trouble with chatter/vibrations new to lathe

postronics · May 2, 2019

We are fairly new to CNC lathe, as we just got our first machine a Milltronics ML16/40 recently. We are familiar with mills as we have had a few for years here at the shop. We are having some massive vibration/chatter issues when boring, and don't know if its our tooling, set-up, feeds/speeds, or just the lathe itself. The part we tried to cut was a piece of 6"x3" mechanical tubing. We needed to take it down to 5.5" OD x 3.152 ID x 4" long. We put the stock in the bison 10" 3 jaw. The pc was 6" long to start. We cut the OD with no issues, when moving to the ID we had tons of chatter to the point we hit the feed hold to keep from breaking the tool. We had the spindle at 400FPM with a .010" IPR feed for the roughing. depth of cut was .030". We used an 1-1/4" Kennametal boring bar with a DNMG432MP insert. We also tried turning the feed down all the way to 50% with no improvement. Our boring bar was sticking out just enough to make the cut. Any ideas? We gave up and finished in the manual, but i would really like to get this machine figured out.

JCByrd24 · May 2, 2019

Have you checked that you're not below center and rubbing? Insert tight?

Guest · May 2, 2019

If your set-up is rigid the first thing to try is lower RPM. On the DOC even if it is per side it is a little light for a 432, you want to go deeper than the nose radius.

Triaged · May 2, 2019

That's a thin wall and not a low tool pressure insert. It's a negative insert, big radius for the DOC, and the M of the chip breaker is for medium machining which means a honed edge not sharp. I'd try a positive insert with R.015 radius like a CCMT31.51 and bump up the speed.

JCByrd24 · May 2, 2019

Triaged said:
That's a thin wall and not a low tool pressure insert.

The wall is over an inch...not what I'd consider thin. Part should be plenty rigid for that cut, though I agree with both of you on light cut for the nose radius. A 431 would be better, or go to 0.050" DOC.

postronics · May 2, 2019

JCByrd24 said:
Have you checked that you're not below center and rubbing? Insert tight?

No, is there a specific way to verify?

I will try a couple different inserts an a deeper cut as well.

JCByrd24 · May 2, 2019

I'm not a CNC operator but you are just looking to verify that your tool is on center or slightly above like you would on a manual. I'd imagine it's in the tool setup. When boring if the tool is below center you have to have more clearance on the tool so it cuts and doesn't rub. Kind of like when turning the OD if the tool is above center the cutting edge will rub, but just not as drastic.

Guest · May 2, 2019

JCByrd24 said:
Have you checked that you're not below center and rubbing? Insert tight?

postronics said:
No, is there a specific way to verify?

I will try a couple different inserts an a deeper cut as well.

It is one of those things that is best demonstrated instead of explained. Unfortunately with my data capped internet I can't find you a You Tube video to watch, maybe someone else can.

Hazzert · May 2, 2019

postronics said:
No, is there a specific way to verify?

I will try a couple different inserts an a deeper cut as well.

Face the stock with the boring bar. If there's a tit left behind at center you're either low or high. You can also pinch a scale between the tool and the work and see if the scale is perpendicular with the tool axis. If if points away the tool is high, if it points back it's low but that test is more common with flat bed not slant bed lathes.

Investing in some proper torque torx (say that 3 times fast..) would solve the is my insert too loose or tight questions as well.

Ox · May 2, 2019

Hazzert said:
Investing in some proper torque torx (say that 3 times fast..) would solve the is my insert too loose or tight questions as well.

---------------------

Think Snow Eh!
Ox

Hazzert · May 2, 2019

Ox said:
---------------------

Think Snow Eh!
Ox

You’ve never used the clicky torx screwdrivers? Usually something like 5 inch pounds.

Triaged · May 3, 2019

JCByrd24 said:
The wall is over an inch...not what I'd consider thin...

Wow! I'm bad at math sometimes!

exkenna · May 3, 2019

Triaged nailed it. The two major causes of vibration is overhang (not the problem in your case) and edge prep (and/or) geometry of the insert.

You're on the right track with the 55 degree diamond but a smaller nose radius and switching to a positive rake insert would make a world of difference. A DCMT 32.51 or DCGT 32.51 would be ideal.

Also, edge prep is everything. If by .030 DOC you mean .015 per side then you are barely getting over the hone on the edge of the insert.This means material is piling up on the cutting edge and causing the loading/unloading condition that causes the chatter and poor finish. If you must stay with the DNMG negative rake tooling try a DNMG-431 in a LP or LF finishing geometry. It will lower the cutting forces somewhat.

Good luck

Jashley73 · May 3, 2019

I actually don't think it's insert related. That's a big boring bar - you can still take "finish" cuts with a medium/roughing insert.

I'd check to make sure something's not loose.

Is the machine a linear-guide lathe? They suck for boring. We have (3) big Mazak's that are really nice machines, but I think the machines are too stiff, and we sometimes get chatter with even big boring bars.

My go-to fix has been to put a layer of electrical tape between the turret, and the boring-bar block. My theory is that this either (1) acts like a small damper, or (2) it compensates for any poor flatness on the bearing surface, on the toolholder block. Regardless, it works way better than it should. Rigidity & accuracy have not suffered either, as we hold some pretty tight-tolerance bores with this setup.

If everything is tight in your setup, maybe try it.

cameraman · May 3, 2019

Jashley73 said:
I actually don't think it's insert related. That's a big boring bar - you can still take "finish" cuts with a medium/roughing insert.

I'd check to make sure something's not loose.

Is the machine a linear-guide lathe? They suck for boring. We have (3) big Mazak's that are really nice machines, but I think the machines are too stiff, and we sometimes get chatter with even big boring bars.

My go-to fix has been to put a layer of electrical tape between the turret, and the boring-bar block. My theory is that this either (1) acts like a small damper, or (2) it compensates for any poor flatness on the bearing surface, on the toolholder block. Regardless, it works way better than it should. Rigidity & accuracy have not suffered either, as we hold some pretty tight-tolerance bores with this setup.

If everything is tight in your setup, maybe try it.

This is the lathe in question (I believe)…

http://www.milltronics.com/wp-content/uploads/2015/07/ML1640-8-1-16-Web.pdf

Basically a 16x40 flat bed lathe that weighs 6000 lbs but has turcite lined boxed ways...

Or this ---> ML16/40 | Milltronics USA - Let's Invent

^^^ (No affiliation).

_________________________________________________________________________________________________

BTW awesome tip about the electrical tape for MAZAKs (I remember you posting that years ago, and always stuck in my mind)… Definitely a dampening effect as those linear rails can be quite stiff/ springy at certain modes of vibration.

____________________________________

Wondering if OP has the auto-indexing turret or has a tool post on his saddle (kind of thing ?)

BobM3 · May 3, 2019

If you are right on the edge (chatter/no chatter) then wrapping some Sorbothane around the OD of the tube might help. Mcmaster-Carr sells it in sheets.

cameraman · May 3, 2019

BobM3 said:
If you are right on the edge (chatter/no chatter) then wrapping some Sorbothane around the OD of the tube might help. Mcmaster-Carr sells it in sheets.

Sorbothane … Amazing thixotropic stuff that NIKE has used in shock absorbing materials in sneakers / shoes (various)… (for years now, exotic polyurethane co polymer (thingy)).

Now in sheets ! Never ceases to amaze me what you can find in McMaster-Carr …

I'm surprised the thixotropic slow move effect of sorbothane is not cancelled out due to higher clamping pressure where it becomes the same as electrical tape...

I'll have to check that out :cool:

(nice one !).[There's a ton of things that could be done with that. :cheers:

(Where's the cat …) ].

postronics · May 5, 2019

cameraman said:
This is the lathe in question (I believe)…

http://www.milltronics.com/wp-content/uploads/2015/07/ML1640-8-1-16-Web.pdf

Basically a 16x40 flat bed lathe that weighs 6000 lbs but has turcite lined boxed ways...

Or this ---> ML16/40 | Milltronics USA - Let's Invent

^^^ (No affiliation).

_________________________________________________________________________________________________

BTW awesome tip about the electrical tape for MAZAKs (I remember you posting that years ago, and always stuck in my mind)… Definitely a dampening effect as those linear rails can be quite stiff/ springy at certain modes of vibration.

____________________________________

Wondering if OP has the auto-indexing turret or has a tool post on his saddle (kind of thing ?)

Ya, that's the machine. We have the 8 position auto turret. I have some imserts due to show up Monday (everything is 2-3 day delivery here) and will hopefully spend the afternoon trying all the suggestions. I will let everyone know how it goes. Thanks for all your time everyone

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