On Tue, 11 Mar 2008 11:01:30 -0700, charlie wrote:

"Tim Wescott" wrote in message
...
More design questions:

I need a stick-like thing, about 30 inches long, to go from a pivot
point to a little electric motor. It needs to be moderately rigid,
light and inexpensive in lots of 50 each or so. It would also be quite
nice if it's moderately attractive -- mill finish aluminum without too
many scuff marks is just fine for this. Prettier is better.

I'm thinking that thin-wall aluminum* tube of about 5/8" diameter will
work well, and I'm getting ready to be shocked once again by how much
things cost these days. While I'm waiting for quotes, I thought I'd
solicit the group for suggestions.

Fiberglass? Carbon fiber? Phenolic? Other plastics? Titanium**?
Extra thin-wall steel? Balsa wood comes to mind, but while it is low
tech I suspect it'll cost more than aluminum. Cooked spaghetti is too
limp, and uncooked is too brittle.

Anyone have suggestions? Do you have supplier names to go with your
suggestions?

* Alloy isn't terribly important. Even dead soft would work, although
harder would be better.

** OK, maybe it's more expensive than aluminum.

pvc pipe? venetian blind replacement rods? wooden dowels? you don't give
a lot of specs on size or what kinds of forces it has to withstand,
whether it has to be weatherproof, etc.

Of course I'm being vague! It wouldn't be a proper posting if I left
everything in!

It's for a table-top thing. It's only expected to work in a shirt-sleeve
environment. The arm's job is to hold a lightweight motor (2oz or so) a
good length away from a counterweight. The motor runs a fan, and the
goal of the assembly is to servo the arm position to a given spot (it's
for control systems training).

A 30-inch piece of 5/8 OD 3003 tubing with a 0.04" wall should weigh
about 4 ounces, if I got my numbers right. To keep the counterweight to
a reasonable size and weight I'd like to investigate things of comparable
weights.

A 5/8 inch tube is a good size to mount the motor -- I can fish-mouth the
end of the tube, drill for a peg, and hold the motor on with elastic. If
I get paranoid about safety I can mount a little ducted fan onto the end
of the tube, screwing into a plug or a large, threaded peg. Down to 1/2
inch is probably doable, up to 1 inch is fine as long as the weight
doesn't grow by more than a couple of ounces.

PVC water pipe is probably much too heavy. Thin-wall PVC tube may work,
but it may have the same problems as uncooked spaghetti. Venitian blind
replacement rods may work, although I'd like tubes for the motor wiring
to travel inside of, and I'd be a bit worried about brittleness. Ditto
dowels, plus there's a problem of looks.

Thanks for the suggestions -- I will consider them.

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html