pic: Wood Coast Drive

[MichaelBick]

Originally Posted by carpedav000

Then why NOT use it for the front/side panels? I've gotten a lot of the benefits of using plywood, but no answers as to why polycarbonate would or would not be better.

Polycarb is denser and less rigid

[mman1506]

Originally Posted by EricH

In this case, plywood takes impacts at least as well as polycarb, and is faster by far to cut and install.

Here are some more possible reasons: Wood is lighter than polycarb for the same size of piece (though it's quite possible that you might need a thicker piece). Wood is actually stiffer than polycarb--take a sheet of birch ply and a sheet of polycarb, hold at one end, shake. Wood is immune to loctite spidering... and it's a lot easier to drill without cracking if you forgot to CAD the holes for the laser.

Note: The above specifically applies to birch plywood, ideally Baltic birch plywood.


That being said, there ARE teams that build chassis out of polycarb: 1714 has been very hard to see for many years because their primary building material is polycarb (or is it acrylic? think it's polycarb). But they have to be very creative in terms of material attachment and stiffening.

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.

[EricH]

Originally Posted by mman1506

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.

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.

[mman1506]

Originally Posted by EricH

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.

I believe you are comparing wood and acrylic.

Originally Posted by EricH

In this case, plywood takes impacts at least as well as polycarb, and is faster by far to cut and install.

Polycarb is much more impact resistant than baltic birch.

Originally Posted by EricH

it's a lot easier to drill without cracking if you forgot to CAD the holes for the laser.

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.

[EricH]

Originally Posted by mman1506

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

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.

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

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.

[mman1506]

Originally Posted by EricH

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.

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

[EricH]

Originally Posted by mman1506

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

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.)

[Sperkowsky]

Originally Posted by EricH

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.

[Ether]

Originally Posted by Sperkowsky

Plywood loses to poly on strength to weight. ... 1/16 lexan ... 1/16 plywood.

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

[Sperkowsky]

Originally Posted by Ether

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

[EricH]

Originally Posted by Sperkowsky

Plywood loses to poly on strength to weight. [...] 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.

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.

[Necroterra]

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.

[Ether]

Originally Posted by Sperkowsky

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

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

[MichaelBick]

Originally Posted by Sperkowsky

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.

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.

[TheOtherGuy]

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).