9 mm Snap-Off Utility Knife – Ratchet Lock, Parametric (FreeCAD)

A printable cutter for standard 9 mm snap-off blades Utility knife handle for standard 9 mm snap-off blades (30°). The blade is locked by a sawtooth ratchet: push the button forward and it clicks out notch by notch; the cutting force can't push the blade back. Press the button down to release it and retract the blade. - 20 notches of 3 mm: up to 60 mm of blade out, and the blade hides completely inside the nose when retracted

- Integrated spring in the carrier: no metal spring, no screw

- Pinch-to-remove rear plug: squeeze the two ears to pull it out and change the blade

- Handle: 129.6 × 13.9 × 11.2 mm

3 possible shells - One-piece "zebra" shell (Coque_Unie_Zebre) – the tested version, shown in the photos. Printed in one go, upside down (top face on the bed), with 45° filleted windows through the floor. The carrier and the button slide in from the rear.

- One-piece plain shell (Coque_Unie): same as the zebra shell, without the windows.

- 2-part shell to glue (Coque_Bas + Coque_Haut): bottom and top shells located by a lip/rebate joint. Not tested.

Parts - One shell: Coque_Unie_Zebre (tested) or Coque_Unie or Coque_Bas + Coque_Haut

- Chariot (blade carrier with built-in spring)

- Bouton (push button)

- Bouchon_Arriere (rear plug)

- One standard 9 mm snap-off blade

Printing - Printed and tested in PLA on a Bambu Lab A1 mini, 0.4 mm nozzle, 0.2 mm layers.

- Cutter_oriented.3mf contains all the parts with the orientation I used for a good print with the zebra shell: just open it and slice.

- The individual 3MF files are oriented too: carrier and button on their side (the spring and the teeth flex in the layer plane), plug flat, one-piece shells upside down.

- No supports needed for the carrier, button and plug.

- A little sanding may be needed to fine-tune the clearances and get parts that fit perfectly into each other (mainly the carrier sides and the button), depending on your printer.

Assembly - Put the blade on the carrier pin (the hole edge is 2 mm from the cutting edge).

- Fit the button foot into the carrier window, above the spring.

- One-piece shell: slide the carrier and the button in from the rear, button pressed. 2-part shell: lay the carrier in the bottom shell, then close and glue the top shell.

- Clip the rear plug in: pinch the ears, push, release.

Files - Cutter-oriented.3mf: all parts, oriented for printing (zebra shell).

- 3MF (oriented) and STEP (assembly position) for every part, plus a full assembly STEP.

- FreeCAD file: the model was generated by a parametric Python script driven by a "Parameters" spreadsheet (clearances, wall thickness, number of notches, spring size, blade dimensions…).

Safety This holds a real, very sharp blade. Always retract the blade after use, check that the lock holds before cutting, and keep it away from children. Use at your own risk. Designed with FreeCAD; the parametric script was developed with the help of an AI assistant.