TinyKnife

Just about the smallest replaceable-blade knife possible

Does anybody really use their pocket knife for more than opening packages?

This project begins with the surge in EDC (every day carry) knives around the mid 2010s. At some point, keychain knives that accepted #11 hobby blades started showing up, but they were fairly expensive due not only to low production volumes, but bad implementation. For example, the TUKK used titanium for the housing? I somewhat understand throwing titanium at random problems, but it's completely unnecessary here and subverts one of the advantage of replaceable blade knives: not only is the blade disposable, but the entire knife itself. TUKK was largely a good design, but a bad product (unless you're trying to maximize profit at $50 per unit). This is more or less a blatant ripoff of the form factor, re-engineered for mass production.

Two primary changes from TUKK are in material selection and blade retention. Titanium is replaced with a thermoplastic for injection moulding; polycarbonate in the prototype for mechanical troubleshooting.
The more interesting change is moving from a magnetic blade lock to a "living hinge" spring. While this decreases materials cost, it more importantly decreases part count, and consequentially assembly (labor) cost. A mere 2 COTS parts and 3 custom parts remain.

I machined a prototype of this living spring design on a belt-driven desktop CNC mill, which bears more resemblance to a stout 3D printer with a spindle motor instead of an extruder. Creative workholding methods were required owing to the small size of the parts, especially the blade slider. The final implementation uses 2mm dowel pins (the same size as the untapped m2.5 screw holes) and a very small stepover on machining operations.

I machined a handful of these for friends and family, and they all remain intact and issue-free three years later.

Identical top and bottom blank further reduces CAM efforts

CNC mill fixture

Intended hand position
I'm not a hand model, 'mmkay

As you can see, this has been my EDC knife for several years now

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