On 11/15/2010 7:44 PM, Joe wrote:

Buy some 1/2" electrical conduit (gray) and slice it down the middle
with a table saw or whatever you have in your shop. Bend and twist it
any way you want it using a heat gun or hair dyer and mount it with
glue, hot melt, whatever, on some plywood or other substrate. The
conduit should be sized about right for a marble. If not, try 3/4"
conduit. Be careful with the heat as conduit takes very little to get
rather limp. Too much heat could distort the hemispherical cross
section, so care is needed. Conduit is made to be bent this way.
Regular PVC pipe is not, so don't bother trying it. With nice 10'
lengths, this method will give you a lot of track.

Conduit's easy, but there's extra credit if you have an open design with
rails.

They are using 1/4" rails which are made from hanger wire (not coat
hangers) inside 1/4" black irrigation tubing. This is just the right
stiffness to be able to bend but that still holds its shape, and was all
very cheap. The tubing adds friction, but that's a good thing because
they need it to run for as long as possible.

At first I suggested they use thin wire to tie the rails to the inside
of a piece of PVC at the proper spacing but then I went to the
electronics surplus store and got them some fuse clips and they bolted
them to the outside of a short piece of 3/4" PVC pipe at the proper
spacing using 4-40 nuts and bolts. The PVC goes into a tee, and the tee
can be rotated on the support pipe and the short piece can be rotated,
so it's not hard to position things and experiment. The rails snap in
nicely, but I suggested that they trim the inner part of the fuse clip
down to the level of the track so there's a minimum of a discontinuity.
I also suggested that they raise the outer track a little on curves for
banking, which they can do with a spacer under the fuse clip.

The one on the right is like what they're using:
http://www.ilsco.com/Images/ProductImages/Fuse%20Clips.jpg

They need to fabricate a lot of the spacers/support thingees even when
they are just floating spacers to keep the tracks the proper distance
apart. Welding would be great but this is their project and I'm not
welding anything for them.

The project can be 75 cm square and 100 cm high. There is extra credit
for loops, and for a design that goes for a longer time.

No funnels are allowed unless there are tracks in the funnel. You can't
slow down the marble with intentional bumps on the track. Apparently
this teacher has been assigning the same project for more than a decade,
and knows all the tricks the students use to try to increase the running
time.

It's hard for me to resist going out there to help them! They are
unrealistically optimistic about the time it will take to complete this
project. These things always take far longer than they expect.