thnx for the help everybody!!!
I don’t believe welding is a good option, it’s just more machinery you need to bring to the competition. and I weld aluminum all the time, just cuz it’s harder dosen’t mean it’s not good, no pain no gain. and correct me if i’m wrong but isn’t there a no smoke or sparks rule, cuz they had that at our regional last year, and i don’t believe that first would approve of welding while people are walking about without protective eyewear.
You can’t weld at competitions. Some events will have welders at the machine shop that can make any repairs to welded parts that may fail.
O ok, but the regional that my team went to last year didn’t have those resources
well i think to go aluminum be caz its lighter then steel but more flimsy :eek:
For those wishing to do some actual engineering calculations to compare materials for a given robot application –
I recommend checking out:
http://www.matweb.com/
Lots of cool information there. You can plug in values to your heart’s content.
John
Lets see… opens “Materials and Processes in Manufacturing” textbook…
First, ALuminum:
"A number of unique and attracitve properties account for the engineering significance of aluminum. these incluce its workabilty, lightweight, corrosion resistance, good electrical and theramal conductivity, optical reflectivity, and ease of recycling. Aluminum has a specific gravity of 2.7 compared to 7.85 for steel, making aluminum about one third the weight of steel for an equivanlet volume. Sost Comaprisons are often made on the basis of cost per poun, where aluminum is at a distinct disadvantage. There are a number of applications, however, where a more appropriate comparison would be based on a cost per unit volume. A pound of aluminum produces three times as many same sized parts as a pound of steel, so the cost difference becomes markedly less. "
Now, Steel:
“Compared to other engineering materials, the carbon steels offer high strength and high stiffness, coupled wit reasonable toughness. They can be magnetically seperated from mixed materials, and are easily recycled. Unfortunatly, they also rust easily and generally require some form of surface protection, such as paint, galvanizing, or other coating. The plain-carbon steels are genearlly the lower-cost steel material and should be given first consideration for many applications. Their limitations, however, may become restrictive. When improved performance is required, these steels can often be upgraded by the addition of one ore more alloying elements…Steel is an extremely useful engineering material. It offers strength, ridigity, and durability. From a manufacturing perspective, its formability, joinability, and paintability, as well as repairability, are all attractive. As a result, steel accounted for half of the material used in a typical 2000 model Japanese passenger car, and will likely continue at this level. In terms of tonnage, steel is the most recycled material in commerce, nearly twice as much as paper, and far exceeding aluminum, glass, and plastics. Its magnetic properties facilitate easy recovery and seperation from other materials. As a result, about two thirds of the steel production in the United States comes from the recycling of steel scrap.”
well there you have it…I knew tackling a course like design and processes of metallurgy and material sciences in college would lead to something. :rolleyes:
In my many years in FIRST, I have always had a sweet spot in my heart for aluminum. I think the engineering and properties that it has makes it beautiful. For any of those who cannot come to a decision over steel vs. alum. Look into aqcuiring this book( however be forwarned, it does cost about $150).
Hope everything works out!, Good luck!
Last year we used 1" x 1" x 1/8" wall square tubing to make a ladder frame (single platform). Each corner and cross member was sandwiched between small 1/8" thick top and bottom plates (about 2" x 3") and bolted with 10-32 x 1 1/4" alloy steel cap screws (typically 4 per bracket pair). Internal bracing used short pieces of aluminum angle on the sides to keep the top clear of obstructions. It was strong, very stiff, and required only cutting and drilling. It could also be reconfigured easily. Best of all, it only cost about $40. in materials. The square tubing is a LOT cheaper than 80/20 or other extruded types, about $1./ft., though probably a bit weaker for it’s weight. I think it was some softer 5000 series alloy, not 6061-T6. Last year was the first time we didn’t have any problem making weight.
If you are on a budget and don’t have welding capability, try it. Buy the tubing at a metal supply place in 20 ft. lengths, not Home Depot! Allen head cap screws are about $8./100 at MSC and the nyloc nuts are cheap.
Consider style and temperature as well. Aluminum “cleans up” very well and looks professional and in my opinion, its easier to weld and not as messy as steel. When you deal with temperature you need to consider how much heat your robot puts out and if the metal will become brittle under the temperature, then again it all depends on how thick your piece is
Metals actually become brittle under low temperatures. And most FIRST competitions are indoors, so it shouldn’t be a concern. Unless, of course, the NJ regional has to be held outdoors this year for some reason.
Heat can affect the temper and ductility of metals, but this is usually pretty darn hot (~500F) So, heat should not be a factor for FIRST robots, really. nothing less than your battery shorting across your frame will generate enough heat to affect the structural integrity of your robot.
And in case any engineer here wants to nit-pick, yes, precipitation hardening aluminum alloys can be over-aged by prolonged exposure to elevated heat. And thus become brittle. But still, most FIRST teams needn’t worry about it.
Isn’t aluminum significantly harder to weld then steel? Steel can be welded with a torch while aluminum requires TIG/MIG because of its corrosiveness.
This statement is just wrong. Aluminum is not easier to weld than steel, and I think anyone who has ever welded in their life would agree with me. Maybe if you’re welding stainless or something
aluminum is sturdy and lite but is easly warped or damaged
steel isn’t light but is stronger
so it’s kinda up to your prefrences and ideas 
dude yeah aluminum is sooo hard to wield and just get right.
Actually it depends on the alloys being used. Some top grade aluminum alloys, such as 7075, are close in strength to low carbon steels such as 1018. However the steels will be stiffer, that is they will deflect less under a given load, than any aluminum. So you can use a stiff heavy material or a lighter not so stiff material for the same job. How much deflection you can tolerate is the descrimating factor. Many times determining the proper material for a job is pretty difficult. That’s why we have Materials Engineers!
We riveted this year, and we loved it.
Go to home depot, and for $15 you can get a rivet gun, and for about $5 dollars you can rivet an entire frame. You just have to think a little bit before you rivet. Do it with reckless abandon, and you’ll suffer. Do it well, and I think you’ll come to love it.
And for that extra bit of permanence - braze. Brazing aluminum is within the financial means and skills of almost every team as long as you follow safe work practices. For the low budget team, Burnz-o-matic makes a cheap kit that works (but the O2 gets pricey if you do too much.) I would encourage you to look up info on welding, brazing, and soldering - find out the difference.
Rivets and Braze - what a good mix IMHO. Low cost and does the job.
This year we used very few bolts on our frame, only the side plates were bolted on to allow access to our tracks. We don’t have welding capabilites, and our frame is made out of boxtube. Very hard to bolt 2 x 4 tubing without using really long bolts, and risking crushing it. Instead we used 1/4" STEEL pop rivets. They are great, almost as strong as a bolt and easier to put in, and you can do it anywhere easily. The only catch is you need a really big rivet gun. Ours was about 2 feet long and looked more like bolt cutters than a rivet gun.
Oh oh… here’s how to get around that. We used the 3/16" rivets. Almost 1/4" 
And you can use a tiny, cheap rivet gun (about the size of an average hammer.)
We did 1/4" rivets last year - that was hard. The rivet gun was actually called the “Big Daddy Rivet Gun”. And you can get (2) 3/16" into a 1" X 1" spot.
Sorry - I love rivets!
Arrow’s website, the rivets we use
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.
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?