The basic concept I got from a guy in this site (check it out if you think its worth looking at the original design https://www.instructables.com/id/Hidden-Blade-Final-version/)
My Idea was the same thing but a few more details like a spring attached to the "bar gear". This way when the bar is locked into position it can shoot out instead of simply sliding out. \
What I forgot to add onto the image is the spring at the other end of the Bar gear so that once you release it and lock it in place so that it doesn't simply hang there or get pulled back. This way it can be retracted and released with some force which lets face it, its exactly what we want to do.
Another Idea I had is to add a break like to the bar somewhere so that no matter how far it goes it can still halt it from moving.
I am trying to make a gear but its becoming one hell of a task. If anyone could please send me a schematics of a gear that is about 1 inch in diameter and a bar gear to go along with it I would greatly appreciate it.
For what you will need for all of this (my design idea)
-Wire (as thick as for bike brakes)
-Lazy tongs (you can make them from interconnected pieces of metal cris-crossing each other)
-Springs x2 (the stronger spring should be the one connected to the wire that pulls the Bar Gear the stronger the more power it will have when it shoots out)
-Materials for the box sealing the mechanism
-Anything to keep the pieces in place (screws, bolts, soldering metal, etc)
I personally like the design made by Techie_Gary so your call
I have not yet put this all together but when I am done I will definitely make an instructable and I will make a full on explanation on how to make gears when I figure it out (Because to get any kind of gears is either a nightmare or just plain impossible)
I hope you guys get some cool ideas with this. And again if you do end up building my design please send me an explanation on how you got it because I would definitely would like to see it. Thanks for reading my first ever instructable. Promise to make it better next time with real pictures.
*** Oh and another thing, I am not responsible if anyone building my design or plays around with my design get hurt or severely injured, I would first suggest making the knife dull and even plastic or wood and not shaped as a knife in any way till you have figured out the kinks in the design. Please be careful your playing with springs and other mechanical movements, and maybe some welding. Wear proper protective wear. Good luck building, I am going to figure out how to do this myself.