Here's a short MPEG video of the Flip Flop in action.
This past weekend (2/20/2000) I managed to make one of these toys and I'll post the instructions here so that others could have some fun too. I made the ladder rails from approximately 3/4"x3/4" sticks cut to a length of 13 3/4". The four ladder rungs were made from eight pieces of 1/8" dowel, each about 2 1/4" long.
In order to drill holes in the side rails that would match, one rail to the other, I first made a Plexiglas drill guide (see diagram below for spacing). In addition to the 8 holes for the rungs, I drilled two small holes for thumb tacks. The thumb tacks held the guide in place on the rails during drilling. I marked the guide with an L on one side and an R on the other, and with the word "TOP" at one end. Be sure that the L is on top when drilling one rail, the R is on top for the other, and that the word TOP is at the top end of both rails. I used a drill stop on my drill press so that the all of the holes in the rails were the same depth. Alternatively, the holes could be drilled all of the way through the rails. In that case be sure to mark the side of each rail into which you drill, as the drill guide will ensure that the holes will match if these sides are facing each other.
Next tap the eight dowels into place on one rail. I used a bit of wood glue on the top two and the bottom two. Add a bit of glue to the top and bottom two holes of the other rail and tap it into place.
I used a scrap of maple for the traveling block, with dimensions of 3 3/16" by about 2" by 3/4" thick. Carefully mark the center position for two 1/2" diameter holes. They should be places symmetrically on the block and be 1 1/16 " apart. See the diagram below for the pattern. Once the holes are drilled, then slots are cut between each end of the block and the nearest hole. These slots must be just slightly wider than the 1/8" dowels -- too narrow and the block will stick on the dowel, too wide and the block will be able rotate too far and will not travel by itself down the ladder. The slots are rounded where they exit the block to facilitate capture by the next rung as the block works its way down the ladder.
Click for larger construction diagram.