Thank you!
Okay, winter time is sled time!
I was thinking carefully about how to achieve a back and forth motion of the arm. I tested several ideas in my mind, abandoned them, came up with new ones. After three days of mere thinking - the build was literally on hold because of this - I almost gave up. But then, during my lunch break, I got it! A rail / sled system that would hold itself in place.
I fired up SketchUp and rebuilt the upper box that forms the torso. Then I created the sled and the rails. On paper everything looked like a walk in the park - but reality turned out to be a complete nightmare! The first rails sat too low, the next ones too high. Then the sled got stuck. I was about to abandon the idea, but after seven or eight test builds (!) I chose to combine the upper rail that sat too high with the lower rail that was too low - and it worked!!
PICTURES!
I started with the sled. It was made of one piece.
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Easy peasy. A hollow tube was inserted. It will hold the shoulder in place later.
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Again water-forming was applied. The flaps are pushed together so that they wouldn't get stuck while being pushed through the holes.
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These are pictures of the white prototype. ONE of the MANY prototypes...
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The flaps are glued to the outside
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and cut to shape.
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The sides are cleaned up by adding a piece of black paper.