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.