After having issues getting locking OTF knives to work well and consistently, I came up with a different design I hadn't seen elsewhere: the out-the-front rack and pinion knife.

The mechanism is super simple. There's a gear rack on the inside edge of the knife body. The back of the knife is also a gear rack. A pinion gear spins freely around an axel attached to a slider. When you push the slider forward, the pinion rolls pushing the knife forward AND rotating it forward meaning the knife travels twice as far as the slider.

With this mechanism, the knife extends twice as far as the slider has moved. Here's a quick demo: https://youtube.com/shorts/fankV-J6fP0

Advantages:

1. Very easy to assemble
2. Very easy to operate
3. Very easy to print

Disadvantages:

1. Fairly long range of motion
2. No locking mechanism

I have very large hands (I'm 6'0") and this knife is around the limit of what's comfortable for me to operate. The mechanism is very smooth, though, so you don't need to apply much force. If you don't have large hands, just try printing it scaled to 80-90% first.

NOTE: You may have to drill out the holes for the top cover with a drill if your printer isn't extremely accurate. They are a tight fit.

Supports: NO, although I do recommend supports for the blade itself. The blade tip can get a little droopy since it's so thin.

Adhesion: No, although you may need a brim for the pinion gear since it's fairly small. I recommend trying to print the gear without a brim first.