pic: Team 3647 West Coast Drive

[Andrew Schreiber]

Originally Posted by AdamHeard

We like 1/16th. Runs like $60 per robot and you get a LOT left over for other parts. Mcmaster btw.

How do you feel about plywood, yeah it is ugly but it is cheap. How effective do you feel it would be? I know that 1114's Kitbot on Steroids uses 3/4" plywood for their electronics mount.

[Ninja_Bait]

Originally Posted by Andrew Schreiber

How do you feel about plywood, yeah it is ugly but it is cheap. How effective do you feel it would be? I know that 1114's Kitbot on Steroids uses 3/4" plywood for their electronics mount.

If you just put some paint on your plywood than it looks almost beautiful. It's a good material for stiffness and weight, especially at 1/2" and 1/4", and most teams that build field elements have some leftovers for things like electronics. You do have to worry about warping, though.

[JamesCH95]

Originally Posted by Chris is me

Also important to this design - rivets are better in shear than bolts.

Care to elaborate on that? For example, I'd find it very hard to believe that a 1/4" pop rivet (steel or aluminum) is better in shear than the shank on a graded 1/4-20 bolt.

A properly designed and setup bolted joint can sustain more shear loading than the bolts themselves alone. The friction between the two parts, as generated by the clamping load of the threaded fastener, carries a substantial amount of load in addition to the bolts. Can the same be said for rivets?

Originally Posted by Andrew Schreiber

How do you feel about plywood, yeah it is ugly but it is cheap. How effective do you feel it would be? I know that 1114's Kitbot on Steroids uses 3/4" plywood for their electronics mount.

Team 95 likes to use 1/2" or 3/8" high quality birch plywood. It is plenty strong enough and is lighter and more visually appealing that 3/4" construction grade plywood.

[Ninja_Bait]

Originally Posted by JamesCH95

Care to elaborate on that? For example, I'd find it very hard to believe that a 1/4" pop rivet (steel or aluminum) is better in shear than the shank on a graded 1/4-20 bolt.

A properly designed and setup bolted joint can sustain more shear loading than the bolts themselves alone. The friction between the two parts, as generated by the clamping load of the threaded fastener, carries a substantial amount of load in addition to the bolts. Can the same be said for rivets?

I looked it up: Pop Rivets vs. Bolts

These are two kind of arbitrary sources, but it shows that while rivets are not quite as strong, they're comparable. The steel 1/4" rivet (shear: 2750 lbs.) beats Grade 2 1/4" bolts (shear:~2200 lbs.). However, the higher grades outmatch the rivets by a lot.

[Ether]

Originally Posted by Andrew Schreiber

How do you feel about plywood, yeah it is ugly but it is cheap. How effective do you feel it would be?

I don't have a link handy, but I recall seeing a recommendation to use thin plywood with fiberglass cloth bonded to it with resin.

[JesseK]

Originally Posted by Ether

I don't have a link handy, but I recall seeing a recommendation to use thin plywood with fiberglass cloth bonded to it with resin.

I remember that too, somewhere. It might have been Baltic Birch though, which is a bit lighter than plywood and commonly comes in multiples of 1/4" thickness.

[Ether]

Originally Posted by JesseK

It might have been Baltic Birch though

Yes, I think you're right.

[Brandon Holley]

The belly pan is the perfect place to use composites. Very rigid, very light, usually very thin and also non-conductive. I highly recommend a fiberglass or carbon fiber belly pan.

Making a composite panel yourself is also something just about any team with a vacuum pump can do.

-Brando

[Akash Rastogi]

Originally Posted by Brandon Holley

The belly pan is the perfect place to use composites. Very rigid, very light, usually very thin and also non-conductive. I highly recommend a fiberglass or carbon fiber belly pan.

Making a composite panel yourself is also something just about any team with a vacuum pump can do.

-Brando

Just make sure that if you use fiberglass for your bellypan that you're not 816 and have it end up being 8 pounds, un-lightened.

[thefro526]

Originally Posted by Akash Rastogi

Just make sure that if you use fiberglass for your bellypan that you're not 816 and have it end up being 8 pounds, un-lightened.

Was 8 pounds lightened, bro. 3/8" Thick Fiberglass Sheet Was Overkill for a bellypan, but you can't beat free.

(Until you're trying to figure out where you have 5lbs of weight that shouldn't be there.)

[Chris is me]

Originally Posted by thefro526

Was 8 pounds lightened, bro. 3/8" Thick Fiberglass Sheet Was Overkill for a bellypan, but you can't beat free.

You can be extremely aggressive with lightening in fiberglass and still retain your structure, because it's just so rigid. Holes with as little as 1/8" - 1/4" of material left around them are possible.

[Mk.32]

Someone asked for a photo of the bearing block, here it is.
Two 500 Hex Bearings press fit into 1/4 alum plates.
Bolt holes are pocketed so it slides about .25inch to allow for tensioning.

[JamesCH95]

Originally Posted by Ninja_Bait

I looked it up: Pop Rivets vs. Bolts

These are two kind of arbitrary sources, but it shows that while rivets are not quite as strong, they're comparable. The steel 1/4" rivet (shear: 2750 lbs.) beats Grade 2 1/4" bolts (shear:~2200 lbs.). However, the higher grades outmatch the rivets by a lot.

I assume they did a straight shear-strength calculation, which is good, but doesn't describe the whole situation.

Friction matters a lot, especially when clamping aluminum to aluminum. The static coefficient of friction is generally over 1, sometimes well over. You'd be looking at (to use my own numbers) 75%*1750lbf (proof load GR2 1/4-20) * 1.2 (coefficient of friction Al-Al) = 1575lb, so it would require 1575 lb of force to overcome the friction generated by the bolt's clamping force before the bolt sees a single pound of shear force, assuming the bolt was properly torqued.

Edit: I looked at the bearing strength of aluminum, assuming 6061 T6 it is 56ksi. Doing the math on an 1/4" diameter hole in an 1/8" thick plate says that anything over 1750lbf will start to yield the material around the fastener, so the excess strength of the rivet over a grade 2 bolt is meaningless because that will not be the mode of failure. The bolt's clamping force now makes it a clear winner.

[craigboez]

Originally Posted by Mk.32

Someone asked for a photo of the bearing block, here it is.
Two 500 Hex Bearings press fit into 1/4 alum plates.
Bolt holes are pocketed so it slides about .25inch to allow for tensioning.

[/img]

Neat design. How do you keep the two plates aligned with each other and the shaft perpendicular to the frame?

[Mk.32]

With 2 bolts and a shaft going though the blocks, they will be aligned when the bolts are tightened. I have seen this type of design before and it seems to work pretty well.