Quote Originally Posted by Muzzer View Post
I guess the resonance you need to damp in a machine tool isn't the ringing of the tube itself but of the structure formed by it. So if you were to stick a large mass (gantry, machine head etc) on the end of said tube and give it a knock, you'd get a much lower resonant frequency as the mass swung back and forth. That's the motion you'd need to damp in a machine tool isn't it? I expect the damping effect of that different structure at the lower frequency is likely to be different.
The tube resonance (or rather resonances) seems to be of some importance for the gantry at least on my machine, being a "raised sides" design, the long (y in my case) axis seems to be well constrained due to it's large attachment area to the base/bed with multiple fixing points and shows very little vibration even when properly thugging through metal. When the z-axis is in the middle of the gantry it will obviously load it and reduce the frequency of the fundamental resonance however it won't affect the 2nd harmonic much at all, similarly for other positions where it's sitting over a node. I found that damping out the higher harmonics with constrained layer damping was quite easy, the lower ones were less easy which is an issue to my mind as on typical sized machines they can fall into the same frequency range as cutter flute impact frequencies and low harmonics thereof.