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.