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The papers I read seemed to say that the major deciding factor was the % of mineral filler, and then the particle size distribution - you seem to need a fair % of small stuff to keep the minimum...

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Being intrigued by this I went digging and found the thermal expansion coefficient. It will vary with the mix (more epoxy = more expansion it seems), but typical figures are 12...16 x 10-6/deg C....

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Kern uses aluminium for the linear rail mounting I believe. They cool / thermally control all the mounting surfaces.

I don't think so.! Some of the best and most accurate lathes and Milling machine in the world use Epoxy granite because of it's thermal advantages and the fact they can build in extra mechanisms ie:...

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This was something I failed to find any kind of definitive answer to.
The conclusion I drew, was it can't be that big a concern, given there are some very big EG machines with linear rails mounted...

Indeed many hobby people put reinforcements in their EG structure. Probably because they don't really know the material and treat it like a regular concrete.
Look at how the big guys do it....

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Trying to stiffen an epoxy-granite structure with interlinked pieces of steel will only lead to issues due to the huge difference of thermal inertia. The steel will expand much faster than the EG.
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Hi EZELab
Welcome to the forum.
I would like to point you to John McNamara's epoxy build( https://www.model-engineer.co.uk/forums/postings.asp?th=139042)
A very thorough and informative thread....

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I believe it will, a fixed gantry has got to be more stable than a moving mass gantry, the only thing is you need more room for the moving table.
I have a fixed gantry and no problem at all.

Phill

How do you arrive at that thinking? most things have the skeleton inside for strength of the main structure and appearance there would be very little point of having steel outside of Epoxy granite...