to be honest I don't think there is a "conundrum" but as it is, nobody seems to go for it and actually make one
there is a thread on the cnczone that goes on and on about mix ratios and filler type and it seems to just drift past logic into an endless pissing contest
I'm sure some care and though is needed for a choice mix but like you say "It doesn't look like rocket science" and the real nack to the job in my opinion is managing to commit and deciding to crack on with making it happen (like anything else)
Cast iron is 250 year old tech for machines and we are using alu and steel because we can't afford to get custom lumps of cast iron done for us and it isn't really feasible to start a smelting foundry in your garden just to make a cnc machine.
But homebrew epoxy granite is feasible. If you use some thin walled steel box section, source a carboard tube for the centre, devise a reasonable mix and do a few experiments to find a good setup that is superior to the empty steel box or alu extrusion/alu plate designs people are using now there is no reason why you can't make a moving gantry with the stuff. Don't forget if you are relying on the epoxy granite for stiffness you can cut down a lot of weight in metal.
A few things to read:
CNCCookbook: Epoxy Granite Fill
You can actually save weight and get better performance using EG:
If you design a beam from EG to have the same deflection for a given force it will actually weigh half what the steel does and have superior dampening charcteristics to boot.
PS If you are wondering about the cardboard tube, it is there to turn the energy in the vibration passing through your beam into heat (the energy has to go somewhere and if you want to dampen a vibation you need to suck the energy out of it and turn it into a different form energy or it will just keep on vibrating your beam or be passed into the rest of your machine) The cardboard tube is proven to work in cnc machine beams and also in things like automotive shafts.
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