Pete, Thanks for the article. I agree the AL/PLY/AL would be a great construction material. I have been considering doing a composite beam for my gantry. I have looked at pre-made carbon fiber sheet stock, but it breaks the budget. Considering the aluminum option too. At this point, I will wait until I have the gantry beam fabricated and can do some deflection tests on it. If it looks like it needs to be beefed up at that time I will bond the aluminum sheet to the exterior faces.
Here is the beam cross-section. 6"wide x 10" tall.
To overcome the lower compressive strength of the plywood for fastening hardware to it all of my hard attach points like the linear rails and racks are mounted to wider 3/8" aluminum bar stock which will have tapped holes for the hardware attachment. Then the aluminum will be bonded and screwed to the plywood. For the connections between the gantry side plates and columns and columns to the gantry beam, I have internal bolt plates to fasten too, so the compressive loads of bolting are spread over a larger area.
About how large is this next machine you are working on?