I entered the design into Google Sketchup. The first thing I noticed was the use of 1/8" thick wood for the ladder. I don't keep 1/8" thick wood in stock, and it is very difficult to plane wood down to 1/8" (it sometimes blows up in the planer). I decided to use 3/16" thick Luan plywood for the ladder instead. This means the 2 notches at the ends of the monkey have to also be 3/16" wide versus the 9/64" called for in the plan.
My brain did not really grasp exactly how the model works. I went ahead and built the 1st prototype. It was hit and miss on whether the monkey would stay on the ladder all the way down, or get stuck, or fall off. I set up my video camera and took some video of the model in action. I then slowed the action down to 0.40X of normal speed so I could see exactly how it worked......
The video in slow motion really explained how this toy works. After the monkey falls into the notch below it, it rotates in the 1/2" diameter hole, until it is hanging down. It actually rotates on past the down vertical position, before it swings back and it vertical. When it falls, the clearance notch in the monkey keeps it precisely vertical, so it falls down onto the next lower rung of the ladder.
I googled problems with this toy model, and found another toy builder who had similar issues. He had no length to his guide clearance notch in the monkey, so it fell off the ladder.
On my 1st prototype, I had the length for the guide clearance, I just made my notch too wide..........so the monkey was not forced to be vertical before it fell down to the next rung. I went into Sketchup and laid out the motion, since I now understood it from the slow motion video.........
I made a new monkey, and was very careful on the clearance slot thickness. I used the big band saw to saw out the clearance notch, then used a hand file to fine tune the opening to just match the 3/16" Luan ladder. It worked flawlessly!!
I am going to go ahead and make a new ladder. I am going to increase the distance between the runs by a 1/4". I am also going to raise the lower rung 1/2" above the base, so it is easier to get the monkey out and do it again. Right now, it is hard to get it out when it is done.
Increasing the distance between runs by 1/4" turned out to be a bad idea. The rungs need to be close enough together, that the bottom of the monkey needs to be seated in the lower rung, before it is released from the run above.
Since the prototype ladder works ok and is painted, I sawed off the bottom base, then planed each side down to match the original 3/16" thick Luan, then table-sawed the width back to the original rung width. I am going to use my prototype ladder.
This model looked very simple to build, but it turned out to be a lot more work than I thought. Because I did not really understand how it worked, I made my slots too wide.........and therefore the monkey was not properly guided when it fell down to the next rung. The fact I used 3/16" thick Luan versus 1/8" thick wood for the ladder also messed things up. I probably should have made the rung thickness a little smaller, or increased the 1/2" hole in the monkey to account for the thicker ladder rungs.
The grand-children should enjoy playing with this one!
Using Sketchup, I changed the ladder design so the rung heights was 7/16" instead of 1/2"............because you need a smaller rung height if you use 3/16" thick versus 1/8" thick wood for the ladder. I quickly made up a ladder to this design. After cutting on the scroll saw and sanding the edges, it did now work. I used the Dremel drum sander to round over each rung edge, top and bottom..............then it worked fine.