A quick post showing the arms for the tumbling acrobat model in place. There are two stops to limit rotation. The legs will fit in the place indicated, possible with a paper hinge rather than an axle. I have reduced the weights in the tube to three penny coins total weight 12g. I'm still worried that they pick up quite a bit of speed as they roll down the tube. A friend of the web site who has a plastic version of this automata says that his model has ball bearings with baffles to slow down the roll. Not sure if I can do baffles for coins but I'll give it a try.
I put together this coin tube complete with baffles which slows the coin down nicely. Might need to experiment with different angles and heights of the bump but even as is, it stops the coin crashing too hard into the end of the tube.
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After spending a day cutting and sticking without much success I've decided to take a step back and try to work out the geometry of this classic tumbling toy.
I've been sent various YouTube links by helpful friends of the website so I've been poring over various online videos. I screen grabbed shots of this YouTube video by Ian B Dunne and looped it to more easily see what is going on.
I've then taken the individual frames and dropped them into Illustrator.
In essence, the tumbler has three parts. the body with the sliding weight, the arms and the legs. I traced the three parts and experimented with different stills from the YouTube video to work out where the joints should be.
I've animated the frames together and included the coin weight to show its motion. You can see it rolling back and forth with the body tube.
There is one point in the motion that might be a problem. When the body reaches the point shown in the still, the arms need to swing over the body to the other side ready for the hands to touch the floor. Gravity won't do the job. Inertia might but failing that I'm hoping that a strip of paper where the purple dotted line is will pull the arms over as the body drops down. Time will tell.
By checking the geometry of the parts in the animation I can see the range of motion that the arms and legs need. When I put the second prototype together I'll use these angles in the hopes that the model will work straight from the drawing board. Hah.
In the tumbling boy automata, who stars in a few YouTube videos, there is a strange 'tail' fixed behind the legs. Rather that using that design I'm going to be using a really big foot.
Tomorrow - back to card.
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I've been thinking about making a paper version of this classical automata.
In the original version, mercury flowed inside a steel tube to change the centre of gravity of the acrobat making him tumble down the steps.
In my version I'm using paper tubes and coins.
The weight of the coins rolling in the long central tube helps move the acrobat.
I've made a start, experimenting with the joints to make them as free as possible and finding a way to limit their arc of rotation. At the moment I have settled on pegs in holes for the joint. The triangular section on the upper surface of the 'leg' works as a movement stop, restricting the rotation to ninety degrees. Looking at the video I suspect that there is a connection between the arms and legs, I'm not sure as it is hard to follow the video but if necessary I'll use paper straps to link them together.
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