Greetings and salutations,
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
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?
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:
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
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.
'Sounds like fun.
I'm going to go with 3, Eric.
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
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
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.
Hugh Prescott wrote:
> Birch plywood aircraft grade available at your local hobby shop in sheets 6
> x 12 to 12 x 48 in thickness from 1/64 to 1/4 all is 5 ply no voids. >
>>Greetings and salutations,
>>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?
>>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.
On Tue, 11 Nov 2003 19:25:42 -0000, "PR"
brought forth from the murky depths:
You're referring to naughty pine ply, eh?
* Scattered Showers My Ass! * Insightful Advertising Copy
* --Noah *
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