[Jared Russell]

We actually use Bosch-Rexroth 20mm extrusion for structure on our robot (this year we used both 20x20mm and 20x60mm). The 20x20 is .269 lbs/ft and the 20x60 is .605.

The strength/weight ratio for T-slot extrusion is similar but slightly inferior (within 15% IIRC) to square tubing, but what you get in return is an easy way to tap the ends of the profile (we generally use a 1/4-20) and the ability to bolt crosswise through it without having to worry about "caving in" the sides of the tube. If you have limited machining resources, it's a good way to rapidly build structure. It is also a great way to let the students get their "hands dirty". But, as noted, expect to pay a slight penalty in both weight and materials cost. If you are using 1" or 30mm profiles, the weight adds up REALLY quickly. Our use of the 20mm profile helps to alleviate this, but even then there are times where we opt for other materials/profiles to get better stiffness or strength for the weight (our 2011 arm comes to mind).

We generally do not use T-nuts or specialized connecting hardware (the exception is for mounting things like optical sensors that will bear no load) because they drive up weight/cost and slide all over the place. Instead we drill through-holes and/or tap the ends of the profile to build solid connections.

In an ideal world, we would rapidly prototype using extrusion and then go for more streamlined square/rectangular tube + gusset plate or even sheet metal for final mechanisms. In the past (including 2012) we often run out of time before we get to this level of optimization (and have also had widely varying levels of machine shop access from season to season).