Hi everyone,
I the mechanical lead of team 5308. We are currently working on the new version of 2015 robot, and we are not sure what kind of aluminum is good for FRC robots. For bent parts, will 5052 the best choice? I will appreciate your help.
Thanks
Material selection is an engineering trade off but a general rule of thumb we go by is the following:
Sheetmetal/Bending (Up to .125 Thick) - 5052
Plates/Gearboxes (.125+ Thick) - 6061
Shafts (Hex or Round) - 7075
Be very careful of the origin of 7075 if you are using it in a high stress application. We used 7075 for drivetrain axles a few years ago only to replace them with 4041 (steel) after the 7075 fractured at the first competition. One of Rosie’s mentors is a metallurgist and has a co worker who is an industry expert in aluminum and traced back this particular batch to China and really crappy grain structure after micro-sectioning and analysis.
Yeah, I’m still really surprised at the things FRC people use 7075 for. It’s aluminum trying to be steel - same strength as (mild) steel, but way less stiffness, way less toughness and way less hardness. (The latter can sorta be overcome with hard anodizing, as long as you don’t actually stress the coating too much.)
7075 could maybe find use in highly stressed structural plates, where you want to save weight on a big piece. But for small shafts & gears, don’t be silly - just use steel!
I agree. Just splurge a little more on steel as it will save you a ton of headaches 
Also note welding aluminum is more difficult than steel, and 7075 can’t be welded at all. Welding will reduce the strength of aluminum in the heat affected zone significantly. 6065 can be heat treated after welding to restore its strength, but you’d have to find an oven big enough to fit your whole weldment. It’s easier to just design the part with a large safety margin, but doing so will eat into the weight advantage of aluminum.
http://www.lincolnelectric.com/en-ca/support/welding-solutions/Pages/aluminum-faqs-detail.aspx
One more thought: While 6061-T6 is the arguable the best all-round alloy to use because of its strength (276 MPa at yield), 6063-T5 is far more common and cheaper at shops like metalsupermarkets.com. Be aware that 6063-T5 is only half the strength (145 MPa) of 6061-T6.
It might be worth noting that the question is, itself, coming from China.
Just another note:
6061 is used best for flat parts.
5052 is great to bend.
It also depends on the bend radius, but for a rule of thumb, bent = 5052, straight = 6061.
For alloys, just make sure you do your research, we use 6061-T6 and 5052-H32.
Here’s a video showing what happens when you make bents parts out of 6061 vs 5052.
FWIW, my team has had no problems with shafts made from Thunderhex. We love the stuff because it fits both hex bearings and hex broached holes perfectly, eliminating the “file to fit” step. It also comes with a 0.2 inch center hole that works well for clearance to #10 screws and for tapping 1/4-20 threads. And it machines very easily, leaving beautiful diameter and face cuts with ordinary HSS tools.
It is 7075. I think Paul Copioli or one of his technical staff could provide some background on how they selected a supplier, and what tests were performed. I only know we have never bent or broken it.
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We use 3003 for our bent parts and 6061 for flat parts that need the extra strength. We only use 7075 when purchased in shaft form from AM or Vex.
We get most of our aluminum provided by our amazing sponsor Waterjet Cutting of Indiana.
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Isn’t that almost the goal of 7075 for most teams? We want something kinda like steel but at 1/3 the weight? Since its engineering there is going to be a tradeoff…
It just doens’t make sense to use steel on most of the robots, teams would almost NEVER make weight.
Common examples and weight differences:
3 Foot piece of Hex: AL .79 lbs steel would be closer to 2.37 lbs!
84T @ 1/2" Hex: AL: .53 lbs steel would be closer to 1.5 lbs!
In FRC using steel over aluminum in a many applications just doesn’t make sense, especially not to our team and a whole lot of others. Also the run time is so short on the FRC robots that we really don’t see failure.
This is terrible advice and I hope people don’t blindly take it. You cannot possibly make a blanket statement like that without way more justification. The one use case that you feel 7075 is justified in is the one case I can’t possibly imagine using it in, on a FRC robot.
7075 is perfectly good enough for almost any shafting or gear use in FRC, provided something stupid like putting snap ring grooves in the loaded portion of the shaft, or otherwise adding stress risers isn’t done. These robots don’t see the kind of duty cycle required to make steel a justifiable usage in all but the most highly loaded, atypical scenarios.
Frankly, the steel gears that are available for FRC use in COTS form (ie: hex broached, 8mm bore 2mm keyway, dog gears, etc) are all inferior in quality to VEXPro aluminum gears.
254 has been using 7075 shafting possibly longer than anyone in FIRST (11+ years now) and not once have we ever sheared a shaft, that I can recall.
So yes…if you want to be absolutely sure that you can mask most possible bad design decisions with material choice, feel free to use steel…just don’t wonder why your robot doesn’t make weight.
You’re free to disagree, but I provided context and a justification.
There’s a fine justification right there. This is a student competition after all. Even the pros get it wrong sometimes (I won’t single out a supplier here), so it’s nice to have some margin.
The difference honestly isn’t worth it for small parts. You’d be looking at a difference less than 1/2 a pound between steel and aluminum for the size of parts I’m talking about, and less if you design the steel piece to take advantage of the improved properties.
If you can buy a COTS part in hard-anodized 7075-t6 and it comes with a good reputation, by all means - enjoy the tiny weight savings (and reduced wear life). But if you are designing a small, highly stressed component in house, I suggest you save yourself the hassle.
LOL you’re making the 7075 part sound inferior… I can’t believe you’re seeing enough wear in 7075 parts to justify steel and your robots must be massively under weight to ignore weight savings.
Cost matters too right?
I really don’t think you will put yourself at huge risk of going overweight by using steel just because you used steel. We use steel parts a lot and haven’t been overweight ever (or even close). Whatever is best for the application (or available at the time). Both the “no steel ever” and “7075 is inferior for FRC” generalizations are both wrong.
To the op: 6061 for almost everything. Mild steel, 7075 Aluminum, or 2024 Aluminum for shafting.
5052 or 3003 for sheet metal.
6061 and 5052 are both very weldable (5052 more so), and we’ve had welded frames made with both before.
It’s easily a 7 lb weight difference if you add up all the gears, sprockets and shafts on an average FRC robot.
We use 2024-T3 (0.05") and 6061-T6 (0.09" and I think 0.063") for sheet metal. I believe we also have some 5052-H32 sheet laying around as well.
For general purpose stuff like superstructures and the like, 6061-T6 is generally the go-to.
For hex shafting, we either use 7075-T73 or 2024-T351 (we’ll either get it from onlinemetals or mcmaster). As mentioned before, we’ve never had any sheared shafts when using aluminum.
Also, we come very close to being overweight every year (partly because of mismanagement of resources, but we also generally build tanks for robots for some reason).
Well you don’t make everything out of steel. In 2015 we needed 6 #35 sprockets. Five 12t and one 15t. A guy had 2 brand new Martin Gear 12t #35 hub sprockets in his back yard and we had one vex pro 15t 7075 hub sprocket left over. As for the last two, one of our sponsors lets us send him a mcmaster PO each season and we got the last two as part of that (also Martin Gear).
So why then would we spend $28 plus shipping and tax to get 5 12t #35 sprockets from Vex to save ~5lbs when we spent $0 on a mix of steel and aluminum. Finally the steel sprockets from Martin Gear will all last longer than the aluminum one thus adding to long term savings. In the end we were nowhere near 120lbs. Weight isn’t everything.
The number of teams that run into the weight limit must be exponentially higher than the number that run into the dollar limit. While weight isn’t everything, spending $28 to save 5 lbs. seems like a tremendously wise investment for most teams.