Hi Nick,

As per your pm:
Your requirements are to cut wood, guitar bodies, and occasional aluminium.

The L shaped gantry in a pair of 90x45 extrusions has been used many times on this forum and will cut your requirements as above. At 900 mm gantry width it is starting to get a bit wide, but I would say still OK.

The raised X axis can also use 90x45 extrusion so long as there are plenty of supporting uprights connecting it back to the main bed.

Would this be good enough or should you use steel? Well, I would say you are OK to use aluminium profile on the gantry and the X axis and still cut the requirements above. If however you want to do regular aluminium cutting then I would upgrade the raised X axis sides to steel.

Here are some aluminium gantry, steel sided machines:
Click image for larger version.  Name: IMAG0730.jpg  Views: 500  Size: 320.9 KB  ID: 22667

Click image for larger version.  Name: IMAG1088.jpg  Views: 507  Size: 371.7 KB  ID: 22668

You wanted to use profile to avoid using epoxy leveling - well I've not found the profile extrusion to be particularly level and flat. You can buy specially machined profile, but regular stuff is not likely to be flat. If you don't want to epoxy level you can always shim the rails manually, but need a master straight edge to compare to.