ok, first attempt using sketchup 8

[irving2008]

so even though both are negligible as you put it.. you would still steer him clear of the easier/cheaper option of an rsj that needs little work over him welding box section and possibly leaving room for him to create errors...

or have i mis-understood... i'm watching this thread as the thought of rsj being an easy way compared to welding box section appeals to me

Not sure i understand your logic... given a lump of i-beam or a lump of box section, you're still going to have to drill holes to mount rails etc. He was suggesting the rsj for the y-axis element of the gantry... not sure what welding is needed???

Also he says he can get a section for a tenner... the cheapest 178x102x19 rsj on ebay is £19.50/m min order 4m.. plus carriage...