SEE THE VIDEO ----youtube.com/watch?v=cmJYksR5X_k

Pinshape doesn't seem to like embedded links so copy the above link and       paste it into the address bar    (not the search bar)!

This is a fully functional 'otf' (out the front) knife (auto blade).  At the push of a button the blade shoots at high speed out of the body.  The push button also returns the blade at high speed back into the body.  This can be repeated in quick repeated successions.  However the blade in this case is a toy version.

As an engineer, I took it as a personal challenge to design and print the fully functional knife using  only printed parts  including printed spring systems.  So no glue/ additional springs/ or anything else is needed.  Assemble it as instructed in the video for it to work - its a simple clip together design.  Smear petroleum jelly or grease on the internals help it to work fast and smooth.

All 6 parts will print together on my Wanhao i3 200x200mm platform, but a smaller platform is possible because the longest part is 140mm.  Takes about 4 hours to set up and print.  Keep an eye on part orientations to keep cavities and extrusions facing upward, it can be made with no requirement for support material.

Part 6 needs to be duplicated 2x, these are the latches.  They are designed to plug into obvious slots.  The main spring and the slider can only fit in their correct positions, and it should be obvious to place the blade correctly.  Anyone with half a brain should be able to assemble it! 

PLA is what I've used, so I don't know if it will work in ABS yet.