Originally Posted by ger21 What he's saying is if you push on the side of the gantry uprights, the 3/4" MDF will flex and the gantry will move from side to side. By at least doubling them up, it will be stiffer. I'm making mine out of 2 pieces of 5/8 MDF, with a 1/2" Hardwood frame in between them. I also epoxied a 3/4" x 3/4" Aluminum tube running from top to bottom, in a rabbit down the center. You want them to resist flexing in all directions.
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Stiffness increases with the cube of the depth and directly with the elastic modulous of the material. Adding a web is better than doubling up. Aluminum web is better than wood web for the same cross section. The joints between all layers of material must be well glued to each other. Otherwise shear forces will reduce the max stiffness possible with any given composite panel.