I Feel the Earth Move Under My Feet

The material selection and structure are a big part of the design process. If you don't get them right, bad things can happen. There is not any difference between a gantry on a CNC router and a bridge. You have a span that you want to cover with a heavy cutter that you are moving back and forth and up and down. Just to complicate things, you are also generating forces with the cutter that are constantly changing direction. Throw in the fact that the cutter is at the end of a shaft that multiplies the forces just to make sure that you are not too complacent.

One thing you can do to make things more rigid is to use more stuff. More material can get you there up to a point but it is a matter of diminishing returns. Things get heavier and you need stronger motors to move them. The stronger motors are heavier. The heavier it gets, the more strain and flexing you get along with more friction and inertia to overcome which makes you need even more massive materials and stronger motors. The proverbial Catch-22.

The real answer is to make things stronger and lighter rather than just making things bigger and heavier. My machine is going to be made of wood. It is strong, easy to cut, virtually immune to fatigue, bonds well with adhesives, cheap, readily available. Dick Newick is quoted as calling it Miracle Fiber W. Wood is stronger by weight than steel and aluminum or fiberglass. Converting wood into plywood fixes the real drawback of wood: Wood is stronger in one direction than the other. In plywood, the grain direction alternates to even it out. One other problem with wood is that it expands and contracts radially with humidity. Sealing out water (or at least really slowing it down) is all it takes to address that problem. Most modern machinery is made of metals but don't let the hype fool you: metals change dimensionally too. Metals expand and contract with heat. They deform under stress. If you go back just a few years, wood was the high tech material. It just doesn't have as good publicist as some of the more modern materials because there is a higher mark-up on other stuff and marketing has spent lots of money trying to convince people to spend their money on more "modern" materials.