This was one of my concerns when I was thinking about building my frame. @Bar had updated the frame a little while ago to use more solid lumber for that stability along it’s length. Originally, the frame had been made mostly from plywood and there were reports of the ply bowing and twisting in high humidity.
This is a top-beam frame, which we have seen the greatest successes with. @Dlang has suggested the best material for the job is LVL, which is a special type of composite structural lumber that has incredible dimensional stability. I have worked with the stuff building houses and they are great! They’re available at most lumber yards and the big home improvement places usually at 10 ft lengths.
Solid wood might be a good option as well, but it still could twist or bow and that would significantly affect your distance between motors.
YES! and this is the route I’ve taken. Compared to wood, steel has much better dimensional stability. It does flex more but as long as you get a good structural section like channel, pipe, tube, or unistrut, that won’t be a problem at the forces we’re dealing with here. I welded a steel tube frame myself, and the machine is very stable.
The easier steel frame to build is the Unistrut frame, and you may want to look into it. It’s a little more expensive than a wood frame. The thread goes into pretty good detail about it.
I like this idea, and think it could be used well with a wooden top beam frame. I think having a visual indicator that something’s off is a great idea.
If you haven’t already found it, Musings on frame design and Frame design options are good discussions on frame construction.