Will plexi-glass work?

[Al Skierkiewicz]

Plexiglass is a tough thing to use for robot body. It does shatter and easily cracks when you drill into it. Lexan works better and is less prone to damage but weighs a ton, heavier than aluminum. No you are not out of the competition at this point. Just think about a little modification. Use some aluminum extrusion to frame out the same dimensions you already have and start bolting it together. Move things bit by bit over to the new frame and voila! a robot is born. If you can't get it done before ship, cut as much as you can now and ship all the parts in the container with the robot. You will have lot's of work at your first regional but you can get it done. Ask for help from other teams, they are required to help if they can. I know that one team at Great Lakes last year, came with a box of parts and built their robot at the regional. "Never give up, never surrender!!"

[Adam Y.]

Mathew - are you sure it's plexiglas and not polycarbonate?

I'm sure if they broke any pieces drilling them it probably is polycarbonate. Those materials are a pain in the but to use.

Also, one thing i saw was people confusing the terms lexan(polycarbonate) and plexi-glass(acrylic). I just don't want this thread to turn into a "Let's argue about what its called thread".

Actually they are trade-names which is why it gets so confusing. The trade names for polycarbonate are: Lexan, Hyzod, and Tuffak. The trade names for acrylic well according to my book there is only one trade name. It is plexi-glass. Go figure. Also I have heard bad stories about using loctite on plastic materials. I heard it actually weakens the materials. Anyone care to find out if this is true?
*hint* Want to see something cool. Take two polarizing filters and stack them one on top of the other. Turn them until you can't see any light coming out. Then point them at a opaque material like lexan. You would be able to see any of the stresses put on the material*end of hint*

[KenWittlief]

thsi late in the season I recommed you use 3/8" plywood for the base and the 2x4 alum pieces that FIRST gave your for the sides.

we built a pratice robot like this in november with parts left over from last year. It only took us 3 one hour meetings, at the highschool, to put it together and get it running

Im certain you could do the same in one weekend.

[Gary Dillard]

Sorry to hear about your problems, but don't throw in the towel yet. It's easy to get overwhelmed, especially in your rookie year, with trying to do too much and seeing alot of things fail. You still have over a week left - see what you can do positively. Ken's idea about plywood and kit parts is a good start - get something driving so you can get some practice. A moving robot is an asset in any match - you can certainly block your opponent's corral and you can push balls even if one at a time. Use the default code - go back to square one and take smaller steps.

BTW - when you get to the competition I think you'll be surprised at how supportive other teams are. No-one will look down on you if your robot is dead, in fact they'll pitch in and try to help you get it started. You will enjoy this event regardless of how you robot performs.

Our rookie robot was named "Kenny" because it got killed every match. But we learned alot and moved on.

And don't use plexiglass/acrylic. Use Lexan/polycarbonate.

[Stephen Kowski]

if you go with lexan instead of plexiglass ask for polycarbonate - same stuff much cheaper.....gl

[caffel]

There is only one plastic strong and tough enough. LEXAN.
All, that is All of the other and cheaper choi ces will crack and let you down. I can tell you this after 4 years of experience.

[Ryan Albright]

any chance of using lexa or plexiglass for a base is nto good. I am guess you are a rookie team. We use lexan to protect the compenents of our robot and for skins. Lexan is strong but not made to be a base. I would not quit yet but i would suggest go get some wood like mentioned about and just build it out of wood. Especially with a game that i think is gonna involve alot of contact between robots like last year youa re gonna need somethign that holds up

[Scott England]

Didn't plexiglass(acrylic) used to be illegal on FIRST robots since it is so brittle? I know some acrylic got mixed in with our excess stock of polycarbonate, found out the hard way when it shattered while trying to be bent. Eitherway, polycarbonate frames have been done before. In 2001 we had a robot with no structural metal
Browsing the gallery, it looks like team 49 also did a robot made primarily out of lexan

So yes its possible, given your time crunch though, wood might be easier to get and to work with. Just get a drive train that runs first and go from there, there's still plenty of time left. Good luck,
~Scott

[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.]

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]