Problem- Feed/Speed to avoid chatter in turning OP

[Driftwood]

I'm doing an unusual job for me, where we have very long overhang for a couple of boring bars and we're getting a lot of vibration. I'm wondering if anyone has experience with these kinds of jobs and what feed and speed you'd recommend?

Material is 316SS Tube, 1.9" OD and 1.5" ID.
There's no OD work, but I need to bore the inside 1.690" diameter and 4" deep from one end.

So I'm using two 1.25" shank boring bar with CNMG 431 inserts (roughing and finishing), extended 5" from the tool holder so that I can still get some coolant pouring down the lengths...

For roughing passes, I'm trying to go at 400SFM, 0.012ipr, and approx 0.030" depth of cut, so 3 passes... (manufacturer's recommended feed and speed for insert) (I settled on the 0.030" depth (3 passes) because I was getting chip build up and plugging in the tube, so now between passes, I shove a bar down the tube to push all the chips down...)

Finishing pass is 625SFM, 0.007ipr and just cleaning up the last 5-10 thou...(manufacturer's recommended feed and speed for insert)

I seemed to be having some success for the first few pcs, but now just getting ridiculous vibration for both roughing and finishing passes... am I out to lunch trying to run at these speeds given how far the tool is extended?

The vibration is reduced quite a bit when I drop the spindle speed to 50%, so I'd probably be able to get rid of them all together if I dropped to 10-30%... but I've got a lot of parts to run, so I'd rather not do that...


I think I've kind of resolved the issue in a bit of a brute force method: I generated some G-code in an excel spreadsheet, where the machine cuts in 50 thou increments and continuously changes the spindle speed. Code looks something like this:

G96 S400
G1 Z-.05 S450
Z-.1 S350
Z-.15 S450
Z-.2 S350
etc...

This seems to be working and lets me run at these speeds and feeds, though it took some trial and error to get the speed and Z increments correct.


But anyway... all I'm really wondering is whether I'm just running way to fast? Any other recommendations for reducing chatter/vibration in this kind of setup? I've read that up to 4x the bar diameter (so 5" in my case, which is what I'm doing), there shouldn't be much of a problem, but that's not what I'm seeing...

Thanks in advance...

[djr76]

Maybe increase your finish pass to .02 - .03 so you get a nice load on the bar. I like your idea with the speed changes, maybe add some feed changes also and see what happens. Im guessing the vibration is coming from the part and not the bore bar, 1.25 should be robust enough to cut that deep. Maybe change the way you are clamping it, can you clamp in the middle of the bar? That way you have an even overhang fore and aft of the chuck clamp zone.

[neilw20]

Can you slide a piece of rubber or something on the outside of the tube?
I suppose you have bar feed and this is not practical.

Maybe running a spring loaded soft rubber roller lightly against the surface would lower the resonant frequency so you can run the machine much faster, above the resonant frequency.

Have you tried a ridiculously fast speed?
It seems that the speed changes can help.

Some more incremental speed changes during Z to cause more speed ramping will sound quite strange.

Can you look with a stroboscope to find where the vibration node(s) are?

Some extended, even sheet metal jaws to dampen the tube maybe an option.