Hello every one I am working on a robot and I am using kinda big motors and I made the body from 5mm plastic sheet and I found that its not strong enough to hold the robot as you see in the pic there a big curve there And I am searching for a cheap idea to make the robot stronger
I would make an X shape using L channel or U channel going from corner to corner on the upper side. It is much easier to bend a flat piece of metal, plastic, wood, etc than it is to stretch or compress the same material. Think of I beams in buildings, the material at right angles causes the beam to have much more strength.
A cheap and easy way would be to mount one piece of L channel on the upper side going one direction, and the other on the underside going the other, to form the X. otherwise you're going to have to find a way to deal with the intersection of the two right angle pieces, which is still possible, just more work.