80/20 versus u-channel

[SteveT44]

I'm designing a 4'x4' gantry style router. My initial thoughts are to use 80/20 extrusions for the table frame with ply or mdf bolted to the top and bottom for rigidity (torsion box). Linear rails will be bolted to the 80/20 to support the gantry. For a little more effort and a lot less money I'm starting to think that structural aluminum u-channel could replace the 80/20 in this application. How straight is u-channel out of the UPS box compared to the equivalent 80/20 extrusion?

TIA

Steve T.

[ger21]

U-Channel at Mcmaster Carr has a straightness deviation of .0125 per foot.

[sdantonio]

I'm working on the design for a gantry router of similar size (mine will be 3'X5') and was planning on going for a welded frame of 1" square steel tubing for the table support structure. But I have been accused of overbuilding my machines in the past.

Steven

[SteveT44]

.0125 per foot. So were talking a possible 1/16" deviation over 4'. I can live with that. Assuming my surplus rails are straighter, I guess I can machine the mdf top flat enough for the average woodworker.

[Corvus corax]

I'm building my first machine as well. I've chosen 6 inch steel I-beam for the frame, and am debating how to go about joining the corners. I'm welding them, as I am a TIG weldor, but I'm weighing the possibility of using heavy steel tubing in the corners, as opposed to mitering the corners of the 4 I beams and welding them together. The steel tubing will reduce the number of complex cuts, as 45 degree angles vertical through I beam, including the webs, is somewhat tedious.