On Tue, 11 Mar 2008 20:47:26 -0600, Paul wrote:
Tim Wescott wrote:
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.
....
How about economy grade hunting arrow shafting? I know its smaller than
5/8 but unless your motor weights a couple pounds or something it should
be fine. It's fairly thick walled so drill and tap each end to
facilitate attaching the counterweight and motor.

The description in an earlier thread (see beginning of any hit at
http://groups.google.com/groups/sear...d8e61b8f8f6fe4 )
refers to "a propeller on a stick. You mount the propeller on a
motor on the stick (light aluminum tube), you counterweight and
pivot the stick so the thrust of the propeller will move it easily,
...." [for use in control systems seminar workshops] so I suppose
TW probably will use a few ounces of DC PM motor with fairly low
current draw.

I think 6063 .75x.75x.06" tubing, at under $1/foot, is probably a
dollar less per beam than arrow shafting, plus being easier to attach
to the axle. In TW's plan, the beam is fixed to the axle, which
rotates in bearings and turns the sensor pot that provides feedback
to the PID controller. One way to save on bearings would be to glue
the pot to the side of a .5" (.840" OD) PVC pipe in line with a
drilled-through pivot hole, and use a fixed axle. Schedule 40 .5"
PVC at .17#/ft is slightly lighter than the aluminum tubing, and
..75" PVC at 0.226#/ft is slightly heavier. Both have about 0.11"
wall thickness in Schedule 40 so are fairly stiff.

-jiw