As I have never worked with this stuff but have worked extrusions I have a couple of thoughts I would like comments on...
Alum extrusions I have worked with tend to be stiff as hell but also tend to be a bit prone to ringing because of it. Building a mill from it I am afraid it might tend to induce this into the part especially if you don't have a very high speed spindle. So trying to look ahead I was thinking of filling the center of the extrusion with something to help add a bit of mass and vibration damping material. I had two thoughts my first was to use a machine non shrink grout that we use when we install big machine tools after the final alignment we grout the rails to insure they don’t move. This stuff is like a fancy portland cement. Its messy and after thinking a bit I thought why not just plug one end with something like hot glue or epoxy about 1/4 -1/2 inch then fill it with a very fine grade sand, vibrate it to pack it in well then hot glue or epoxy the other end to seal it in. My thoughts were you would get the ease of working with the extrusion but the sand would help dampen out the ringing that thin walled material tends to have and a bit of mass would be added to the overall build...
Just a thought …
Mark

Go for it. So long as you don't have to cross drill into the loose material, it shouldn't hurt. Increasing the mass of the machine may require a little more oomph from the axis motors to move it, but more mass is a good thing, IMO.

I've wondered how much non-symmetric bracing would also help reduce the number of harmonic frequencies that the machine would respond to.

Do anybody know where to get information on vibration damping properties of various materials?

Do sand really work? I'm also planning to fill extrusions with something.

one thing we do is fill the extrusion with expansion foam

Dave

I think your on the right track with sand. They sometimes use it in speaker boxes to dampen vibrations at lower frequencies (I.e. sub woofer boxes).

And hey, if it doesn't work, you can always drain it...

Carlo

I wonder if you could fill an extrusion with molten lead? It wouldn't melt the aluminum, and would have great damping ability. My only concern is it might take the temper out.

Keep in mind that vibration in mechanical systems is similar to resonance in electronic circuits. Mechanically the mass is the inductance and the capacitance is the speed at which vibration travels in the material ( or is it the other way around :) ). Try adding a damper ( car shock absorber , not chunk of rubber) or mechanical resistance. Sand should work . The idea is transform the energy into heat and let it dissipate

One idea I have been toying with has been threading a rod through extruded ALum stock filling the tube with sand and then closing the ends of with a plate and tensioning the rod till it is tight and straight . Movement on the aluminium is dampened through the sand against the tensioned rod.

I haven't seen this tried , so it may be a complete waste of time , love to know if it works before I try it myself.

D

Try using earthquake and vibration damping as search terms in google.

Forgot to add: Try different types and amounts of resistive material . From what I have read it does not have to be sand , mdf dust or microballoons seem to work as well. In addition a packed tube may not work as well as a half or three quarter full one.

D

Doh! NOW I see the edit button