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Chatter during circular interpolation

HuFlungDung

Diamond
Joined
Jan 19, 2005
Location
Canada
1996 Haas VF3

I noticed a while ago that I get chatter in the cut when doing fairly heavy cutting (for a Haas). This chatter (tool screams) seems to be centered exactly on the 90° and 270° quadrant lines and spans +/- about 15 degrees each side of the quadrant line. Then the tool quiets down for the section of the path between the areas I described.

I was doing some clean circle operations today in C1018 plate, 1/2" thick. Part is held in vise. 1/2" 4 flute (evenly spaced) carbide endmill, full depth of cut, about .075 stepover per orbit, feedrate 35ipm, rpm 3500.

Varying the rpm and feedrate does not seem to be the cure all, although I suppose if I really slow way way down, it's going to quit sometime, but I'm not about to slow way way down :D

At the end of the clean circle op, I take a finish pass with about .0075" material to remove. I speed up the rpm to about 4500, cut the feed to about 20 ipm and the resultant hole is beautiful and round within a couple of tenths (as checked by chucking the part in the lathe and viewing the runout with a DTI).

A couple of months ago, I was roughing some aluminum plate, using Iscar insert endmills and in this setup, the plate was clamped directly to the table. So that takes the vise out of the equation. This was heavy plate but I noticed the same tendency to chatter. Because I was using a high speed milling strategy, there was considerable 'clean circle' type spiralling motion going on, and I noticed this same phenomena about when and where the tool was in the arc when the chatter seemed to peak. It was similar to what I described at the start that is happening today.

Anybody heard of such a thing? Could there be something 'loose' in the Z axis linear rails that would permit a certain amount of slack in the XZ plane but not in the YZ plane? I cannot visualize what kind of mechanical slack would account for such a strictly defined chatter area.

Normal X or Y linear cuts don't seem to cause any particular grief that I have picked up on with typical machine usage. Something about circular interpolation seems to be key. The fact that the machine makes a nice round hole (on a light cut) maybe is a distraction to me from learning the truth.
 
On my Fadal VMC15 there are settings g8/g9 that control if the tool pauses between moves. Sometimes I'll forget I have a g9 in the program and when the tool pauses it will chatter. I change to the g8 to eliminate the pauses and all is good. Maybe you have something like this going on.
 
That is a good point. I checked the spiral routine being output by my CAM software and the place where the arcs change radius while spiralling out is at 90 and 270 degrees.

I changed the programming to use the Haas G13 to do the spiral, but the same chatter issue continued to occur. Of course, I don't know if the G13 logic does essentially the same thing as my CAM does.

For the record, as I watch the machine do the interpolation, it does not appear to pause, but it is difficult to tell if it slows down. I varied the feed between 10 and 50 ipm without really solving anything. It's just stubborn and wants to sing at those two areas :D

Tomorrow I may just backplot my toolpath, rotate it 90 degrees and see if it makes any difference where the chatter occurs.
 
I've noticed peculiar things at the 1/4 circle marks before (12, 3, 6, 9 o'clock)
Since yours are occurring at 3 & 9, I would suspect the x axis.
See if you can watch the following error on the diagnostics page. This may not be the right nomenclature for the Haas machine but you get the idea.
If it's not a servo motor or amp problem (gain or bias adj.) check the gibs in the x axis just for the hell of it, might be time for an adjustment or the ole girl might be getting old and she's telling you to take it easy.
Come to think of it, it may not have gib adj. on box ways (?) - check to be sure.
If not, do a quick backlash check on x axis. Perhaps the ballscrew nut is loosened and "giving" just at the time the axes are changing.
Or as I said above, she's just wore out.
 
It's got linear ways, so no gibs, I guess. So if I were looking for worn components, how would I distinguish between a bad Y thrust bearing, versus sub-size balls in the X axis ball trucks? The current backlash settings are still quite low, and the roundness of a slowly interpolated hole seems to indicate no lumps on the quadrant lines from axis reversals. That is part of the conundrum, that it does not mill out of round to any significant degree on a light cut.
 
We had a brandy new Mazak come in last year that did the exact same thing ( obviously not acceptable ). After many many :toetap: visits they got it fixed. And this is something that could happen by wear or being loose no matter what brand machine. Simple on the new one because when they finally tracked it down one of the linear trucks was loose so when it was pushing slightly harder in the cut in the X direction it would move the head very slightly side ways causing chatter in the same area you describe. Maybe yours has loosened or worn a little. More than likely loose. The thing that tricked them was when a small cut was taken it was perfect, also they did a ball bar check many times but they were looking at x,y at that point and it had nothing to do with x,y. Sorry if I didn't explain it right, hope I did.

The following statement that you made was the key "The fact that the machine makes a nice round hole (on a light cut) maybe is a distraction to me from learning the truth.
 
I had this happen on two of my machines.

The first I just ignored as I thought it was a serious problem...Ball Screw or Ball Screw Nut Replacement or linear Bearing...I just did not want to put money into a 91 VF1 and then finally when it showed up in the accuracy I made the call to Haas. They told me to check the thrust bearing nut as they can turn loose. Snugged it up as per spec and she was back to her old self.

Soon as I had the 1st indication of this on another VF-1 i pulled the covers, ploped and indicator down and saw it was the same deal...snugged it up and I was back in biz.

Hopefully your encountering the same problem.
 
They told me to check the thrust bearing nut as they can turn loose. Snugged it up as per spec and she was back to her old self.

This was the cause of the exact same problem on my hurco not six months ago. It would certainly be the first place i'd look.

Gregor
 
When cutting the threads are you starting at the bottom of hole or the top? I thread bottom up during circular intrpolation. That way the chips move up and out with the cutter. Don't think this would cause the chatter problem, more FYI.
 
When cutting the threads are you starting at the bottom of hole or the top? I thread bottom up during circular intrpolation. That way the chips move up and out with the cutter. Don't think this would cause the chatter problem, more FYI.


Doug.....whatever you're smoking :smoking:....pass that thing. :D
 
what was the outcome of this problem Hu?

curious if you ever figured this out, i have a 96 vf2 that is having issues, im going to post a new thread though. let me know if you can!
 








 
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