Hidden blade
I decided to 3d model and print a functional hidden-blade similar to those seen Assassin's Creed:
The goal with my design was to have a ring on your finger with a rope like in the picture. When you pull the rope the blade would toggle between being extended or retracted. I however paid no attention to its looks, only functionality.
My design in the end had a bit too much friction however, making it impossible to activate the blade using your wrist when it's strapped to your arm. Below is a video demonstrating that the system did work properly apart from this:
Design process
I based the design off of some mechanics I had seen on youtube in the past. Throughout making the design in Fusion 360 I kept printing and testing the new mechanical parts I modelled to verify they worked as expected:
I also made sure to design everything parametrically, which allowed me to tweak tolerances and other values if after printing it did not turn out to work well. Using some of these parameters I altered the travel distance of the spring loaded parts to increase the force, and I also made the whole blade longer at some stage. The model consisted of quite some parts in the end, many of which are moving parts:
Below are some more videos of intermediate stages of the project, which I originally posted to instagram.