I built mine out of MDF, but it does flex. The modulus of elasticity for MDF is about 400,000 psi. The modulus for wood or plywood varies by type, but both are in the range of 1,500,000. Flex is directly proportional to that number, so the MDF will have almost 4x the flex of plywood.
That said, MDF is less expensive, can be cut and tapped easier, and if you build something like a torsion box, it will "solid" enough. The flex in my machine is in the gantry. I will probably re-make it in plywood or add a lot of stiffners. |