Lathe Chatter Tool Tactical Design

Vera Donlyn1959 · Friday, November 26, 2021

CNC Lathe Chatter fix, but......

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When doing a finish pass on a CNC Lathe and chatter is a problem the solution may be slow the speed and increase the feed. That doesn't always work though. I have found using the spindle override and moving it back and forth so the spindle speed slows and speeds up will break the chatter. Is there any G code that anyone has heard of that will vary the speed like I mentioned? This is my first post and looking at some of the other one I have already gained some knowledge. Oh I'm running an Okuma Lathe.
im having the same problem, I upped the feed and slowed the rmps, also i found that if you have alot of stock hanging out the back of the jaws, he vibration will cause chatter, so what i did was cut material about 4 inches long and ran about 4 parts per bar. im courious if there is a G code to do that though.
That's not a very good way to fight chatter, the finished size will vary from the different speeds, if I have chatter problems a positive rake 1/64 radius tip tool will usually solve it, as well as slowing down, but once you find a sweet spot stay there and don't vary it, also sometime you have to take a heavy finish cut to keep the part loaded, but it's hard to maintain a tight tolerance doing that.
I think the bar mounting method is quite important. Take the old Armstrong 'sidehung' bar holder made to be clamped in a lantern style toolpost. Back when I started out, that's what I began with, and performance was lousy.
Currently, my big old cnc lathe has an old style turret, with 1.5" dia holes to insert bars into. The turret is only 2.5" thick. The holes are not in horrible shape but it is easy to slide the bars (or bar adapter) into. But, setscrewing a round bar into a round hole is a crap mounting method. One point of contact on the setscrew, and most likely, an infinitely narrow line of contact on the side of the bar opposite the setscrew. Its a recipe for vibration, and... it does. I have to use extremely short bars, and I have even cross drilled the turret holes for setscrews at 90 degrees to the main ones, but even those do not help a lot. I think 2.5" clamping width (turret thickness) is just too short.

By comparison, I use 40 position toolposts on my manual lathes, and we can bore easily with steel bars at 6:1 overhang if necessary, and they behave quite well. All except for 3/4" bars, for some reason, that diameter wants to sing.
But, setscrewing a round bar into a round hole is a crap mounting method. One point of contact on the setscrew, and most likely, an infinitely narrow line of contact on the side of the bar opposite the setscrew. Its a recipe for vibration, and... it does.

I'll have to disagree with that statement, while I agree that you can take a boring bar that was chattering badly when hung out at a 4 to 1 ratio in a regular CNC round tool block and put it in a manual lathe at a 6 to 1 ratio at the same speed and feed and it may work fine, I think it has less to do with how the bar is held and more about the rigidity of the machine, I've tried holding boring bars in a CNC using the square type holders where the bar sets on it's flat bottom in an attempt to get rid of chatter and it didn't help a bit, I really thank the difference is because every manual machine I have used is a little "loose" and has the ability to soak up some vibration in the clearance of the ways and that CNC's turrets are so rigid and massive that the vibration has nowhere to go so it sets up a ringing in the bar, this is just my theory anyway.
one thing you are forgeting...
changing speeds and feeds is only 2/3 of the equation...

How about final pass Doc ?

Increase that to break the chips and decrease chatter...

Slower rpm + higher feedrate + heavier finish pass = MUCH HIGHER chance of breaking chips and getting chatter to go away...

PS : changing speeds during a final pass will change the finish look and sizes... Not good...
Phil,
I've run an okuma and had found the same trick. Constantly changing back and forth the spindle rpm override, kept the chatter away. As far as affecting finish, technically it changes but, it's not that big of a deal. The feed stays the same so the feed lines accross the part will still be evenly spaced. The only thing you're really changing here is the surface speed. And we're only talking 10% here. It's just enough to keep breaking up the harmonics. If ya got the money to spend, a de-vibrator bar works awesome. You have to need it bad though to justifiy the cost of those babies. Same with a solid carbide bar. That's if were even talking about a bore here. Could be OD work, he never said what it is.
Here's another trick we found on our Okuma. When the finish boring bar is just on the verge of chattering, when your getting that slight "singing" that you can barely see in your part, we would loosen up the back screw, and only hold the boring bar with the front screw of the holder. Like magic, chatter is gone.
Thanks Dave for agreeing with me that the spindle speed thing works. By the way I've used this on both OD and ID. The tolerance doesn't seem to be effected. I do use a positive 1/64 tipped insert. I also agree with the use of set screws to hold boring bar can be a problem. I'll experiment with leaving more material for the finish pass also. Thanks
Hey phil...
I have used the spindle speed overide many times during a finish pass, to see what speed was best... And whenever i changed the speed, i did notice that chatter would dissapear for 1 or 2 seconds... But i do not think, that is the way to deal with the problem...

I mean, it IS a way to do it... But its not THE way to do it...

I guess, if you are lucky enough to run only 1 lathe at a time , thats ok... Sort of...

But i run a lathe and a mill at the same time...
I cant just stand there playing this the speeds while my mill just sits there...

I think the BEST way to fix your problem would be to play with speeds, feeds, AND DOC!!!

Dont forget the DOC...

In your G71 canned cycle... Play with the U.xxx
If you are using ( FOR OD ) U.0156 then try U.0312, and if that does not work, try U.0468
And so on...

Move up and down in 1/64 incraments untill you find the right DOC...

I HAVE NEVER had a chatter issue, that i could not get rid of by playing with the speeds, feeds, and DOC...

Good luck
solar you kill me!
i remember when you came on board with no experience and now you "HAVE NEVER" had a chatter issue.
i challenge you to turn a 36" dia. low stage compressor vane made of RENE 90 which is 90% colbalt and deal with the "chatter" issue.

i have turned hundreds of feet per night on various materials on large diameters from 30" to 48" in forgings and cast steels, titanium ,RENE 70 and RENE 90 and stainless of various alloys and every one has its own machining characteristics.

"Young Weedhopper return when you can walk on rice paper and not shread"...jim
I know what you're saying, toolmaker, but actually, Solars advice is pretty valid, I think.
Solar, chatter can usually be worked out by the exact methods that you mentioned, but sometimes when all else fails, you do what you have to, to get through a job.
A few yrs ago Kennymetal was pushing a new handheld electronic device that was to be used to finger out'cher harmonics issues. It was maybe the size of a GPS unit or such...
Anyhow - (as I remember from the ads) you would tell it what speed yuhr running now, and maybe diameter. ???? Not sure if DOC was a perameter or not. ???

Then you would hold it up to the operation and let it "listen" to the chatter. It would then tell you a recommended speed to change to.

Really seemed like a kewl little unit! I haven't seen it advertised in a cpl yrs now. ???

-----

If messin' with the RPM can get you where you need to be - you certainly don't need to stand there with the potentio (?) meter. (dial) A cpl minutes of editing the prog in your trouble area should be able to take care of it.

Usually a smaller R tool will work.

Think Snow Eh!
Ox
That's what I meant. Program it to constantly vary the RPM or the feedrate or both.
I was thinking more along the lines of a short length on an OD turn where you could get through it relatively quick as the original post was aboot.

What your talking aboot would take a LOT of programming! Maybe some custom macro could be written to do what you need? (I've got the macro book - now to get a round tu-it to read it. LOL!)

Think Snow Eh!
Ox
I have not used this but I was aware of the codes for the spindle speed variation control function. This is off of a 100E control. M695 turns it on and M694 turns it off. Then the following parameters need to be set for the amplitude, cycle and timer. Parameter word 114 sets the amplitiude of spindle variation, range is 0-100. It is a percentage. Parameter word 115 sets a cycle of spindle speed variation, range is 0-255 in units of .1 seconds. Parameter 116 word 116 sets an interval timer, the range is 0-125 and the unit is .1 second. Then the following variables are added to the program to execute the parameters. VFLTQ, VFLTP and VFLTR. Again, I never used this feature, but I know exactly what you are going through. Hope this helps.
Toolmaker...
I didnt say i "NEVER HAD A CHATTER ISSUE"
Please dont say things like that...
I said i "NEVER HAD A CHATTER ISSUE THAT I COULD NOT FIX WITH SPEED, FEED, AND DOC"

Yes, i have had all sorts of chatter issues, but they are ALWAYS resolvable with these 3 variables...

Also there is a 4th variable... Insert nose radius size... One time i switched from 1/32 to 1/64 and that removed chatter as well...

I have never had to sit there and play with the spindle overide for an entire run of parts, just to get rid of chatter...

hahaha

You crazy!!!
no my late buddy Bob was crazy we called him Crazy Bob and i really miss him he got electrocuted a coulpe years ago.
but back to the chatter problem.
you have not and probably will never run RENE material add the the fact this stuff is mean nasty and Hard the low stage compressors has over 300 interupted cuts so the tool is being loaded then unloaded then loaded then unloaded ETC. so how would you suggest eliminating the chatter issue VIA programming because these guys have 30+ years of dealing with this issue and never have found the cure.

so heres the challenge to you Solar solve a problem that you have no experience with...jim