Hi Doddy,

The idea of the 1mm rebate is to bring the box section together and minimse filing or grinding to get the joint to a right angle. I was not thinking of a weld seam inside the box section, just the 6 big plug welds. The frame would have four of these joints and the gantry two. I think one beam of 100x50x5 box would be strong enough for a 600mm gantry width.

Rob