which is better aluminum or steel????

[sanddrag]

Originally Posted by Andrew

How come no one talks about riveting? Yet another alternative to welding (permanent) and bolting (removable but heavy).

We used tons of rivets on the arm. It was all made from flat .040 sheet metal (aluminum) that was lasercut then bent picture In some (higher stress) areas the rivets worked a little loose over time. While it did come out very nice, I still think extruded aluminum box tubing is the way to go for any frames or arms or anything. With the sheet metal we were able to make a very thin wall box for light weight but with extruded box tubing you could just surface (machine) down the sides to make it thin too and it would be stronger because it is one piece.

As for the issue of cracking welds, if you have a good welds then you will not have a problem. If the welds are not coming out nice, then of course they are likely to crack.

You can see a picture of our frame and the nice welds (lower structure, upper structure was a rush job) here picture For the frame itself, it is only 11 lbs. It is all 1/8" wall tubing. Thinking back, we probably could have gotten it down to about another 1/2 lb by going a little narrower on the tubing on the side rails.

The only steel in our robot besides gears, sprockets, and chain is in shafts. All the shafts are steel but some of the larger ones are bored out through the center for weight savings.

[Captain Rich]

Check out different alloys of aluminium. It makes a difference, but costs more. I think the kit frame was an alloy of sort, although I'm not sure what it was specifically. Anyone know?

[Cory]

Originally Posted by Captain Rich

Check out different alloys of aluminium. It makes a difference, but costs more. I think the kit frame was an alloy of sort, although I'm not sure what it was specifically. Anyone know?

something in the 7000 series, I think.

[mtaman02]

I would go with Aluminum - Its easier to play with ( Weight Wise ) Steel is just too hard to lighten up, If you build the frame the right way and Identify and take care of the weaker parts of the frame then everything will be alright. Depending on what stock you use as well will help a robots strength

[sanddrag]

Originally Posted by mtaman02

Steel is just too hard to lighten up

Actually, I view the situation as quite on the contrary. It is much more effective to take more wieght out of steel parts because they have more mass per unit volume. Say you want to take a pound out of 1/4" aluminum plate. You'd have to drill 50 1" holes! But if that was steel you'd take out a lot more than one pound with that many holes.

But all the weight you can take out of steel did come from somewhere. It comes from the fact that a steel part is heavier to begin with. So, you can start light with aluminum and if you are still over than there's not a whole lot you can do. Or you can start heavy with steel and if you are over (which you probably will be) you can probably remove a lot. So, it is a tradeoff.

Now, for "ease" of lightening, I suppose you are correct afterall in saying steel is harder to "lighten up" because it is much harder to cut, drill, mill, turn, etc. But, you do get more lbs off for your increased work time.

It is all about tradeoffs.

[jonathan lall]

Originally Posted by sanddrag

Actually, I view the situation as quite on the contrary. It is much more effective to take more wieght out of steel parts because they have more mass per unit volume. Say you want to take a pound out of 1/4" aluminum plate. You'd have to drill 50 1" holes! But if that was steel you'd take out a lot more than one pound with that many holes.

But all the weight you can take out of steel did come from somewhere. It comes from the fact that a steel part is heavier to begin with. So, you can start light with aluminum and if you are still over than there's not a whole lot you can do. Or you can start heavy with steel and if you are over (which you probably will be) you can probably remove a lot. So, it is a tradeoff.

Now, for "ease" of lightening, I suppose you are correct afterall in saying steel is harder to "lighten up" because it is much harder to cut, drill, mill, turn, etc. But, you do get more lbs off for your increased work time.

It is all about tradeoffs.

How do you figure it's a tradeoff? You're saying a steel robot (or component) would get more pounds off in the lightening phase, but wouldn't it be far heavier to start off with anyway?

Let's say the aluminum robot I wanted to make would be 150 pounds before lightening. If I were to make a functionally identical robot with steel, it would weigh 250 pounds (the ratio of aluminum to steel's weight of course varies, but it's generally between two and three times as much). Now, bearing in mind you will be able to put a few more holes in the steel robot or use thinner steel, the strength gain from steel (and don't forget in addition to this the higher cost and machining time) would not be enough to use appreciably less material.

In other words, this is no tradeoff, but is very one-sided in aluminum's favour. Bearing in mind this example is a bit impractical in a robot-building sense, a square-inch hole in a steel plate versus a square-inch hole in an aluminum one will obviously not bring the former's weight below the latter's. Which is probably why most everyone uses aluminum on their frames I would say.

[JVN]

Originally Posted by Captain Rich

Check out different alloys of aluminium. It makes a difference, but costs more. I think the kit frame was an alloy of sort, although I'm not sure what it was specifically. Anyone know?

I happen to know ... AND
It is listed in the Chassis documentation found here:
http://www.ifirobotics.com/kitbot.shtml

The chassis is made from 5052, H34 Aluminum.
Great stuff, welds niiiiiice.