Large Beltesian 3D printer - that is a cartesian with a belt-driven Z axis.
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Top Rail - add a top rail on the other end of the Y beam, and double-up the X axis motors. While it works, the machine isn't rigid enough at the top end of the Y rail. Works fine for ~100mm tall parts, but more would be better.
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Thicker bed surface, test other surfaces - the bed isn't quite rigid enough. Making it thicker should help. Joining, bending and moving a thicker belt aren't the easiest, but hopefully there's a happy mediun somewhere.