pic: Wood Coast Drive

[z_beeblebrox]

[cdm-description=photo]42200[/cdm-description]

[rutzman]

I heard at World's that you were limited in the size of the pieces that you could cut by the size of your laser cutter. Can you handle larger pieces now, or is this more of a theoretical design?

[z_beeblebrox]

I heard at World's that you were limited in the size of the pieces that you could cut by the size of your laser cutter. Can you handle larger pieces now, or is this more of a theoretical design?

All the pieces fit within our laser.

[AdamHeard]

I would remove all the diamond pocketing in the mock "rails". It cuts the fiber up so much, and it'd be hard to justify the weight it saves for the decrease in strength for a more traditional FRC game with contact.

[Necroterra]

Yeah, taking a second look at it, the belly pan definitely wouldn't survive a contact game. I think you could design the gearbox crossbrace to handle that though.

[z_beeblebrox]

I would remove all the diamond pocketing in the mock "rails". It cuts the fiber up so much, and it'd be hard to justify the weight it saves for the decrease in strength for a more traditional FRC game with contact.

That makes sense; will do. Edit: Removing the holes only cost ~.5lbs; not that big a deal, given how light the chassis is anyway.

Yeah, taking a second look at it, the belly pan definitely wouldn't survive a contact game. I think you could design the gearbox crossbrace to handle that though.

The intent is to epoxy 1/8" plywood plates to the center of the belly pan, making it one piece. I'm planning on adding a brace between the gearboxes.

[asid61]

Very cool. 31lbs with a 6-cim drive is very impressive, too.

[dtengineering]

When you take a thin layer of fiberglass and epoxy it over wood the strength and stiffness goes off the chart... you'd have to figure out a way to fillet the corners a bit to allow the glass to flow smoothly over the edges.

Actually, once you got good at glassing you could replace the plywood with balsa and go for...

Oh, wait. You said this weighs 6.5 lbs? Yeah, I don't think the weight savings will be worth it. Wood is pretty amazing on it's own.

Jason

[carpedav000]

I'm new to the idea of wood construction, would it be worth it to replace the front and side panels with polycarbonate?

[Scott Kozutsky]

I'm new to the idea of wood construction, would it be worth it to replace the front and side panels with polycarbonate?

The main advantages of wood (IMO) are that it's dirt cheap and can be laser cut very quickly and easily. Polycarbonate is neither.

[carpedav000]

The main advantages of wood (IMO) are that it's dirt cheap and can be laser cut very quickly and easily. Polycarbonate is neither.

While both are true, I saw that someone was concerned about cutting the fiber up too much. I don't necessarily understand what that is, so I was guessing that is was something to do with structural integrity. Polycarbonate is (IMO) much more suited to take impact than plywood is, hence me asking the question in the first place.

[mman1506]

While both are true, I saw that someone was concerned about cutting the fiber up too much. I don't necessarily understand what that is, so I was guessing that is was something to do with structural integrity. Polycarbonate is (IMO) much more suited to take impact than plywood is, hence me asking the question in the first place.

Polycarbonate cannot be cut on commercial laser cutter. It releases dangerous fumes when melted.

[z_beeblebrox]

Polycarbonate is (IMO) much more suited to take impact than plywood is, hence me asking the question in the first place.

Plywood's not bad at impacts; it's springy and returns to shape when metal dents. Until our safety captain forbade it, I demonstrated that by hitting our demo swerve module with a hammer. We also ran over our prototype drivetrain with a car, with only minor damage.

We also use wood since it's so fast to make; we get turnaround times less than half an hour on our laser, while we'd have to cut Lexan by hand.

[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.

[EricH]

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.

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.

[MichaelBick]

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]

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]

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]

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.


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. (https://www.youtube.com/watch?v=Hsls5ZPCUnE)



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]

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]

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]

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]

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]

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

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

[Sperkowsky]

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]

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]

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]

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]

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.:p

[philso]

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]

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 (http://www.instructables.com/id/How-to-Build-your-Everything-Really-Really-Fast/?ALLSTEPS#step4).
-8-32x5/8" or 1 1/8" Torx screws, 8-32 square nuts from McMaster.