My plate has finally arrived. The first piece had a bend in it:
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I think it must have been hit by a forklift in the warehouse. Anyway called Smiths and they exchanged it the next day, the new piece is perfect.
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I've roughed out some bits for the Z axis on the table saw.
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I got a deal on this plate because it was originally an offcut piece, however I have way too much material for this machine. I certainly have enough plate to build the gantry entirely from plate and not bother with the extrusion, has anyone done this or know of any build threads (I have looked)?

From plate I'm guessing I would form a channel 160x80 with the back open, question is would this be sufficiently twist proof?