[MechanoMan]

I'm looking at building something along the lines of what they show on CNCRouterParts, using a 1018 cold-rolled steel flat bolted into the 8020 to run bearing wheels on.

I want to build at least an 8ft X-axis. 8ft is a common size for a steel flat, but it won't do because the carriage itself has some width, and you want some margin to work with 8ft plywood, the bit needs to be able to cut past the end of the sheet.

Can multiple pieces be joined? It does seem like kind of a bad idea, although the basic stiffness is being provided by the 8020, still, the discontinuity seems like a problem since that surface is both supporting and guiding the gantry weight. The bearings could wear on the interface and it may be inaccurate near there.

[jscavitto]

if you are working with full sheets I would go with 10ft csr. I would not do a butt joint as you would most certainly see it in the finish piece when you hit the joint.

[CarveOne]

I would also recommend using a one piece rail. Most suppliers will cut it to the length you need. There will be freight charge of course due to the length. It will be a tedious job to get the butt joints matched and aligned well enough to work properly. The CNCRP carriages need very accurate guide rails.

Depending on whether you use the long or short carriages, you will need to allow about 12" extra to cut a 96" sheet end to end plus some extra over travel.

CarveOne