ok, first attempt using sketchup 8

[Jonathan]

Ok, would i be better using Ali extrusion, or are you talking about steel box section. What size would you suggest

I was thinking steel box section, but aluminium extrusion is pretty good too. If you're happy to do some welding then steel box section is the cheapest and strongest option for making a frame. The standard length is 7.5m, so it would be prudent to design the frame to use an integer multiple of 7.5m! I used 60x60mm, 3mm wall thickness box section for most of the frame and 100x60mm, 3.6mm wall thickness for the two lengths the X-axis rails mount on.

How long is the piece of RSJ you've drawn for the gantry?

I'm still confused on the axis direction, by "Y" are you referring to the bearings the longer gantry is riding on

I always call the axes on the gantry Y and Z and the other axis X. So by that definition in your case X is shorter than Y.

How do you use the multi quote option, i clicked it and nothing happens

The what.. never seen that! I just copy and paste the [QUOTE] tags in the relevant locations. I've put them in on your post for you, so if you click 'Edit Post' you can see how...