Second layer
 

Our second layer mounting on top of the first layer is going to be made of 1/2" thick plexiglass. The mentors are saying we should just go ahead with it. Is this going to be a problem?

Re: Second layer
 

Could you post images or better describe your situation? I'm not exactly sure what you're asking.

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For general robot applications, ensure the "plexiglass" is polycarbonate and not acrylic.::safety::

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The reason for this STRONG recommendation is that acrylic plexiglass very easily shatters. Many teams have used acrylic plexiglass, but they almost always use it just once, then they learn.

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I'm sorry: Did you say 1/2"? as in 0.5 inches thick? :eek:
Goodness that's overkill, way too thick for anything I can think of. I think the thickest POLYCARBONATE (not acrylic! Never!) we use is just under 3/16, and that's a major structural element of the robot.

Print this thread out and show it to your mentors. We're sure it is overkill.

Oh, and not plexiglass. Not ever. Even 1/2" thick.

It's confirmed that we have acrylic plexiglass. We used a dremel on it and had some minor hallucinations from the smell. It's a half inch thick. I don't think we have a chance on getting a substitute for it by bag and tag day. We'll probably be practicing with for a bit and get a copy of it with polycarbonate by our regional.

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I don't know exactly what you're using it for, but is there any possibility you could use plywood instead?

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Probably a good idea. Two things:

Lexan is very strong, and it bends instead of breaking. 1/8" lexan is about as strong as 1/16" aluminum. So you could not possibly need Lexan thicker than 1/4".

Also, take careful notes on that piece of plastic, so you can pre-drill and cut things, making it an easy retrofit at competition.

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I can think of something. Though even then it was only 1/4". http://www.chiefdelphi.com/media/photos/21476


Oh, and we were thinking of using plexiglass (or rather, "unidentified clear plastic") on the robot this year. A quick "smack! test" from a mentor left several pieces on the floor, and we found something else right away.

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Amazon.com is an good source of small sheets of polycarbonate. Much of it ships Prime.

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You can buy Lexan/polycarbonate on McMaster-Carr and ship it next-day air, too....

But at this stage in the game, I'd much rather use thin plywood than thick acrylic plexiglass. Plexiglass is a "no, no, no, no, never, never, NEVER" item in our workshop. Acrylic plexiglass shatters easily when subjected to FRC-level forces. .5" wouldn't give me the slightest bit of reassurance over .25".

Lexan good. Plexiglass bad.

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Heh. 1075 built our electronics on a plexiglass panel in our rookie year. That was the last time plexiglass ever made an appearance on a 1075 bot.

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Home Depot usually stocks a good selection of both acrylic and polycarbonate (lexan) in their windows section if you need something fast. Yes acrylic will shatter but with a piece 1/2" thick it's going to take an aweful lot of force. If you have the weight allowance I would say go for it if you have it in stock. If not, home depot for lexan/polycarb.

Re: Second layer
 

It is not clear to me what application you are using this for. It sounds like an outside layer of something that you would want to see through, so the suggestions here of lexan are good. We use 1/8" for side panels all the time. It will bend a lot in impact but rarely breaks, and when it does break it doesn't shatter.

If you don't need to see through it, corrugated plastic is a nice alternative as well. We often use 1/2" corrugated and it is light, sturdy stuff. We had a slide made of it last year that stuck out about 13 inches past its attachment on the frame members. You could whack that extension with a hammer and not do any significant damage.

So the acrylic broke today when one our members tried dremmeling hacksawing and hammering it to get it the right shape. It's all for the good now because we bought half inch plywood which is much lighter. We'll get us some lexan or polycarbonate and trace around the plywood after the deadline. Our second layer on top of the electronics needs to be clear so that the inspectors can see our circuit board. Thanks for the help!

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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.

Re: Second layer
 

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.