For this project you will need some stuff:
-A Scooter handlebar, one with that button that you press and he come up again, or any tube with that button in it,
-Any pipe/tube, better be an aluminium/steel one, copper is too soft.
-A spring, i got mine from a car handbrake asembly, the apring that operates the handbrake button.
-And most important thing, the Blade. you can get any knife you want and cut it in half, or you can do this really professionaly and order some throwing knives from amazon or ebay.
when you have done this, you should hammer into the other end a round screw or anything round, so you can regulate the pressure of the spring,and get some better shots.
To make a blade you will need a scooter handlebar, or maybe a folding chair leg, the important is that it has that button you press and it comes up again, and that it enters the tube with little or no spacing(there should be just a small movement into the tube).
Then you will have to cut your blade, and weld it/glue it to the tube with the button. I used epoxy metal glue to glue it in place, and its pretty steady. When you've done this you are pretty much done, put your spring in and start shooting!
NOTE: This is considered as a weapon in most parts of the world, in some parts its legal, in some its not, if it is unlegal in Your country you do this at your own risk! Dont point it to humans or animals, and think two times whenever you shoot at something.