Wooden Combination Lock


A while back, I bought plans for a wooden combination lock from Matthias Wandel. It’s a fairly complicated piece, and he assumes you have a pretty well-equipped workshop, requiring a table saw, a jigsaw, and a drill press. I have a coping saw and a handheld drill - not hardly enough. I decided to see if I could use the local library’s laser engraver to fill in the gaps in the workshop.

This was my first project using vector cutting. Everything I’ve done before used raster engraving, where the laser draws a picture on top instead of cutting through the material completely. I had to convert my plans from rasterized pictures (where each pixel has a certain value) to vectorized picture (where lines are defined and stored mathematically). An easy way to think about the difference is to think about a picture of a circle. As you zoom in on a raster circle, you’ll see larger and larger squares that represent the pixels in the circle. As you zoom in on a vector circle, you’ll continue to see a smooth curve no matter how much you zoom. Wikipedia has a good comparison.


Once the plans were converted to cut-able vectors, I started the printing on some econowood.


Here’s the pieces all cut out and given a quick coat of mineral oil and polyurethane.

Aaaand the front view of the assembled model. The one visible screw here on the combination dial didn’t quite catch on the sides of the front of the dial (with the numbers). I’ll pretty the thing up once I get a fatter screw. The second circle in the dial controls the combinations rotors, while the lever on the right controls the bolt-release shaft. Check out the first ninety seconds of Matthias’s demonstration video to see it in action. If a picture is worth a thousand words, a video is worth… … (checks math…) more, probably.

More pictures, with a brief explanation of how it works:


From the side, we get a better view of the inner workings. There are three combination rotors on the rotor shaft. Each rotor spins freely and has a tab (to catch or be caught by the other rotors while spinning) and a notch (to allow the lock to open when all three rotors are in the correct position).


After two spins of the combination dial, all three rotors are caught by the tabs.


After putting in the correct combination, the notches in the rotors are lined up.

Lifting the lever pushes back the bolt...


And opens the lock.

Ta-da! A working lock!

Again, this was my first cutting project, and I will happily label all my mistakes as learning experiences. First, the whole raster-vs-vector distinction was unfortunately extremely relevant. I lost a lot of precision using rasterized plans in the first place. Pixels can only hold so much information, and you’ll have blocky artifacts whenever you’re doing something high-precision. I could have been more thorough in making sure that the raster-to-vector conversion kept things the size I was looking for. As it turned out, all of the inside holes on the pieces were just a little bit too large. Glue can fix a lot, but the pieces that weren’t glued in place (like the combination rotors) are too loose. The middle rotor in particular will sometimes get caught with the first rotor when it shouldn’t. Tighter tolerances would’ve helped out there. Moral of the story: don’t try to make circles out of pixels.

In the end, I’m super happy with the project. It looks great and works as a good conversation piece. I like the idea of making contraptions out of wood (next up: wooden robots?), and I’m looking forward to what comes next.