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Master combination locks are a very simple mechanism, and I wanted to build a see-through one to help explain it to friends. Some vendors offer commercial cutaway master locks, but I figured it would be easy enough to construct one myself.

To remove the back of the lock I used a dremel tool with a grinding wheel. Eventually I was able to remove the thin and shiny back cover, after which the thicker back plate was easily pulled out, revealing the inner workings of the lock.

Starting with 1/8" acrylic from a home goods store, I traced out the back plate and made rough cuts with a coping saw. Next, I bolted the two pieces together through a hole I drilled for the cam alignment pin, and then performed finish cutting and sanding around the acrylic with a jeweler's saw and some 40 grit sandpaper. Don't take the protective film off of the acrylic until all of the fitting and filing is done.

The main spindle that the disks rotate on is press-fit in to the back plate, so this was the most difficult part of the construction. I removed it from the backplate by dremeling away the back and pressing it out, revealing a hex-shape that didn't look easy to press fit in to the acrylic. Instead, I carefully drilled a #43 hole in the end of the shaft (the drill size for tapping a 4-40 thread) and used a 4-40 stainless bolt to self tap the soft aluminum shaft. This allowed the spindle, once fully assembled, to be affixed to the acrylic backplate with a 4-40 screw and washer.

The final installation is missing one piece that aligns the hasp properly, but since this doesn't look like it has any impact on the locking mechanism I'm happy leaving it out.

master lock close-up of the locked lock close-up of how the lock looks when the correct combination has been entered close-up with the combination almost dialed