Introduction: The Prisoner's Puzzle Lock

About: Awesome Gear I've designed myself.

I made this lock based on a rope puzzle where two participants unlink themselves without cutting the rope, or loosening the knots. It's is an entry in the Puzzles Challenge. Thanks for considering me for your vote!



• 1/8" Vinyl coated steel wire

• Towing Hitch Pin

• 1/8" Aluminum crimping ferrule

• 3/16" Mild Steel

• 5/16" Shackle Master Lock


Fusion 360

17/64" Drill bit

5/16" Drill bit


Ferrule Crimper

Water Jet cutter




3/4 Socket

Step 1: Drill Out a Hitch Pin

I drilled two holes in the hitch pin, one for the Master Lock and one for the steel cable. The Master Lock hole is 5/16", the cable hole is 17/64". You just need to make sure the aluminum ferrule won't slip through. I also included my crude sketch.

Step 2: Cut and Crimp Steel Cable

Makes sure you cut the cable long enough to pass around the lock. My total cable length is about 16". Pass the cable through its hole in the pin, then crimp it. Otherwise it will be difficult to thread through.

I cleaned up the protruding cable ends on a diamond wheel so nothing would poke anyone. I also crimped the end of the loop with pliers. This helps it through the locking hole on my gate.

Step 3: Design in Fusion 360

I designed the shackle in Fusion 360, the file is included in the beginning of this Instructable. I measured the distance around the cable as if it were passing over the pin. I then rounded up to the nearest tenth. This gave me a little clearance but not enough to make the solution obvious.

I right clicked on my sketch and saved it as a DXF file. Now off to the Water Jet.

Step 4: Water Jet

I set up my material in my Water Jet and uploaded the file. I'm using a Protomax by Omax. If you don't have access to a machine like this you could use a hole saw (like what's used to install deadbolts on doors). In that case, you would use bar stock instead of a flat sheet.

Step 5: Form and Tumble

The metal was too thick to bend cold. I heated it up with a propane torch then clamped in a socket with vise grips. I hit the shackle in the jaws of my vise to bend it into shape.

Next, I ran it in my tumbler to give an even finish and smooth over any sharp edges. This is completely optional for the finish, but if you don't have a tumbler, use sand paper to take down the edges.

Step 6: Solve the Puzzle

Place the lock by passing the cable through the locking hole of your gate. Weave in the shackle and add your lock. If you can solve the puzzle, you don't need a key.

Puzzles Challenge

Second Prize in the
Puzzles Challenge