Fitting belts within your frame

[Cory]

Originally Posted by asid61

+1. 6063 has a much lower tensile strength and stiffness than 6061. If you take a hard hit to the side, 6063 bends much easier. It's cheaper, but it's not worth it; just move to 2x1.

This is somewhat misleading.

6063-T6 has a 15% lower yield (35 ksi vs 40 ksi). Two identical cross sections in 6061 and 6063 have the same stiffness because stiffness is a product of Young's Modulus (which is basically 10,000 ksi for all aluminum alloys). Welded joints will be 40% weaker than in 6061, but any annealed aluminum is already incredibly weak regardless of alloy.

Our experience using square/rectangular tubing is that there is no difference. Quality 6063 should machine more easily than 6061, but I know that 973 has gotten some before that was far gummier than even 6061 (which is an impressive feat).