This toy is made from a wooden spool and is powered by a rubber band. I used to make these as a kid and they are great fun. When you wind it up it will creep forward and climb over small obstacles like pencils, erasers and electrical power leads. It goes for quite a long time on a single wind up of the rubber band. As a small kid I also liked to place the tank on its end and imagine that the turning pencil was the tank's turret. Cotton-spool tanks are easy to build if you have the right parts. No special tools are required and you can probably build one in an hour.
· An old cotton-thread spool like the one shown in the figure. You are going to cut notches in the flanges so old wooden spools that have large flanges are the best but you can get by with a more modern spool that has small flanges. If you have a spool with small flanges you can choose to glue 40 mm diameter discs of 3-ply plywood or stiff cardboard onto the ends of the spool. If you do have to use a modern spool try for a wooden one because you will need to put nails into it.
· A rubber band that is a fraction shorter than the cotton spool, or a long rubber band doubled over.
· A pencil.
· Two small nails (10 mm long).
· A match
· 12 mm off the end of an ordinary candle stick.
To build this you will need a triangular file, a hammer, some string and a pointed knife (pocket knife is ideal).
The tank is complete! Wind it up by some amount and put it on the floor. The pencil will rotate around till it also touches the floor and then the tank will roll forward. Put a small obstacle in its path and watch the tank climb over it. It should move quite slowly - if it unwinds rapidly you need more wax on the end of the spool or a smoother contact between the candle stub and the spool. If the tank does not roll forward well enough use two rubber bands instead of one. If the rubber bands keep breaking and it still rolls forward too slowly, chamfer the face of the candle stub that rubs against the spool. That is, reduce the area of the candle where it contacts the spool to give it less friction.
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