ok, first attempt using sketchup 8

[deannos]

Well seen as your using steel for the frame then I'd probably look towards using that. Simplest way would be an I beam RSJ type beam.
Would get 180x100 for about £30 if your buying steel for rest of machine. You can buy 1500mm length from Metals4U for £37 plus postage.

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I like the idea of using the RSJ, less for me to do. I assume i wouldn't need to box this in and it can be left as is, would it need support or stiffeners at each end.