Hi Ard,
Granite and epoxy can be considered isotropic for the purposes of FE. Its mix will be isotropic as well. The issue is establishing an accurate youngs modulus to use. Typically the rule of mixtures is used to estimate an E. Its Poisson ratio will be 0.25 - 0.3 and this will not make a real delta either. The strains in machine parts are so small these things don't matter. Machines are designed to be very stiff. Stresses are in the order of 1-2Mpa so strains are tiny. Granite varies from 40-60GPa stiffness. Epoxy is about 2-3GPa. Have a look at CSA cement I think its superior. I'm about to make some parts using this. Epoxy is expensive but not stiff. CSA is 2200kg/m3 and 23-30GPa stiffness. Look up the Milli thread it gets discussed there. Peter
FE is used for non-isotropic materials all the time. I do alot of composite analysis for yacht structures. Orthotropic solvers just aren't on desktop CAD yet as most people work in metals.