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Hooray! Better now than in a collision on the field!

Okay new problem. My mentors (and me) are fairly new to FRC. This being our second year and only one mentor has technical experience with cars. They seem fine with 3/8"lexan which we already have. I feel that's way overkill given its weight and how much prefabricated materials we can bring. The thickest at home depot is 0.093in. Is that suitable for taking FRC level hits?

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Could you post a photo of your application? There is much more to knowing if a material will work than it's thickness and a benchmark of "FRC level hits".

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If weight is not an issue (i.e., you are underweight) then go with what you have.
Otherwise 0.093" Polycarbonate is quite strong and, more importantly, tough - it takes a beating without breaking. FRC "hits" are not all that bad since everyone has bumpers. This isn't battlebots. You'll see.

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We use .125 Lexan because it's the thinnest we can get at cost from a sponsor, but would use .093 if we could.

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Sorry I have no pictures. Its because our robot is bagged and tagged :/
It would basically sit on top of the frame with bolts securing it. Its so the ball doesn't interfere with the electronics. Nothing is secured on it.

I have another question :D
Would a sheet of lexan unmodified except with marks in sharpie made for future drilling and cutting count as COTS?

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What is... the definition of COTS?

Note that the above goes on to further clarify and provide examples. So, not COTS--it's been modified.

However... what you're REALLY asking is "can I carry this sheet of Lexan, marked up with Sharpie, into the pits and NOT have it count towards Withholding Allowance?" And the answer lies in the definition of Fabricated Item, which is, in short, "any component or mechanism (and a sheet of Lexan would be a component) that has been altered in any way, by any method, into any semblance of a Robot part's final form". (Heavily paraphrased, of course.) Guess what, by a strict reading... it probably is a Fabricated Item. But... that's a little trickier to justify, particularly given some past rulings on whether, for example, cutting a piece of metal for transport only would make something Fabricated or simply a different size of raw material. (Those rulings also don't apply to this year, BTW.)

As an inspector, would I pass it and should I pass it are not necessarily the same thing. I might be inclined to pass it, but whether I should pass it is not clear--it's clearly not COTS as it is, but I might have a hard time justifying it being a Fabricated Item (as the markings are rather easily removed).



So, here's what I would do to make absolutely sure you're legal: Make a drawing. Copy the sharpie lines onto paper (you can scale them--in fact, you'll probably want to scale it down), and get dimensions between them and write those down--just make sure you can re-copy all the lines and locations at the proper scale later. Erase the sharpie lines on the Lexan. As soon as you walk into the event, use the drawing to re-copy all the lines and cut/drill at your convenience. (By the way, it's good practice to have a drawing any time you build something like this.)

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3/8th polycarb is nearing bullet resistant territory (for sufficiently small bullets). It's wild overkill for anything in FRC.

.093 is, in broad terms, strong enough for FRC. You could, for example, whack a supported piece with a hammer without much permanent deformation. It's seriously tough stuff. Thickness of polycarb sheet in FRC is usually driven by stiffness requirements more then outright strength, and .093 is going to feel pretty 'floppy'. That may or may not matter for any given application.

Acrylic is actually a pretty cool material. It's super transparent (second probably only to glass among common materials) and very hard, so it makes good optical elements. It's fairly easy to form, super easy to glue and cheap (about the same as polycarb). I can't think of any reason to ever use it on a FRC robot, but it still has a lot of utility.

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Slight problem. It shatters into countless sharp, hazardous pieces when hit with sufficient force. Most inspectors will not allow acrylic on the field as protection or other roles on the robot with a potential for impact.

Polycarb just gets a web like appearance of cracks when hit, but it stays together.

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You may consider PET-G, it's nearly as strong and nearly as resilient as polycarbonate, but a fraction of the cost.

Pretty sure Andy knows about it's brittle nature :rolleyes: he's just explaining what it is useful for. Some people get scared to use acrylic ever because of FRC experiences, when in fact it is very good in the proper application. For example: it's nearly UV-impervious, and lexan is not. They each have their place.

Also, I have seen polycarbonate shatter on FRC robots before. It's not nearly as common or catastrophic as acrylic, but it does happen.

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You might reread my whole post, which I end with a statement to the effect of 'don't use acrylic on a robot'.

I'm familiar with acrylic. I use it a bunch at work at a medical school. It's widely used in medical research for it's good optical clarity, chemical resistance and ease of assembly without fasteners. The ability to solvent weld pieces of acrylic together is awesome and, with some practice, easy to do (polycarb can be solvent welded but with much poorer results). It's also easy to cast, doesn't break down under UV sterilizing bulbs, and ages/resists scratches well (there's a reason why safety glasses are so easy to scratch- polycarb is soft!). Sure, it shatters but when weight isn't an issue you can use thick cross sections that are pretty resilient to even the most klutzy post-doc.

Naturally, I'm aware that it's totally inappropriate for FRC purposes and I can't think of any contingency were it would be better than polycarb or another plastic/wood. But FRC is pretty niche and, in the big wide world of engineering, every material has it's place. I don't, for example, often use polycarb for making apparatus because it doesn't like some of the cleaning agents used in labs and breaks down under UV sterilization. There are no awful materials, just awful applications of materials.

That and James can attest I'm a natural born contrarian. It's been a while since we had a good metric vs. imperial thread for me to espouse the virtues of the furlong in, so I'm feeling a bit punchy.

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Why not just stick with plywood? We have built entire robots out of plywood before and it will easily hold up for the season. Polycarb looks cool, but other than that I really don't see an advantage. If possible stick with high quality baltic birch plywood.

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Plywood would be kind of hard to see through.