Re: pic: Wood Coast Drive
 

Polycarb is denser and less rigid

Re: pic: Wood Coast Drive
 

I think your mixing up polycarbonate (lexan) and acrylic (plexiglass). Polycarb is extremely impact resistant but cannot be cut on a commercial laser cutter. Acrylic is very brittle and can very easily be cut on a laser cutter.

Re: pic: Wood Coast Drive
 

In what way? I happen to be comparing WOOD and polycarb, thank you very much. The comment about the laser cutter was in a statement about WOOD.

Re: pic: Wood Coast Drive
 

I believe you are comparing wood and acrylic.

Polycarb is much more impact resistant than baltic birch.


It's very difficult to crack both polycarbonate and baltic birch while drilling. Out of the two baltic birch is easier to crack.

Again you can not laser cut polycarb with a commercially available machine.

Re: pic: Wood Coast Drive
 

My understanding was that acrylic is easy to drill, polycarb needs a special drill bit or it'll crack, and birch merely splinters if you make a mistake--not crack.

I never, EVER, said you COULD cut polycarb on a laser. If you can show me where EXACTLY I said that, by QUOTING, then I'll retract this statement. Otherwise, leave any discussion of cutting polycarb by laser out.

Re: pic: Wood Coast Drive
 

As I said earlier you are mixing up polycarbonate and acrylic. Acrylic is difficult to drill, polycarbonate is very easy to drill.

Re: pic: Wood Coast Drive
 

Ah. (Makes me wonder why acrylic is found with FRC teams--and their "unskilled" drillers--around, but that's beside the point.)


Now, for the impact resistance: polycarb vs birch. Polycarb deflects, birch absorbs. They're about the same, but I'm going to have to give that to the plywood on strength-to-weight ratio. The main issue with birch ply is that it absorbs by getting dented or, eventually, splintering. (Acrylic never even enters this discussion, as the "standard" test for identifying unknown clear plastic is to clamp firmly and hit with the biggest hammer in the area--if it doesn't break, use the polycarb, otherwise it was acrylic.)

Plywood loses to poly on strength to weight. Lexan is crazy strong. I stood on a freely Standing peice of 1/16 lexan and it just bent I stepped off bent right back into shape no problem. I can't say the same for a piece of 1/16 plywood.

Re: pic: Wood Coast Drive
 

FWIW: That's strength to volume, not weight.

Yea but in order to get a similar strength you would need much thicker wood which may be heavier then the thinner poly carb

Re: pic: Wood Coast Drive
 

Which is lighter? Think about that one for just one moment.

You're confusing strength and strength-to-weight.

Here's an example: Steel is stronger* than aluminum. But aluminum tends to be the material of choice** in applications that need low weight, despite being weaker. Why is that?

Because aluminum, in general, is stronger for the same weight. Some alloys of steel are very light. Most are not. Use aluminum and you get a lighter weight, even if you have to use more material to do it, for the same strength.


So: if you got an equal weight of plywood and polycarb in the same general shape, which would break first?


*I do need to point out that this is a generalization--there are, in fact different kinds of strength, and because of that, any declaration that X is stronger than Y is dependent on application. In this particular case, it's true mostly across the board.

**Excluding exotic materials or odd applications, of which there are plenty.

Re: pic: Wood Coast Drive
 

In 4 competitions, the only thing we broke on our wood robot was the polycarbonate switch covers. ;)

Of course they took the brunt of the stress and weren't boxes.

Re: pic: Wood Coast Drive
 

The point was, your 1/16" test doesn't resolve that question.

Re: pic: Wood Coast Drive
 

You are confusing material properties.

I could build a robot out of rubber and it would never break. It also would be completely unsuitable because it has no rigidity.

Re: pic: Wood Coast Drive
 

For reference, baltic birch plywood is ~0.65 g/cm^3, while polycarbonate is around 1.2g/cm^3 and aluminum is ~2.7.

There are many more factors that determine what material we use than simply density and strength. As Cal mentioned, we use plywood because it is incredibly cheap and easy for our team to work with. If using polycarbonate makes sense for your team and application, go for it.

The cutter also leaves a fairly smooth finish on the sides (structurally, though, the lightening patterns aren't really justified).