Yes, the inserts are completely buried inside the EG, ~10mm under the surface. This leaves the surface plain EG without steel islands sticking out of it. This is important because I will hand grind the EG to make it flat for the linear rails. Then I just drill through the EG and drill and tap the insert in precisely the right place. That's how the X and Y axes rails were attached.
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It would work but the rebar is cheaper...