Chassis Materials

[JesseK]

What's the most common grade of aluminum among FIRST teams -- 6063?

[MrForbes]

Which alloy you use depends on what you're doing with it. 5052 is good for making sheet parts that require some forming (such as bending), while 6061 is commonly used for extrusions and parts that will be machined, not formed.

[EricH]

Aluminum tube with a plywood base in between (for electronics and motors). We've never had a problem in my memory.

[AndyB]

269 has used 4130 Chromoly Steel now for the past 4 years and hasn't looked back.

It has three times the strength with about the same weight as aluminum.

We have primarily used 1/2" for areas with little side load or where weight is a concern. 3/4" is used on our chassis although I think we've used 1" as well in the past.

http://aircraftspruce.com/catalog/me...4130square.php

We are surprised that we are still the only team, to our knowledge, to be using this stuff.

The only downside to this stuff is it goes through drill bits and drill batteries like crazy. I think we replaced our bansaw blade this year 1 or 2 times this year as well. We just bought a corded dewault so drilling should be much easier. We always use .035" thickness. We have never had a break.

If you buy this stuff, they give you welding rod as well. Although I don't know if we've ever used it.

[AdamHeard]

Originally Posted by AndyB

It has three times the strength with about the same weight as aluminum.

It's about 3 times heavier.... but, it is offered in thinner wall thickness than aluminum usually. But still, it's a good deal heavier.

294 used the kit frame in 05, a welded 2x1 extrusion frame similar to 4/22/60/254/968 in '06 and a plate sandwich concept (similar to 233 in 2006) for '07 and our current prototype.

[lukevanoort]

Originally Posted by JesseK

What's the most common grade of aluminum among FIRST teams -- 6063?

I'd wager that 5052-H34, 6061-T6, 6063-T52, and 6105-T5 are the most common alloys: 5052-H34 because it's in the kit; 6061-T6 because it's an excellent balance of price, availability, and strength; 6063-T52 because it's cheap and available (not too strong though...); and 6105-T5 because that is what 80/20 makes their extrusions out of. Personally, I like 6061-T6 for the reasons I've listed, but I wouldn't say no to some nice 7068 or 7075!

Originally Posted by AdamHeard

It's about 3 times heavier.... but, it is offered in thinner wall thickness than aluminum usually. But still, it's a good deal heavier.

I dunno, I think it depends heavily on your heat treatment. From some numbers I'm looking at on MatWeb, it seems that some (hardened) 4130 steels are about 2.9 times the weight of 6061-T6 aluminium, while being 3.5 times stronger. So, its strength to weight ratio seems a bit better. However, annealed 4130 and 4130 that hasn't been hardened that much are a different story; the less hardened versions are only about twice the strength.

[MrForbes]

Steel is 2.8 times as dense as aluminum, it doesn't matter what alloy. But strength DOES depend on alloy and treating, and you can find steels that are several times stronger than some types of aluminum...and you can also find aluminum alloys that are several time stronger than mild steel! so you have to be pretty careful in you comparisons, to make sure you are comparing apples to apples, and oranges to oranges.

1501 has used chrome-moly steel tubing.

If you are interested in keeping your robot from falling over, go heavy on the chassis, and save the exotic materials and super lightweight design for all the "up top stuff", such as arms, etc.

[cooker52]

But don't go to heavy, otherwise you will come up at the last minute drilling holes in everything.

As for us, we did use some chromoly this year. For the most part, we use 6061 aluminum. We use the sheets of aluminum and bend and curve it into our robot's form. This technique of making a uniform body is called monocoque and it is very light weight and durable.

Here's a picture of last years (2006) robot.

[AdamHeard]

Originally Posted by squirrel

If you are interested in keeping your robot from falling over, go heavy on the chassis, and save the exotic materials and super lightweight design for all the "up top stuff", such as arms, etc.

Good advice for rookies, but I still cringe every time I hear you say that.

I prefer to lighten everything as much as possible AND keep the weight low. That way your not wasting any weight just to get a low CG. Some teams are amazing at that (469 and 1114 come to mind this year)...

If you lighten your frame by 5 lbs, and that frames CG is 3 inches above the floor, and you then mount 5 lbs of something anywhere below that 3 inches, your CG is lower.

But in the end, there's nothing wrong with your idea.

[M. Mellott]

Team 48 has used welded aluminum box tubing for our base frame for years, 1/8" wall for the outer box and 1/16" wall for inner bracing. Aluminum takes some practice to weld, both to weld it with proper penetration without burning through it, and to weld it without warping the frame. We've had ours welded up at a local machine shop before, but last year one of our team member's father who works at a steel mill offered his services.

We feel the base frame, the foundation for your robot, is very important to make as solid and strong as possible, even if it means spending an extra couple pounds. That way, it's one "sub-system" that we don't have to worry about down the road, even after many matches. The last thing you want is to have to tear apart your robot to repair a bent/broken frame in the middle of a competition--it could make for a long weekend.

[joeweber]

The 2004 and 2005 season we used HDPE. Pictures @ http://team1322.org/pictures.htm It can be heavey but it has alot of give. This was befor bumpers, other robots would just bounce off with out damage. We found a local place that would weld the plastic together.Our robot was completly plastic except the arm. The plastic was very easy and safe to work with, students could cut it and not get shavings in there skin or cut themselves on the edges. You could use a jig saw, router, hand saw, and drills with ease. We still use HDPE (High Density Poly Etholean?) as mounts for motors or structure supports between aluminum frames. This seasons robot we used two blocks 2.5 x 5 x 6 in. and cut it into a fun looking "Y" to get around some items so we could mount the motors in a particular spot. The stuff is strong and easy to work with. Also you can use it as a low speed axel support or make pillow block out of it. Look close at this picture @ http://team1322.org/DSC00703.JPG you can see the motor mounted to it.

[redbarron]

Team 1270 used 6061-T6 4"x4" box tubing (3/16" wall) last year and we are using it again this year. We integrated the AM Transmission with the whole drive train into each side tube. The CG of the bot with the tube 113 inches in the air was in the center of the robot and about 7 inches above the floor, we didn't add any extra weight or heavy materials to the base just kept as much of the weight as low as possible.

[falconmaster]

http://www.creativepultrusions.com/

Pultruded Fibergalss

1/5 the wt of steel/ 1/3 of alluminum for same strength!
Electromagnetically transparent
Non conductive
non corrosive

the best stuff!

[JesseK]

Originally Posted by falconmaster

http://www.creativepultrusions.com/

Pultruded Fibergalss

1/5 the wt of steel/ 1/3 of alluminum for same strength!
the best stuff!

What's the cost compared to 6061 Aluminum? I'm thinking along the lines of 1" square tubing that's 1/16th thick for aluminum (since we know it's strong enough) vs. 1" square tubing that's 1/8" thick for the fiberglass.

Also, what do you use to reliably "weld" the components together? Also, being that you're from a NURC team, how well does this stuff hold up under water?

[falconmaster]

If you sand it epoxy works great. We use 1/4" bolts with 3/8" lock nuts, and it works great. We use all the same size nuts and bolts. You need stainless for the underwater bots!

In most cases the cost is more, but not that much more, especially if you count in the labor time and repair time you save.
1 1/2" x 1 1/2" x 1/4" is $2.74 a foot

1" x 1" x 1/8" $1.20 per foot

6" c-channel $7.34 per foot

The water has no effect on the material at all!