I have to build an altar for a new church, which will have a pedestal base and a torsion box top that will be 96" long and 36" wide curving to 30" at the ends. The top will be 4" thick. If the grid work were made from 3 1/2" basswood and covered with 1/4" mdf, how thick should the basswood be? Also, what should the spacing be to achieve as light of a top as possible without sacrificing strength?
(Furniture Making Forum)
From contributor Z:
I often use 1/2 veneer core ply for my ribs, and make the cells about 5x5". Also, when you press the MDF on, make sure you use a thick caul on top or you can get scallops in the MDF.
1/2 or 3/4 material is fine for the core. If you can use vacuum for the glue up you will be better off, but as mentioned above, you need to watch for the skin being forced into the spaces. use a pressure switch and turn the vacuum way down. You only need a firm even pressure to hold everything while the glue sets.
I have a 10' x 4' torsion box that I use as an assembly table made exactly as described. It has 3/4" ply ribs, half-lapped at 8" spacing, and 1/4" MDF skins. It lies loose on top of a low bench, and remains flatter than the bench it sits on.