Two C-beams would "sandwich" the Z-Axis. The reasoning is to eliminate z-deflection that would occur when spindle is pushing lower into material. This would allow higher Z gantry and longer Z-axis travel if desired. Also potentially increased rigidity of X-axis with second c-beam. Due to lack of CAD/Sketchup skill here are a couple of pictures to hopefully illustrate what it would be.
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View attachment 12526 View attachment 12527