Step 8: Machine a Polarized Magnetic Key
Why magnets make this lock so special...
The binary magnetic key is what makes this combination lock a bit more special than just having a tumbler you spin with a magnet; because unless you know the appropriate pole combination, it won't deactivate the locking pins. So you can't use any old magnet to turn the tumbler. Also there is no way for someone to figure out the correct combination from feedback as to whether any given pin needs repulsed or attracted.
Of course one could use trial and error with all the different possible combinations, which isn't many with only two locking pins. Once again I wanted to prove my basic mechanisms would work. If you were to add multiple tumblers to the design or add more sets of magnetic locking pins that would require a larger number of magnets on the key you could of course increase the difficulty of figuring out the combination.
However I felt this design secure enough for my purposes. The more likely first obstacle someone would meet trying to unlock the safe would be to even know there is a safe behind the picture and that it uses a magnetic lock.
To make the key...
I used four 0.375" x 0.75" Neodymium Grade 50 magnets for the key. They are a bit powerful to be used as a free floating key, which I am sure could be remedied by using more powerful magnets in the tumbler and using weaker magnets in the key. But I figured that after I had built the set-up.
I machined the key out of 3/4" thick HDPE plastic. I drilled all the way through with a 3/8" end mill. If you were to use thicker plastic, or wood you could drill to depth and only have too stop them from coming out one side instead of two like I had to. Although I suppose you could avoid using anything to keep them in by drilling the holes to an interference fit with the right size drill bit; I did not have that drill bit.
If you have been following the polarity combination and direction that I have recommended for your lock make sure that when you insert the magnets into your key that you make sure the orientation is the same as the technical drawing, otherwise you will be putting the magnets in backwards.
Stop your magnets from popping out & don't scratch stuff...
I chose to put a piece of adhesive backed craft foam on one side and some clear packing tape on the other to prevent the magnets from slipping out of the key. So far it seems to be adequate, although in the future I may decide to do something more substantial for the covering, or make another key with interference fit holes for the magnets.
Whatever material you choose to make the key out of, be sure it won't scratch the plexiglass. You don't want to scratch up the beautiful surface that's in front of your picture; not very covert either. You could use glass for your picture frame to make it more difficult to scratch. Just be sure your magnets are not so strong that they break the glass when they attach to the lock. This is why I used plastic.
Design side notes for making the key...
I chose to remove most of the material to reduce the key to as small as I could. I did notice after I took the extra material away I had to change how I spun the key to ensure I kept it spinning around the center axis of the tumbler. The key was originally square and did not provide the same challenge of keeping it on center. Of course, the increased difficulty in using the key is a security measure as well. At first it took me several tries before I could actually work the lock so I could open the safe. But after 15-20 minutes of practicing with the safe open and noticing how my movements effected how the lock moved I got a technique down that allowed me to open it the first time just about every time.