Hi,

If it weren't so, filling cavities of a machine tool would have no effect, because the cavities are "not in the path" as you say.
BS. The filling material must adhere to the inside of the section, and then any flexure in the tube is transmitted to the filler material which in turn dissapates energy. This is the problem with filling tubes
is that the filler material seldom adheres sufficiently to the inner of the tube to effectively participate in any damping. If you want stiffer.......then make the walls of the tube
thicker. When all said and done if a structure is stiff enough is does not vibrate worth a damn anyway.

and if I was subconsiously looking for an excuse to mold some epoxy granite, I found one ! ;-)
If you want to try epoxy granite then try it, trying to suggest that it can dampen the vibration of a machine sitting on top of it is not correct. You could just as well make a cube of epoxy granite
and sit it on the other side of the workshop for all the vibration damping it will do on your machine.

Craig