Ignatz, pretty sure I got it right. Someone else please chime in.
Dave was talking about solid wood. It will swell and shrink quite a bit through out the year. Not a good choice for making a machine. Furniture makers do all kind of tricks to allow this movement. Do not try to stop it; you will end up with cracks. Solid wood also has the problem of edge (long) grain and end grain. End grain can not hold glue. This is why dovetails and biscuits and rabbits and dados etc. are used to always try to glue edge grain to edge grain or to add mechanical strength because the glue will not. On the plus side it is beautiful.
MDF will grow in thickness when it absorbs moisture and not return to its original size. It is compressed and always wants to become un-compressed. It is also too heavy for its own strength and needs to be supported. On the plus side it is cheap, consistent, and stable if sealed.
Plywood is the most stable of these three. Every other layer has the grain running in the opposite direction. It does not change in size (for all practical purposes). Baltic Birch ply (as I have always known it) is the king of plywood. It has nine plies (layers) to the half inch. This is almost double the count of normal plywood. It is guaranteed to be “void free” meaning that there are no knots or missing pieces even in the internal layers. It mills extremely well. You can see it by looking at Ger21’s machine. Woodworker’s use it for drawers and jigs. I even think the edge looks pretty. You have to buy it from a wood store. The stuff I get is metric.
Steve  |