Originally Posted by rweatherly 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. |
Actuall wood has several moduli according to which grain direction one is considering . MDF has little strength other than in compression because it is homogenious by it's very nature. Now comsidering plywood, and this varies widely as far as quality is concerned . Probably "baltic birch " with good quality wood used in multple plys is probably the best but one must remember even though it is stronger than MDF only half the thickness is working to the best advantage [longitudinal plies]. So in the long run as I uderstand it solid wood still is a better choice provided the design takes into account of it's movement . Basically across the grain with changes in humidity and to a much lesser degree temperature . Regarding finishes,whatever you do to one side it is essential to do the same to the other side otherwise you end up with a unbalance moisture absorbtion situation causing stability problems .
Someone mentioned getting 2 by 4's from home depot .Most construction materials are not dried to the correct level for precision construction as discussed on this group . If using solid wood it should be either kiln dried or air dried correctly to the correct moisture content for your area .I find even then it is good practice if possible to leave it for a time in the environment where it ia going to live before milling it . That being said bearing in mind every time it is cut ,planed etc The internal stresses have to readjust,the stresses at the new surface being zero. So as far as I am concerned I cut
less and less material off the closer I get to the required dimension and each time leaving it a day or two to stabilize between milling operations .
I am probably wrong here but I always though the oft mentioned "torsion box" was exactly that, a closed structure with everthing glued up including the end caps. What a lot of folks descibe to me are open box beams which deform under torsion loads and do not do as well as torsion boxes. as far as the ballscrew structure which transits the gantry along the bed I would imagine box beams are sufficient because the torsional loads would seem to be pretty low ....Please feel free to critique I have already made my major boo boo regarding butress versus acme threads, major brain fart ....mjh