Steel as a material

[nuclearnerd]

It's all in the design. Aluminum isn't as much lighter than steel as you might think. It's only 30% the density, but it's (generally) only 50% as strong, so you can afford to use smaller pieces with steel. The ratio of strength to density is called "specific strength". For instance, a 0.75" x 0.75" x 0.065" (thin) wall tube is roughly as strong, a little less stiff, and only about 30% heavier than a 1" x 1" x 0.095" wall 6063 aluminum tube.

Welding (most grades of) aluminum takes away half of its strength, whereas mild steel stays relatively constant, so the steel welded frame would end up slightly stronger. Plus any hack with a mig welder could weld steel for you, compared to aluminum where you need a TIG welder and a decent amount of experience. Steel is a heck of a lot cheaper than aluminum too! These are good reasons to give (thin wall) steel a careful look.

[Kevin Sevcik]

Originally Posted by nuclearnerd

It's all in the design. Aluminum isn't as much lighter than steel as you might think. It's only 30% the density, but it's (generally) only 50% as strong, so you can afford to use smaller pieces with steel. The ratio of strength to density is called "specific strength". For instance, a 0.75" x 0.75" x 0.065" (thin) wall tube is roughly as strong, a little less stiff, and only about 30% heavier than a 1" x 1" x 0.095" wall 6063 aluminum tube.

Welding (most grades of) aluminum takes away half of its strength, whereas mild steel stays relatively constant, so the steel welded frame would end up slightly stronger. Plus any hack with a mig welder could weld steel for you, compared to aluminum where you need a TIG welder and a decent amount of experience. Steel is a heck of a lot cheaper than aluminum too! These are good reasons to give (thin wall) steel a careful look.

My boss has a similar opinion when he was mentoring a team. He loved teaching the students how to weld up their frames. He was also mildly surprised that they were always very close to over weight. My primary arguments against him were that steel is a fair bit harder to cut and drill, and its difficult to get sections with walls thin enough to be comparable to aluminum. Getting something equivalent to 1x1x1/16 aluminum is not easy. And we've used smaller than that for lightly loaded structures before.

[MrBasse]

Our lightest robots ever have had full steel superstructure built from 1/2 inch square steel tube. In 2011 we were ~85 lbs. and in 2013 we were right around 105 lbs.

We found that with a steel frame we used so much less material for the same strength.

Another benefit is when you hit something wrong, you just bend it back. Aluminium sure doesn't play as nice in that regard.

We use aluminum alot now simply because so many nice COTS items interface with common aluminum extrusions.

[MrForbes]

Another feature of steel is that lots of folks can weld it, even me! and I have little skill.

We have made two robots with steel frames. As mentioned, you want to use relatively thin steel...16 gauge square tubing is good, and easy to find. But also as mentioned, we don't know what it is you're trying to make, exactly. Sketches are very helpful when asking questions like this.

btw, most cars have been made mostly of steel for about a century. It works.