Hi Monk,

I've created a spreadsheet to work out simple arbitrary shapes I values - will post it sometime.

Anyway, to recap, your L shape was somewhere between:
40x120x3 Z bending is 0.066mm, X bending is 0.020mm, twisting is 0.0019mm
80x120x3 Z bending is 0.043mm, X bending is 0.004mm, twisting is 0.0005mm


Actual value for L shape is:
40/80x120x3 Z bending is 0.052mm
which is closer to the 80x120 than the 40x120

But oddly this is the same deflection as:
60x120x3 (0.052mm) which is half way between width 40 and 80. I wasn't expecting that, and neither was Irving with his guess of 55. Double checked it. Can't find anything wrong.

Enough maths, looking forward to seeing the ballscrew options. . . .

Ah, yes, adding rails will increase stiffness but don't bother working it out, get the structure right underneath and you're away.

Barry