Which kind of Aluminum is a good choice
 

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

Re: Which kind of Aluminum is a good choice
 

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

Re: Which kind of Aluminum is a good choice
 

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.

Re: Which kind of Aluminum is a good choice
 

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!

Re: Which kind of Aluminum is a good choice
 

I agree. Just splurge a little more on steel as it will save you a ton of headaches :)

Re: Which kind of Aluminum is a good choice
 

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...qs-detail.aspx

Re: Which kind of Aluminum is a good choice
 

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.

Re: Which kind of Aluminum is a good choice
 

It might be worth noting that the question is, itself, coming from China.

Re: Which kind of Aluminum is a good choice
 

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.

https://www.youtube.com/watch?v=KgeP__CSiSw

Re: Which kind of Aluminum is a good choice
 

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.

Re: Which kind of Aluminum is a good choice
 

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.

Re: Which kind of Aluminum is a good choice
 

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.

Re: Which kind of Aluminum is a good choice
 

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.

Re: Which kind of Aluminum is a good choice
 

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.

Re: Which kind of Aluminum is a good choice
 

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.

Re: Which kind of Aluminum is a good choice
 

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.

Re: Which kind of Aluminum is a good choice
 

It's easily a 7 lb weight difference if you add up all the gears, sprockets and shafts on an average FRC robot.

Re: Which kind of Aluminum is a good choice
 

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).

Re: Which kind of Aluminum is a good choice
 

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.

Re: Which kind of Aluminum is a good choice
 

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.

Re: Which kind of Aluminum is a good choice
 

I'm aware and that said our drive train was all Vex Pro hardware thus anodized/Teflon coated aluminum was abundant. I get it and I certainly have little against aluminum wherever it could make sense. But I will not discount the use steel for weight alone.

We do a lot of things on our robots the way I mentioned in my other post and have saved thousands over the past two years on actual money spent on parts. Our 2014 bot was worth (by our BOM) 2.6 times what we spent on it.

Edit: With regard to money I don't mean to say that we are trying to stay away from breaking the BOM limit. The point is to save more of the cash we get for parts each year.

Re: Which kind of Aluminum is a good choice
 

+1. The counterarguments might have sold me 36 hours ago, but not now.

As part of our pneumatic-actuated H-drive project, the students (for good reason having to do with floor clearance and bolt head thickness) wanted to go to a 6" dualie strafe wheel on the TB-mini. The tank wheels are 4", driven 8.45:1 in an AM 14U2 chassis, with two 4" wheels stripped of tread used as spacers. To compensate for the greater diameter, we decided to do a lower gear ratio. I knew that we had a 10.71 TB2 in stock from our 2014 (Aerial Assist) robot, so we located it and swapped out the gears (we wanted to use the long hex shaft). I told the students to move the shaft plate labeled 8.45:1 to whichever gearbox got those gears; they accepted this suggestion instantly, and took it as their own. (minor victory!)

Through 2014, we used keyed round shafts rather than hex, so the AA gearbox had a keyed round shaft. However, when they put the 8.45 gears together with the keyed round shaft, they couldn't put the pieces together. In particular, the shaft would not go cleanly through the bearing; I suspect they used a bit of force getting it out in the first place. They recognized that the shaft was slightly bent. We located a new matching shaft, and they put the 8.45:1 gearbox with a round shaft together. I pocketed the shaft for possible use in our "first four years" shadow box.

Yesterday evening, I verified that the mangled shaft is steel; a magnet sticks to it. I can't see using aluminum of any grade in this capacity. The shaft in question was not used in the drive system; it was used to swing a "hammer" that "kicked" the yoga ball. Despite always supporting the other side of the hammer, somewhere along the way, the shaft deformed, most particularly near the key slot. Curiously, checking the hammer, there is less deformation of the aluminum hub than of the steel shaft.

Re: Which kind of Aluminum is a good choice
 

This isn't a reasonable argument against using aluminum shafts at all. There are tons of different ways that a motor driven "hammer" could load a shaft improperly such that any material would bend. For example, shock loading a long, 1/2" keyed shaft could potentially bend any material. You're putting a ton of force on that tiny area of the keyway. I also suspect the shaft was loaded improperly in some other way, but without details on your setup I can't really comment. But this isn't necessarily a part of your robot with less stress on it than the drivetrain.

This just further reinforces Cory's point - you shouldn't use material to compensate for a bad design decision, because the difference in strength is somewhat marginal anyway and you can still screw it up. 7000 series aluminium shafting isn't really that difficult to use, and instead of steering away from a common sense way to save at least 5 pounds per robot (once you add up all the gears, shafts, etc), you should steer away from bad design decisions that load shafts poorly. I guess if you're still convinced, I'll enjoy having 5 more pounds of mechanism than you do, or 5 pounds less mass to accelerate.

Re: Which kind of Aluminum is a good choice
 

Metal, Metal, Metal. First robots could stand to have a little fiber in thier diet.

Re: Which kind of Aluminum is a good choice
 

Our experiment with wood shafts this season unfortunately didn't work out.

Re: Which kind of Aluminum is a good choice
 

use ironwood but don't try to heat treat it.

Re: Which kind of Aluminum is a good choice
 

Have you tried 3D printing a shaft? I hear plastics are all the rage these days!

Re: Which kind of Aluminum is a good choice
 

Our 2014 robot disagrees...



In 2014 we built a 12 wheel gear drive that utilized a total 22 7075 gears from VexPro and 10 lengths of 7075 hex shaft for the idlers. These were all driven by 3-CIM Ball Shifters geared for 3.25fps overall drive speed in low gear. After over 60 matches of dragging other robots across the field during the season, and various off-season demos since then, all the gears and shafts have virtually no visible wear on them and run like they're brand new.

Had these parts been made of steel (including the gears in the gearboxes), our robot would have been about 12lbs overweight, just in the drive system.

Compare to our 2007 robot which the 2014 design was based off of, which did use all steel gears and was so heavy virtually every other part of the robot had to be made out of carbon fiber-foam composite.




[/2cents]

Re: Which kind of Aluminum is a good choice
 

One thing to note is a lot of aluminum extrusions sold as "architectural shapes" including many you'll find at home centers such as Home Depot are not 6061, but rather 6063. Aside from being about half the strength, 6063 does not cut, drill, or machine nearly as nicely as 6061. It does polish to a nice mirror shine though.

Re: Which kind of Aluminum is a good choice
 

You definitely want to keep an eye out for this. 6063 is gummy and awful, and should be avoided if possible.

Re: Which kind of Aluminum is a good choice
 

Back to the original question: if you're going to fold or weld the metal, get 5052, especially over 6061, 6063, and 7075. All of the high strength aluminum alloys are dependent on appropriate heat treating in the final configuration, and lose a lot of that strength when welded or plasticly (permanently) deformed. If you have the appropriate ovens and quenching baths that can hold your robot chassis, go ahead and re-treat :sigh:*.

As to our 2014 kicker shaft - yes, I concur that it was a brutal abuse of any conceivable material. The point is that, unless you really have worked out the details with a wide margin, a few extra ounces of steel in those key spots is way better than having to replace a mangled aluminum shaft between matches.

Along these lines, on the "importance of doing it right the first time": The down-geared TB-mini I mentioned a few posts above was assembled on Thursday with the axle plate backwards. That is, the plate was bent towards the wheel rather than towards the motors. It took four students (three rookies and one veteran who was not one of our lead/pit crew people last year) about two hours to make the swap-out, even after direct advice as to what had to be removed. Even though we always have our best guys and gals in the pits, I'd much rather over-engineer a key component than have our robot miss a match (or two or six) because our drive system or a key manipulator is out of commission.
This is pretty much where we were in 2013 (Aerial Assist), late in the regional. Our Frisbee trigger/lifter broke (metal fatigue), and two hours of work in the pits (including myself and our lead mechanical mentor, as well as several students) did not resolve the issue. It was more than a year later before we had our 2013 chassis frame tossing a Frisbee again.

* This emoji ought to exist, so I'll use it anyway!

Re: Which kind of Aluminum is a good choice
 

I agree that it is important to "do it right the first time". To me, this means thoughtful design, including designing for maintenance and engineering for the correct amount of strength for the application.

For example, a "few extra ounces of steel" won't help poor design. In the case of the shaft example, it is important to load the ends of the shaft instead of the center. If the shaft is correctly loaded, as many people have already said in this thread, a 1/2", 7075 shaft will usually be strong enough. If even more strength is needed, geometry is far more important than material. I'd rather have 1" diameter, 1/8" wall, 6061 tube for my axle in "key spots" than a steel or 7075 aluminum 1/2" shaft.

Re: Which kind of Aluminum is a good choice
 

I keep hearing on CD that 6063 is awful for FRC and just bad in every way. So why is it so common, and why does it exist at all? Is there someplace where 6063 aluminum is a good choice?

Re: Which kind of Aluminum is a good choice
 

Because frankly it isn't nearly as bad as people make it out to be. I know at least here in MN, most everyone doing custom aluminum is using 6063 (because getting anything else is quite difficult). Yes, it isn't optimal, but then, few things are, especially in FRC.

Re: Which kind of Aluminum is a good choice
 

It's significantly easier to find and cheaper, but weaker and harder to machine. Depending on the profile, we use both 6063 and 6061 aluminum tubing. Most of our rectangular box tube and flat plate is all 6061 while our c-channel and angle is a mix of 6063 and 6061.

Re: Which kind of Aluminum is a good choice
 

If you can get 6061, there's no reason not to. But for the past several years on 2791 we built robot frames out of 1/8" wall 6063 just because of availability and they've been fine. If you're pushing it, go to 6061 (i.e. thinner wall thickness, high stress areas, etc).

Re: Which kind of Aluminum is a good choice
 

I dunno, every time I've had to machine 6063 I've ended up with ugly holes and galled aluminum all over our bits. Lowering the RPM on the drill press and/or using cutting oil helps, but not much.

Re: Which kind of Aluminum is a good choice
 

I've worked a bit with it and can testify that it's definitely worse than 6061 in terms of machining, but I've made it work before. Pockets are terrible, but I haven't seen it very bad on drilling.

Re: Which kind of Aluminum is a good choice
 

Every material can be bad if it's used in the wrong application - materials selection can be an incredibly complex process because you'll always need to make some sort of trade-off depending on your objectives and constraints (cost, availability, appearance, machinability, strength, recyclability, the list goes on and on). The shape and thickness have an effect on the suitability of the material for the application as well. There is no universal material that is good for every application.

As others have said, depending on the heat treatment, 6061 is stronger and harder and may seem like the better material for almost all applications. But, if you don't need or want that kind of strength, 6061 may be overkill and 6063 may be the better option because of the lower cost.

The reason why 6063 is so readily available at hardware stores is beacause it meets the needs of most people who shop there - people looking to do home improvement projects. 6063 is widely used in home applications where appearance is key and strength may not be necessary (windowframes, some furniture applications, etc). 6061, while still useful for some home applications, is more widely used in industry for things like machine parts and structural components, and most people looking for those parts will likely be going through another vendor that is not Home Depot.

Again - this is all very, very generally speaking and the exact choices you would need to make regarding the material are entirely dependent on the application.

Another good, very similar comparison of materials often used in FRC is polycarb (Lexan) vs acrylic (Plexiglas). Acrylic is cheap, shiny, and can crack easily upon impact. Yet, it is still used as glass for hockey rinks, because when it's thick enough, it can more easily handle the impact of a puck (though not always...and then things like this happen). Lexan is stronger and more impact-resistant, but is more expensive and scratches easily. Which would you want to use for a panel on a robot? Which would you use for a panel that's part of a pit display or a trophy?

Re: Which kind of Aluminum is a good choice
 

The fundamental pieces of the most successful subsystem on our 2015 robot was manually-milled from 6063 2x1x0.125 tubing. The problem is that it took an expert mentor & student combo an entire Saturday for just the two ~68" rails. The bit likes to heat up, which means we slow down and add extra cutting fluid via squeeze bottle & paint brush (we use Tap Magic, cinnamon flavor).

6063 isn't all doom-and-gloom. We've been able to make any precision piece we want with 6063. It's just slow. And 1/2 the price in some cases. Since it can gum up a bit when going too fast, I have reservations about CNC'ing 6063 - but it's probably doable. It also welds just fine - our frames have been 6063 since 2013 and have come back 99% fine each year.

Re: Which kind of Aluminum is a good choice
 

Tap Magic is more for aiding chip removal and keeping a tap moving. It's not quite as good at cooling per say.

If you want to cool something you would use a water soluble coolant in a spray bottle. The photo (taken 2 min ago 10 ft from me) shows some in a machine. When we cut in a hass we literally "flood" coolant which aids cutter life, chip removal, dimensional stability etc. On the Bridgeport we generally fill a spray bottle and spray constantly while cutting. Helps a lot with aluminum.