Build a way-cool marble racer

There's no better way to bump into a little adventure than the road trip. A few times each year, my mother and stepfather would get in their car for a week of touring Pennsylvania's Amish country. They always had a starting point and a few favourite destinations, but freedom of the road means getting off-track and stopping on a whim, as they once did at an Amish market in the village of Bird-in-Hand. As an auctioneer, unusual objects are my stepfather's stock-in-trade, and one such object caught his eye at this market.

It was one of those ingenious mechanical toys that's simple but mesmerizing. Load a handful of ball bearings in the top trough, tip the lever, and watch as one ball after another zips down the ramps and spins the pinwheels. My stepfather picked up one for his own amusement and since then I've built several replicas of this clever toy contraption.

My plans are fine-tuned for oak so I recommend using this wood. Other woods can work, but different densities mean you'll have to adjust angles and sizes to get a smooth reliable action. And though this is a toy, the mechanics demand precision—measure, cut and drill carefully and keep your assembly square. Use 5/8" steel bearings—they have enough weight to keep the moving parts moving; glass marbles are too light. I got my bearings at Atlantic Bearing (905-876-1105) for 65 cents each for a minimum of 20. Remember that small parts, like bearings, make this toy unsafe for very young children.

Begin by milling the troughs according to the sequence shown on the plans. Cut the uprights, box parts and cover plates. Assemble the mitred box, then cut 1/4" grooves in the box and the base. Glue uprights to the box and base, making sure everything stands square and parallel.

Next, attach the side rails, which are ready-made cove moulding. While they make the machine easier to load, especially for children, they are optional.

Fasten the upper trough to the base and box uprights using #4 brass screws. Be sure to predrill and countersink generous holes since these screws are small and break easily. Don't use glue. Add the middle and lower troughs followed by the middle uprights.

The project has three sleeve-covered axles: one for each pinwheel and one for the lever. Drill 5/32" dia. holes for the axles, preferably using a drill press so they're accurate. This is especially important for the lever, the most mechanically sensitive piece.

Install the pinwheels and lever on their axles and sleeves. If the action isn't smooth enough, you can make fine adjustments by filing the sleeves a bit, adding a small washer as a spacer, etc. Cut the brass cover plates for the trough ends and fasten them with #2 brass screws in predrilled screw holes. A fine hacksaw blade works well for rough-cutting the brass parts before final shaping with a bench sander.

All the brass parts should be easy to find—I bought them at my local hardware store. The shape of the metering rods is crucial, so use the bending jig shown on the plans. Install the little brass guides that hold the rods, and plug the rods into the lever. Test the action and adjust if necessary.

Then take everything apart for finishing—I used urethane. Remove any dried finish from the holes and reassemble. Use screws to hold the structure and epoxy to hold the axles and sleeves in place. Now there's nothing left to do but load up the machine with bearings and get rolling.