through to the free end of the bar and make the whole bar resonate. A proper steady will damp that vibration.

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I better double check with my man on the hardness tester!!

Also the bar is only 20" long. 5" sticking out the front and 2-1/2" tall jaws. Is this long enough to cause that kind of trouble?

James H Clark said:

Is the machine just sitting on the floor or is it anchored and leveled?

JH

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Feet are epoxied to the concrete. Machine would have been leveled during installation.

Am i way off wondering about what a crash might have done?

tonymor said:

I better double check with my man on the hardness tester!!

Also the bar is only 20" long. 5" sticking out the front and 2-1/2" tall jaws. Is this long enough to cause that kind of trouble?

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20" long shouldn't be undue cause for a vibration issue so long as the jaws are in good shape and bored to fit the stock properly. If the jaws are worn bell mouthed, then the high tool pressure can still make the work squirm around in a 3 lobe fashion which will amount to vibration.

HuFlungDung said:

20" long shouldn't be undue cause for a vibration issue so long as the jaws are in good shape and bored to fit the stock properly. If the jaws are worn bell mouthed, then the high tool pressure can still make the work squirm around in a 3 lobe fashion which will amount to vibration.

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The jaws are in pretty good nick but might give them a skim to make sure.

IDK any causes, but ..
you might pre.thread with a smaller tip.
Then repeat it.

Unless running high volume, the secondary op might just work perfectly and leave great results.