I needed to cut a slit in a length of soft tubing for a project. I wanted the cut to be straight, and not slice my fingers in the process.
The slot will hold a single-edge utility razor blade. It's designed for a 0.3mm thick blade. After installing the blade, the slitter can be secured in a vice, which will both clamp the blade in place, and free up your hands to feed the tubing through it.
This one has a 6.2mm hole for the size of tubing I was working with. But the Fusion 360 model is provided so you can easily modify it.
(The weird two-tone colour scheme is just because I was using up the dregs of a spool of white, and switched to blue when it ran out.)
The author marked this model as their own original creation.