Quote Originally Posted by craigrobbo View Post
I know there maybe an issue of it coming loose - My other thought was flexible grout (similar weight to concrete).
That's the problem - it shrinks a bit when it sets and likes to 'move' and 'creep', so you could end up with it distorting the frame over a long period of time.

Epoxy granite is much better, except for the price of course. There's a huge discussion about it here:

http://www.cnczone.com/forums/epoxy_granite/

...however bear in mind their objective is to use it for the actual frame, not just to fill in which they require a good surface finish amongst other things. This clearly isn't an issue with just using it to fill in, so you probably don't need to worry about that and as such can just use a simple recipe:

http://www.cnccookbook.com/CCMillEpoxyFill.htm

Either way it's a great idea to add mass to the machine frame for strength and damping.