A brand new machine build. 2x4 size or something in that region. :)

[Jonathan]

Also Jonathan I know you have FEA software so if you have the time run realistic loads through and see what you get.

Ok, I've done a very quick simulation as I don't think it adds much to the discussion since I'm only comparing the stiffness of sections for which the equations are already well documented. Granted the rails add a bit of stiffness, but it's not that great in the whole scheme of things.



That's run with a 20N force, so nothing serious, at 200mm from the bottom of the gantry. Deflection at this distance (i.e. the tool) is 8.3um for channel and 1.5um for box section.

Before everyone starts pointing out flaws - this model is just a quick simulation to demonstrate the principle, not the exact machine, except for the UNP160 dimensions which I got here, so the exact numbers are not important. What's important is to note that just using the same size RHS made the deflection over 5 times less than with UNP160. This model assumes the ends are perfectly rigidly fixed, so in reality it will be worse.

The rails are already mounted on top of a welded on piece of steel, which will be machined flat to true up the rails.
A Datum edge, is that a straight edge I can mount one rail along, so it will be true to the surface of the machine?

Partly that, but also to help mount the rails straight and parallel.