I'm writing for the first time (but certainly not the last) from Olin College. As part of the desire to give students a taste of the true engineering experience (which seems to include near-impossible deadlines), we have been given $300 and 2-and-a-half weeks to design and build a miniature (24" wheel base) solar car. We've put together a basic design for a chassis and are planning to order materials to build it sometime tomorrow.
At the moment, the chassis consists primary of a large plate that we plan to make from a honeycomb panel (something like item 9080K43 from McMaster; 3/8" thick, aluminum panel with polypropylene core). The questions that I pose the the great minds of the newsgroup are the following:
1) Can this kind of panel be cut using a waterjet cutter? (I've read some stuff online that cast a shadow of doubt in that direction.)
2) Does anyone know of any place other than McMaster where we might get it? Preferably someplace that will provide it cheaper (or as a "sample") but can still deliver in a hurry. (Yes, I know I'm dreaming...)
3) Are we completely insane for planning to use this stuff for a chassis in the first place?
Thanks,
Eric Gallimore Son of Axolotl
P.S. - Yes, this is my first solo post, but we don't have a Usenet server here yet, so I had to have my father send it for me.
I'll try number 3 (regarding number two, Hexcell and others make a variety of cores, mostly for aerospace, but I have no idea where you'd get it).
It doesn't sound like your application is particularly tough, but joining aluminum honeycomb material is always tricky. First, take a look at what the FAA says about repairing aluminum honeycomb. It will give you an idea of what will work at joints, because you have to remove some core material to make a joint:
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If you're going to get good strength without adding weight, you'll probably want to use epoxied joints with lapping of the aluminum skins. Look up PAA anodizing. If you can arrange for that at school, that's the way to go for getting a good epoxy bond on aluminum. If you can't do that, then use a scratch-bonding method. If no one there knows how to do that, come on back and we'll explain.
The first car made out of aluminum honeycomb was the original Ford GT40, and, while it simplified designing the tub (the great panel stiffness allowed a simpler structure), the joints drove the price of the thing through the stratosphere. Producing curves is one reason it's been dropped in favor of fabric/resin composites for more recent designs; the difficulty of bonding joints is the other. It seems unlikely that you'll wind up with a weight advantage in a 24 inch long solar car using aluminum honeycomb, without some very fancy engineering. You probably could make a chassis more easily by bonding together sheets of high-density polyurethane foam and covering it with Kevlar. If you need more stiffness, use carbon fiber. If weight isn't that critical, use S-glass cloth (you'll need an extra layer of it over what you'd need with Kevlar).
The best bet for a resin is vinylester. It will give you a good bond in any of those fabrics without using vacuum bagging. Epoxy would be Ok without bagging but it drools like crazy and it's challenging to keep in it place on a shape that includes both horizontal and vertical surfaces. Vinylester is not as strong as epoxy, but it's easier to work and it's stronger than polyester. And, unlike the latter, it won't require mat between layers of cloth.
Anyway, this sounds complicated but there is nothing easier to glue and shape and form than high-density foam. Laying on the cloth can be trickier, but minimize sharp angles and it will be simple. The total job probably will be a LOT quicker, and quite a bit easier, than assembling a unitized structure out of honeycomb-core aluminum.
Or you could use some of both. If your design includes, say, a flat floor pan, you could make that from the aluminum honeycomb and then bond on foam-core composite for curved shapes.
Cutting the stuff will be the least of your problems. Picking up the suspension loads on a flat plate chassis which resists ordinary bonding and fastening techniques is going to be the real challenge.
Frankly, I'd be tempted to low tech this with balsa wood and silk span. The materials are cheap, easily available, light weight, surprisingly strong, and the techniques for fabricating structures with them are well established (model airplanes). Structural foam is a more modern variant, and extremely easy to cut and shape using a hot wire cutter.
But another good alternative would be to fabricate a space frame chassis from small diameter tubing, or fold up a backbone chassis out of aluminum sheet. Either of these construction methods make it easy to pick up suspension loads, and they use well established fabrication techniques.
I like the backbone design because it is quick to do, and very stiff for its weight. You could weld the structure, but aluminum pop rivets are perfectly adequate for your application. So special tools and skills are kept to a minimum.
My initial thought on where to get honeycomb was also Hexcell. I still have my Hexcell skis. ;~) Get ahold of the corporate public relations people, they'll likely donate a bit for educational use. If you still plan on using it. The joints should not be a big deal. The car is only 24" long, how much loading is it gonna get? Not likely to smoke the tires.
When you say miniature, is it going to be something somebody sits on or in, or is it just going to be a remote control model? Either way, I'd be inclined to make up a wooden torsion box chassis and save the budget for the running gear. You can get thin plywood door skins for the skins, light spruce for the sides and reinforcing on the torsion box and it's all cheap, light and readily available, e.g., Home Depot. If you wanted to splurge, you could use thin Baltic birch plywood for the box skins, harder to find on short notice. Use thin metal plates for powerplant and running gear mounting point reinforcing. Wood was good enough for the early airplanes, after all.
If you hadn't noticed, the budget and time constraints are there to teach you that high-end high-tech isn't always the way to go. Unobtanium that's perfect for the task is no good if you can't get it in time or at a price you can afford. Sometimes you don't work for a govenment contractor that when the first billion is burned through, you can go get another. Sometimes when the budget is gone, so are you...
Yeah, if it doesn't have to be sexy, I'd go with the wood, too.
I have an old composites engineering book here, maybe 15 years old, that lists the panel stiffness/weight ratio of different composites. In the medium-high range is high-density foam-core S-glass/epoxy composite. Two notches up is foam-core Kevlar/epoxy composite. In between the two is birch plywood.
IF you're going to use the fluted core, stop by a signshop and ask for a piece of alumacor, or similar. we have it here in 1/4, 3/8 and 1/2 inch thicknesses. Like most things mcmaster sells, their fluted core material is expen$ive, for what they want for a 2x4 sheet, I can buy a 4x8.
Honeycomb can be waterjet cut (abrasivejet). However, the cuttting process will be slower than normal because the jet will want to skip to the side as it cuts the thin walls. The fastest way to cut it would be to put the nozzle at a slight angle so that it hits the sides of the walls instead of the top. Either way, it shouldn't be a problem if you are willing to wait a bit longer for the part to complete.
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