My 16mm film slitter

It may not be fancy, but it does an excellent job of slitting 35mm film down to 16mm.
The basic design was modeled after my friend Kamil Horac's slitter.

Mine is not very scientific. I just decided on a basic size and cut the outside pieces. Then I decided how tall to make the inside pieces while leaving enough room for the razor blades to be covered - except for the tips sticking up to cut the film. (Have them stick up enough that you can add felt and they still be high enough to cut the film) I then added felt, which will mash down when pressed by the block and press the film over the blades. It's easy to put the felt on if you just glue it across all three slices while bolted together and then use a razor blade to slice them apart after they are dry. You can take one of the inside wooden slices and lay the razor blade on it at the right position and use spray paint to mark the holes so you know where to drill to place the dowel pins.

The blades sit on dowel pins so they will be in the right position. Take a razor blade with you when you buy the dowel pins and get one that they fit snugly so they won't wobble around.

Drill a couple of holes through the whole thing and run bolts through it to hold it together. You can experiment with the center slice to get the film strip the correct width and then, if the tray width isn't the correct width to hold the film straight, just cut a new outer inside slice to get it right. It's not rocket science and it might take a couple of tries to get it the way you want it. A table saw makes it a lot easier. I made mine by cutting slices off a two by four I had laying around.

If you really want to get carried-away, you can add another blade and position it so that you cut a 16mm slice and a 9mm Minox slice from the same roll at the same time.