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Adam Harris
FabAcademy Projects

April 30th 2014:   Mechanisms

The discussion today focused on designing mechanisms


You can download the different iterations of the project here.


    My final project is a hand with living hinges that moves.  To this end, I finally got around to testing different sizes of living hinges to see what would work best for my design.  I really wanted a 180degree bend in the finger elements.  My initial design had 3 living hinge joints per finger to mimic an actual hand. 

    When testing this design, it was found that the hinges simply weren't large enough to allow the full range of motion I wanted.  I went a' researchin' to find out more about living hinges.    I came across this site which goes into detail on how to design living hinges.  I laser cut the sample designs on that website in 1/16th" 3-ply plywood to see how they would work with my material.  I had also experimented with turning the servos on their sides to allow then to push and pull each finger, not relying on the wood's springiness to lay the fingers down.  I must say, this didn't work as expected and I found the springiness of the fingers alone would suffice to lay the fingers flat when the servo was in the correct position.  First, I started with some scrap wood and cut a single finder and used wire-wrap wire affixed to it to actuate it.

I then made a second iteration with 3 finders that used fishing line with one end tied to a servo and the other end tied to the finger tip to actuate the hand.  To clean up the design, I strung through spacers in each finger.

    On my third iteration of the design, I cut a full-sized hand. Notice that I went back to the servo mount being perpendicular to the palm surface.  This saved a lot of space.  One thing I found out is that while the fingers living hinges worked great, the thumb immediately snapped off.  The reason is that the wood grain on the top and bottom layers of the plywood are in parallel to the living hinge cuts of the fingers, but they were at an angle to the cuts on the thumb.  This caused the  thumb to curl out of alignment when actuated.  When I tried to fix this, the thumb broke off.  I think what I will do next time is to make the thumb separately so the wood grain will match up, and then mount it onto the palm.  This will also make the design waste less wood when I cut it.

    Overall I learned a lot this week (it actually took me a couple of weeks to get this week's assignment done due to work and other responsibilities).  I have a much better idea about how to pull off my final design.  You can see the video of my tests so far below. 


(Click this link if embedded video is not working)


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