pic: Wood Coast Drive

[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.

[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).

[Bertman]

Don't limit yourself to plywood. Several years ago we made this:

http://www.chiefdelphi.com/media/photos/32054

The frame is red oak, uses standard joinery, weighs in at 7 pounds and has been abused by our team for all that time. It has run into walls, jumped curbs, and carried 200 pound students.

Wood is affordable, works with no special tools, is strong, resilient and smells good.

Spend some time examining it and look up some simple boat building techniques.

[EricH]

Originally Posted by Bertman

Don't limit yourself to plywood. Several years ago we made this:

http://www.chiefdelphi.com/media/photos/32054

The frame is red oak, uses standard joinery, weighs in at 7 pounds and has been abused by our team for all that time. It has run into walls, jumped curbs, and carried 200 pound students.

You should REALLY bring that to an offseason with the pictured attachment. Human player challenge: Beat that robot at throwing noodles.

[philso]

Originally Posted by Bertman

Don't limit yourself to plywood. Several years ago we made this:

http://www.chiefdelphi.com/media/photos/32054

Too bad your team did not attend the Lone Star Regional this year. An Anime convention was scheduled at the same time. Many of the Otaku were wielding giant cardboard and foam swords and shields, showing of their "fighting skills" in the park across the street where we were having lunch. You could have brought your robot for some target practice

[M. Mellott]

I have a few questions...

- What kind of spacing do you use for the tab/pocket joints?
- What are the cross-shaped cut-outs under each hole (anti-rotation slot for a nut)?
- What type and size fasteners are used?

[z_beeblebrox]

Originally Posted by M. Mellott

I have a few questions...

- What kind of spacing do you use for the tab/pocket joints?
- What are the cross-shaped cut-outs under each hole (anti-rotation slot for a nut)?
- What type and size fasteners are used?

-No standard spacing. We try to keep one at each end and a couple in the middle of long parts to keep them from bending. Usually about 6-12".
-Yes. See this.
-8-32x5/8" or 1 1/8" Torx screws, 8-32 square nuts from McMaster.

[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

[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.