Steel is good for many things and a frame could be one of them. I always choose what to make mechanisms out of based on what the design needs are not necessarilt what is lightest, because sometimes stell is lighter than aluminum for the given purpose. One of my favorite tricks is to us 4130 aircraft grade round steel tubing for a structure if it's under high loading because it would take a lot more mass of AL tubing to get the same strength and durability. Just remember to consider wall thickness....
Squirrel: You may want to use this as well as it is easily weldable. We usually TIG but you should be fine MIGing it.
Also McMaster carriers it:
http://www.mcmaster.com/#grade-4130-...tubing/=5bx00i
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2011 Championship Finalists/Archimedes Division Championships w/ 2016 & 781
2010 Championship Winners/Newton Division Champions
Thank-you 294 & 67
2009 Newton Division Champions w/ 1507 & 121
2008 Archimedes Division Champions w/ 1124 & 1024
2007 Championship Winners/Newton Division Champions w/190, 987 & 177 The Wall of Maroon
2006 Galileo Division Champions w/ 1126 & 201
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