ok, first attempt using sketchup 8

[routercnc]

Hi Jazzcnc,

I totally agree with you that I beam would be sufficient for cutting wood. I just wanted to show Deanos the improved performance of RHS to explain Jonathan's point about how it could be better if you wanted to make it do more in the future.

I also can't fault your logic and argument on the cost side because that is very important for a home build.

Just for completeness, and not to start a war (!) I've done the calcs for the sizes you mention:

___________________I beam____RHS
Width _______________100______80
Depth _______________180_____160
Flange/wall thickness ___8_______6
Mass (kg) ____________34______32

Tool deflection (mm)______0.009___0.00067

[all assuming 150mm between gantry centre and tool cutting tip - the moment arm trying to twist the gantry]

So the I beam still looks reasonable.