Quote Originally Posted by routercnc View Post
Your estimate of stiffness increase for supported rails over unsupported (2.8 to 6.8 times better) neglects the part that the supported rail is bolted to (often a thick plate for Z axis), which is the main reason for doing it.
Yes, that's what I meant by the rail being 'floating'. I'm thinking of mounting my Y-axis rails either on 50x50x3mm steel box section, or 80x80x3 aluminium so I'm going to calculate both to compare. At risk of straying off topic are the sides of box section parallel enough to mount rails to directly?

Can you link me to somewhere that details how to calculate the twisting?

Other CAD s/ware, such as AutoCAD, CATIA will give properties including second moment of area, so for complex shapes this is another way to do it.[/QUOTE]