Carbon fiber sheet

[MillenniumFalsehood]

I'm looking into buying some carbon fiber sheet after Wonderfest (take a wild guess why ) in order to start work on a walking robotic skeleton. I'm aware of the ungodly expense of carbon fiber, but this skeleton has to be very strong and *very* light. I've done some calculations based on an aluminum frame, and at the weight it'll be at (at or below 50 lbs for the skeleton and mechanical systems in one leg) the jointed areas would be too weak to handle the stress of movement for more than a handful of months due to aluminum lacking a lower fatigue cycle limit and the thinness of the required panels. Lots of testing and trial and error will be necessary for this prototype, so this is a big problem. Other materials have similar problems. This is a rather large machine, so the weight will be pretty high.

So what I want to know is, where is a good source for this stuff? This site looks promising:

http://dragonplate.com/ecart/categories.asp?cID=65

But if there are better alternatives I'm game. I'm really interested in the "quasi-isotropic ply" layering technique they use. From what I gather outside their site, this layering technique produces strong sheets which can handle stress well at a variety of angles in the plane as well as bending, so I'd say this is the way to go.

[kenlilly106]

Have you considered Kevlar composites or even fiberglass? No sense in going the Ferrari route cost-wise when a Buick will work.

Another thing to consider is that if you make the legs and body out of a very rigid material this could negatively impact the joints as well, ex. any bending motions that would normally be taken up by the legs will now get transmitted to the leg joints.

Ken

[MillenniumFalsehood]

I hadn't considered either of those, actually. Doesn't fiberglass have pretty much the same problem as aluminum, having little to no lower fatique cycle limit? By contrast carbon fiber has a fairly high one. I'm not familiar with the characteristics of Kevlar. If it's similar to carbon fiber in behavior, I'll consider it instead. I'd prefer to get it in re-formed sheets, though.

That's true. However the skeleton needs to be very rigid, since it needs to support the weight of the robot, the design of which will have most of the weight slightly above the CoG, near the hips. It's four-legged, so that'll help. The joints will slide past each other and have lubricated ball bearings between the joint halves, and they won't have to bend very far, so torquing won't be a big problem.

[kenlilly106]

There are different styles of fiberglass, the spray on chopped stuff like you see on bathtubs, or the sheet layup seen on aircraft, you can even get honeycomb fiberglass panels that are light and stiff.

One of the new advances in fiberglass is what they call 'pultruded' shapes, the glass fibers run lengthwise through the the shape, you end up with something that looks like a structural steel shape but is made out of fiberglass, these shapes are very strong and lightweight, I've used them before in harsh environment applications where wood and metals will not hold up due to the chemicals involved but the resin used in the pultruded shape (and there are several resin choices) makes the part effectively inert to the environment. Another benefit was due to the light weight of the parts we only needed a small crew to set everything in place, no crane was required.

You can get preformed Kevlar sheets and composites over plywood, to the best of my knowledge they don't make Kevlar structural shapes.

Ken

[MillenniumFalsehood]

How's the weight difference between Kevlar/plywood composite, fiberglass, and carbon fiber? Weight and strength is one of the top concerns with this build, cost not so much (the cost can be amortized over time; I'm in no hurry to build this thing and it will probably take months, if not years, just to get the skeleton constructed. S'matter of fact, I drew that infamous triangle of cost-time-quality when I started this project a few years ago and chose cost and quality, because for me, as a hobbyist, time is no object). If the differences are negligible, then I'll probably change to Kevlar or honeycombed fiberglass sheet.

I really want sheet material so I can cut it to fit my blueprint rather than have someone do it for me or deal with the fumes of fiberglass resin and the complexities of casting it.

[kenlilly106]

Without knowing your design and target weights I can't make a recommendation on what material to use.

There are a lot of variables that affect the weight of each material; number of layers, resins used, etc.

In terms of strength to weight carbon fiber is probably going to be your best bet, but that performance comes at a premium price.

My advice would be to make up a spreadsheet listing the various material options, their respective weights, strength specifications, and price and look at the big picture that way. Would it be worth paying 2-3X the price for a small gain in performance? That's something you'll have to decide.

Ken
(engineer)

[MillenniumFalsehood]

Thanks, Ken. I'm in school trying to get a mechanical engineering degree. I'll probably be better equipped to design this thing after I get my degree, but I figured it would be better to start now when I don't have quite as many responsibilities.

Thanks for the help. I need all of it I can get.