ACM vs Polycarb

[cynthiachinworth]

Hey Robotics Folks! I’m considering ordering our intake parts in ACM; does anyone have any experience with this? Is it FIRST legal?

[Lexeo]

Is it legal? Sure.

Is it a good idea? That depends. If you are trying to do an over-the-bumper intake, you will likely hit another robot or a field element.

Polycarbonate is nice because it tends to be complient and bounce back into shape. ACM won’t have that advantage.

If your intake is mainly in frame … It doesn’t matter a whole lot.

The only thing to remember with ACM is to make sure you have no exposed sharp edges. So make sure you clean up your work.

[RogerR]

It would likely be legal (most disallowed materials are disallowed because they can be dangerous to use or work with).

It may not be a great material choice, though. Polycarbonate is usually picked because it’s compliance allows it to take impacts without permanently being damaged; if designed properly, it should bounce back into place after a big hit. ACM (at least the stuff I’ve handled) is very rigid, but isn’t very tough, so it will likely be permanently damaged by a hit.

[Britt]

We have been playing around with ACM for a bit as a structural component and I can talk about what we’ve run into so far. its place is basically a drop in for polycarb where you might need to save some money, and/or weight not using aluminum because you don’t need its strength. ACM is not quite as strong as poly, but it is more rigid and half the price… but birch plywood is half the price of ACM…

for us it really is just the best electrical board material ever, easy to mount new hardware on and stiff enough that connections aren’t getting pulled on. also makes the chassis really stiff. outside of that the only reasons we might use it comes down to saving $, it comes pre-painted and looks clean, and its fun to learn and experiment with materials.

ACM:

• ~$130 for 6mm 4x8 sheet (we got it free from an old sign shops scrap pile)
• incredible rigidity to weight ratio (stiffer than 1/8 AL at the same weight ish.) aka great electrical board material
• cannot be clamped down tight with bolts because it isn’t strong in compression, but large washers and lower clutch on a drill and it doesnt really matter, just have to be careful.
• fast to drill into and machine, adding electrical holes or match drilling into a part is no issue. making a prototype is really quick.
• can be bent easily to add a lot of rigidity
• can be bent easily… it will never be a part of something that is ever outside of frame perimeter because it will just bend, ACM does not flex back at all. you will have to manually pull it back into position, which is thankfully easy.
• any repetitive bending load over time will cause it to slowly bend out of shape if not supported enough, but its really easy to add a support later and bend it back and it will be like nothing ever happened (can send a picture of this on our catapult from bunny bots once in the lab if anyone’s interested)
• its surface is like a whiteboard so you can draw stuff out and erase it easily (its fun, and useful for electrical)

Polycarb:

• ~$260-300 for 1/4" 4’x8’ sheet
• is elastic, will flex right back into position, great for impacts
• also easy to machine
• “can” be bent into a more rigid shape, its just not easy.

Baltic Birch plywood:

• ~$50-60 for 1/4" 4’x8’ sheet
• will crack and break, not very rigid, is not elastic
• also easy to machine

[Gdeaver]

Ah, but take a piece of Baltic Birch 1/4" plywood. (not home center stuff) and laminate carbon fiber cloth on both side. You now have a great structural material.