Will plexi-glass work?

[pras870]

I agree, if you absolutely must use a plastic, use lexan. It is more durable, and while it does splinter and shatter, it is less likely to do besides plexiglass. If you use this, you have to support it, using plexiglass or lexan will no support is a sure way to have a base be totally demolished. Supporting the base with aluminum extursion at stress points (if you can, ask an engineer to evaluate your design and offer advice on where to support it) is a very good idea.

Now Ken is also right. There is absolutely nothing wrong with using plywood as a base. In fact, one team that comes to mind that uses a wood base every year and is one of the well known teams in FIRST is Team 173 Rage. Every year they use a wood frame (which I'm not mistaken is supported by aluminum brackets). It may splinter, but with the brackets, it's an easy fix. Just put a sheet of aluminum or steel over it, bolt it on, and you're ready to go. I bring up Rage, because they are one of the well known teams that uses a wood base, and have so for years.

And yes, if you turn up at your regional with your robot unfinished, it's guarenteed other teams will come over to help you. No team is going to look down upon you, in fact most teams will do everything in their power to make sure your robot works and is able to compete. That's the great thing about FIRST, everyone is willing to help each other to make sure the competition is as good as it can be and everyone has a fighting chance.

[ngreen]

I don't know how physical this years matches will be but last year we shattered the side of a lexan robot in our first practice match (they ran into us). I still have a piece of it.

I would use wood and the aluminum for the kit. You can definitely get something driving and it will cut down on the repairs you will have to do at competition. You can compete if you can make something that can drive every match.

[tenfour]

You all are crazy. Lexan can't be shattered, or damaged. Maybe if you buy it down at the local rip off house home imporvement store, it will.

Horray for Lexan!?!?!?!

[maclaren]

Originally Posted by tenfour

You all are crazy. Lexan can't be shattered, or damaged. Maybe if you buy it down at the local rip off house home imporvement store, it will.

Horray for Lexan!?!?!?!

I have seen lexan shatter but it takes quite a bit to shatter it. The circumstance was in battle bots. A spinning bot got slammed in to the 1/2" thick bullet proof lexan wall and took a chunck out of it the size of a human head. It was pretty spectacular.

But that is way out of the FIRST robotics league.

[Eric Bareiss]

This is team 1038 from 2003.

http://www.chiefdelphi.com/forums/pi...gle&picid=3592

[Adam Y.]

You all are crazy. Lexan can't be shattered, or damaged. Maybe if you buy it down at the local rip off house home imporvement store, it will.

Lexan can easily be damaged. The sun is just one of the many things that can and will weaken it. Loctite is another.

[Kaelia]

Polycarb + Heat gun = Messy death. We tried to bend it like that for our control box last year. Oooops.

Other than that, it is a great material to use...however, I don't see why there is so much worry about break-age. The competition this year really isn't as violent as last year's, and I personally think you'll be just fine with Plexi unless you're planning on playing bumper cars with the other bots...

[dez250]

PLEASE DO NOT USE PLEXIGLASS/ACRILYC (sp??). The volunteers and field crew will thank you if you dont use plexi, i personally spent a good 10 mins with another staff member at the NJ Regional this past weekend cleaning up 1/2 a bot's plexi frame that exploded on contact with another bot while in auton mode. It shatters into small shards like glass will and needs to be all completly removed due to the hazard to the game balls...

[ShadowKnight]

as everyone else has mentioned, plexi has a tendency to break..well, more like shatter. If someone so much as touches the stuff...Id' hate to be you guys if you had a match in less than a few hours

[Phasmatis568]

Originally Posted by Eric Bareiss

This is team 1038 from 2003.

http://www.chiefdelphi.com/forums/pi...gle&picid=3592

Our entire base was constructed of lexan. With this we managed to hold up against anyone who tried to push us off the ramp.

This year our robot used lexan plating on an aluminum frame. We decided to play king of the hill on the chin-up platform. The lexan on our robot is merely scratched. You dont have to worry about it breaking in this competition.

[Grommit]

I don't want to repeat anything that's already been said... so I won't mention plexi-glass and the fact that it is completely horrible for use as a structural component on a robot.

But I will say something interesting: We have a cart we've been working on, which is motorized, to carry the robot around on. We gave our 12" wheels to team #1043 before we realized that we might have wanted them for the cart. (A good move, they made very good use of those wheels). However, to get to the point, we actually tried replacing those wheels with 12" roughly cut wheels of lexan instead. Ugly, but they could withstand the pressure, and weight isn't an issue for a cart. We will end up putting the pneumatic tires on, but the cart would have functioned, we expect, with the lexan.

[Al Skierkiewicz]

For many years, we used laser cut lexan wheels on the robot. They gave us limited slip on the carpet and allowed some side turning for tank drive. We never had one break but they did wear down.

[Adam Y.]

I agree, if you absolutely must use a plastic, use lexan.

I disagree there are few other plastics that are better suited for a frame than lexan. Polyethylene is probably one of the better plastics to use for frame materials. It is easy to machine and very durable. The names to look for are HDPE (same stuff used in the fields) and UHMW. My best guess is that the gearbox and mounts are some form of polyethylene.

[petek]

I'm sure that if you've gotten this far in this thread, you know that plexiglas (acrylic plastic) is not a good structural material. What might be missed is why you would use one material over another, and how you'd choose the "best" one for your application.

[A little disclaimer here: I am not a mechanical engineer or materials expert - but I do have a tattered degree from the school of hard knocks.]

First - the term "plastic" tells you something right off about these materials: they can deform, move, flow and/or creep by nature. This may be a good thing if you want low friction, impact resistance or high toughness. It also means that in general, their strength may not be the greatest. (There are exceptions to every rule, but we're talking generalities here.)

So, going back to a couple of plastic materials mentioned in this thread, what's so good or bad about acrylic and polycarbonate? Acrylic is great for clarity - you can polish it so it looks like crystal glass. It has pretty good machineability with the right tooling, too, but those tools require a different profile than what you use for metal or wood. It is pretty hard and stiff, but does break easily (notice I compared it with glass earlier). Personally, I really like working with the stuff, but I can't think of anywhere I'd use it on a robot!

Polycarbonate is what they make bullet-proof windows and jet canopies out of. You can bounce a chicken off it at very high speeds. Or another robot at somewhat lower speed. Great flexibility, too, in thin sections. Not so nice to machine, as it tends to grab the cutter and needs coolant to keep from melting, but sheets can be sheared and bent just like sheetmetal. You shouldn't expect holes tapped in it to hold a lot of force, either in tension or shear - use through bolts, nuts and washers. So this is good stuff for the bumpers, feelers, side covers and things that go bump on a robot.

Polyethylene (HDPE, UHMW, LDPE) is great as a sacrificial wearing surface, chain guides for example, and for chemical resistance. It's okay for lightly loaded bearings, but it's a pretty lousy structural material.

One way to look at materials for structures (like robot frames) is their strength to weight ratio. If we compare the ultimate tensile strenght divided by the density we get a picture of why people use steel for fasteners and really strong structures, aluminum for weight-sensitive structures, and plastics for the rest. (The numbers are the ratio of the UTS, in MPa, to Density, in g/cm3)

ASTM A574 Steel: 160
6061 Aluminum: 122
Polycarbonate: 58
Acrylic: 51
Delrin: 48
HDPE: 29
Teflon: 14

[End of lecture]

[MathewSmith]

well thanks for all your suggestions! i beat it into my team that we needed some kind of metal. we used alumminum and we are still using the plexi glass for walls and the base and the top. I suggested using the stuff that you guys told us to use...but we diddn't. and we are getting our electrical stuff mounted and all that. so i think we can have this done by thursday! i hope we can get it done! Thanks for your help!

Mathew Smith
Team 1352