Tiny Folding Scalpel

A small (5 cm) knife handle for use with Swann-Morton No. 3 blades. I couldn't find anything quite like it so I made one. One of those “Just because” projects that don't want to leave you alone until it's figured out and finished.

I find it especially useful for gripping closely to the blade for very fine control, for instance, when trimming a small model. Having a long handle is not always an advantage, as using a normal No. 3 handle for some tasks can make my thumb sore as it can press against the back edge a lot.

The blade folds out then backs up a little into the handle to lock itself in an open position.

Tested with blades 10 ,10A, 11, 11P and 12. Some others should fit.

With hole for a small keyring or lanyard.

Variations

1. For No. 10. Also fits all blades listed above, but 10A is not protected too well when closed. 11, 11P and 12 are slightly wobbly in open position.
2. For No. 10A only. Covers the blade edge fully in the closed position.
3. For Nos. 11, 11P and 12. Similar to 10 but a slightly different shape inside so that these narrower blades can lock open without any play.

Printing Notes

• Make sure the back end face of the model is facing the print surface so it's pointing upwards.
• Using a brim my help with bed adhesion as the area of the back face is quite small.
• PLA works fine but there is a risk of the screw hole cracking if the fit is too tight, so the more brittle types such as silk may not work so well.
• PETG works very well but I've found it best to set the XY size compensation to about -0.03mm to widen the blade channel a little bit.
• Transparent filaments can pair well with gyroid infill to create a fish scale look.

The included .3mf file has all three variations, with default settings for printing 20% gyroid infill plus reinforced holes. The included .gcode files are exports from this.

Alternatively set the .stl in a group to 0% infill - this was useful for me during the development when using transparent filament so I could see inside and better understand some fit issues, but it looks really good anyway and seems strong enough so it's a nice option to try.

Assembly

• An M2.5 x 5mm screw or similar is required to fit the blade to the handle. It can be a self-tapping screw instead if you have something of the right dimensions.
• A screw with a shallow head is best so it doesn't protrude too much.
• The screw does not need to be driven in too tightly - you want it tight enough so it is secure but not so tight that the blade will jam in the closed position and be difficult to open out.