Simple design for a relatively small 3D printed knife that uses an Xacto Blade. Superglue is likely necessary to reliably hold the blade in place for frequent uses.

Only 3 pieces, with a variation to have the keychain ring go through the front to reduce the change of the blade coming out while on a keychain. Short print time and minimal filament. Glue or PLA welding recommended to keep the lid on and the blade in the housing.

FDM  PLA is what I used, something a bit more flexible might work better for the springs, they are very stiff. The forward and backward motion require a lot of force, but some sanding should help that

I have several variations saved, if different versions are requested I will post them.

Please be safe, this design can launch the blade depending on tolerances and can snap the blade forward. I tried to design it safely, but if you add a sharp blade it can be dangerous. More of a proof of concept and fun project than a serious tool.

PS I forgot to rotate the models, the body and lid don't need supports, the top needs them for the thumbswitch