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We all have experience of saving coins into a piggy bank as a child. A saving coin teaches a child a good saving habit to manage his/her personal asset. Japanese invented a coin stealing toy bank to make this experience even more entertaining. The bank looks like a small box. When a person places a coin above the toy bank triggering a switch, small animal paw comes out from the box and drag the coin into the box.
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I was interested in the mechanism behind this cute toy and recreated this coin bank using materials at my home and Karakuri book. Although the shape of the scope of this project has changed because of the recent COVID-19 pandemic crisis, I did my best to successfully finish the course.
Design Concept Generation
In the beginning, I don't know what is inside the coin stealing bank. I found a Youtube video that shows the inside of the box.
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We can see the machinery inside is combination of a few different mechanisms.
3 mechanism should be integrated into the system:
- Opening of the coin box door using a cam mechanism. The follower of the cam lifts the box door up and allows the arm to reach the coin. Then it will close when the arm is returning.
- 4-links mechanism pushes the arm to reach it to the coin. There will be a crank attached to the shaft at point O, which drives link 2. The link 3 is the arm touching and dragging the coin. The link 4 will be a spring to help returning mechanism.
- The arm return mechanism using spring need to design. I need to use a spring with enough stiffness. Thus it toggles the returning mechanism when the copular point (the end of the arm) reaches the coin.