The red vertical rod
behind the bug slides
the marble off of the
hook and onto the
Marbles, Tracks and Construction More details
The tracks are all made of 1/8” diameter steel welding rod, brazed together. Semi-circular
crosspieces keep the track spacing and give places for supports to attach. When one piece
of rod is used, then next one is butted to it and brazed end-to-end. I made a bunch of jigs
that hold the cross pieces in position for brazing to the track. They clamp on with 8-32
Allen cap screws. Tighten the screws with a 9/64” Allen ball driver, with a screwdriver-type
handle. This tool will reach into and around a lot of tight places, and tighten the screws
even when it is 25° out of line with the screw. RBS construction creates lots of tight places.
I have changed the circuit diagram to accommodate a change in the power supply for the stamps. I had a 5 volt supply to the
Vss pins of the stamps (pin 21 or 37). I also had a problem with stamps resetting. This would cause a spectacular "crash" of
the helicopter. Sometimes it all would work for quite awhile, and then reset. I tried all kinds of things, but when I supplied a
higher voltage to the Vin pin (pin 24 or 40) and then relied on the stamps' individual internal regulators to give them the right
voltage, then my problem went away. The stamps are very susceptible to reset due to brownouts. I had the stamps on a
separate supply and thought that this would work with 5V, but once in a while this was no good. Now they have their own
supply, a 9V wall-wart that drops down to 7.2V with a resistor.
Marble switches: the
one on the right
waits for two
marbles, which go
through the marble
separator to the left.
Track can be bent by hand, pliers and vise. A wonderful bending jig can be purchased at Micro-Mark. Flower pots make
good round forms in varying diameters for bending the rod. A great idea I learned a little too late for this RBS: fill a big
traffic cone with cement. When it hardens you have a heavy, stable place to bend many different diameter curves. All
methods are useful, as each situation may require a different bending technique.
The tracks need to be banked so that a marble going around a curve does not fall (or fly) off. This is directly affected by the
weight of the marble and the speed it is going when it hits the curve. The weight should be controlled by marble selection.
The consistency of weight is more critical if the marbles bounce, leap, or get shot from a cannon. Buy 100 marbles, weigh
them, and sort them into piles. See what weight range has the most marbles, and with luck there will be 20 marbles that are
within 0.2 grams of each other. Use these “official” marbles to operate the RBS. Now save a slightly heavier and a slightly
lighter marble for testing. An official marble may negotiate a curve almost every time, but the lighter and heavier ones find
the curves that are banked almost right, and help you figure out whether they need more or less bank. Test each foot or two
of track as you make it, as it is a lot easier to change the angle of bank before too much track comes after it. The track
spacing affects the speed: wider makes the marble roll slower, and makes banking less critical. Narrower spacing makes the
marble roll faster, but the angle of bank on the curves is more critical, and it is easier for the marble to leave the track. I used
15/16” diameter marbles (they are listed as 1”), and the inside distance between tracks is 3/4”. It is a good idea to have
selected the marbles before making the tracks.
Brazing is very easy to do, using an oxy-acetylene torch and brass brazing rod. It is similar to soldering, but is very strong
and not as watery when melted. It is easy to make joints that are smooth and nice-looking. Use plenty of flux, made into a
paste and applied with a little brush. . I use Harris Stay-Silv White Brazing Flux. After the joint is made, the flux will be very
difficult to remove. Just scrape off the part that the marble touches, and leave the rest there. After a week or two this flux
will have picked up moisture from the air and softened, and a wire brush in an electric drill will remove it fairly easily. The
wire brush will seem to barely affect it if it is used too soon after the joint is made.
Many times only a few places need to be brazed, so that those joints can hold things in place while the next joints are made.
A handy timesaver is a “gas saver”. It gives you a hook to hang your torch, where the weight of the torch causes some
valves to turn off the torch. A tiny pilot light stays lit. When you need the torch you pick it up, pass it across the pilot flame,
and go. The settings that control the flame are just the way you left them, and you save a lot of time in turning on valves,
lighting the torch, and adjusting the flame.
The main frame around the whole thing is made from welded 3/4 “square steel tube, with ½” square struts. Welding is not as
easy as brazing, but with a grinder and some Bondo and some paint, even my amateur welds look fine. After everything is
made, remove the flux, and lightly sand it. A sandblaster would be perfect if you have access to one. Then spray paint the
whole thing with a darker background color, and then brush paint just the tracks with a lighter, brighter color. The paint on
the tracks holds up pretty well. It will wear off of some very small and narrow areas, but the overall “look” is still there.
After it is painted, the tracks are waxed with automotive paste wax.
The marbles are lifted to the top with hooks brazed onto number 41 chain. To braze the hooks onto the chain, the chain is
first cleaned with white gas in the spots to be brazed to remove the grease from the chain. The white gas is then removed
with acetone. The spots for brazing are then ground to expose fresh metal with an abrasive disk or wheel, and now they are
ready for flux and brazing. A motor turns sprockets at 10 rpm, to power the chain to the top.
RBS’s are a magnet to children, and if they can touch, they will. I covered the whole thing with acrylic: 3/16” thick on the
sides, back and top, and 1/4” on the more frequently removed front. The floor is the wood top of the cabinet underneath.
Over this is a slightly slanted floor of 3/16” polycarbonate (unbreakable plastic). This is shaped to lead wayward marbles
into the marble rescue device. It is odd that many people do not even notice the slanted plastic floor, and think that there
must be hidden magnets that lead the marbles into the rescue spot.