NUT-T Coin Trap

Once I searched coin trap puzzle on the net and this card-size puzzle showed up.

It is called The Centrale. Beautifully made yet simple, but I just can't find the inside of this puzzle. Some believe that the inside has the same mechanism like 4 Steps Visible Lock using steel balls inside.

Well, then I have my own vision using materials around me. Scrap wood, bolts and nuts. That's why I called it NUT-T Coin Trap (read : nutty) which represent T-shape bolts and nuts as the locking mechanism inside the puzzle.

1. Scrap wood
2. Bolts and Nuts (2 pairs)
3. Little piece of bamboo chopstick.

Tools I used :

• Pencil
• Ruler
• Saw
• Files
• Pen knife
• Wood glue
• Power drill + 5 mm drill bit.
• Varnish (optional)

Get your Bolts cut to 8 mm total length from the top (the head + four threads). In the making process, I adjusted everything to fit in, so the measurement doesn't have to be exact anyway.

Firstly, print out the design at original size. That design was drawn with vector app according to the real size so that the bolts and nuts will fit in. Cut out all the holes so that we can use the papers to transfer the design to wood. We cut the paper at original size. When we transfer to wood -- draw the holes based on the paper -- the size will be smaller because we are drawing at the inside of the holes. That is okay. No worry. We will then enlarge them to fit the materials in (bolts, nuts, coin).

How I cut the small holes? Pen knife. Use any tools you have or you love. Enjoy your cutting time. Do it slowly. Plywood is soft and easily cut or sliced.

Details about the cutting are described below :

The bold text and bold underlined text will be used as glossaries on next steps.

Bottom Board

• Cut the nut track half way down (about 2 mm of depth).
• Cut the chopstick track half way down (about 2 mm of depth).
• Cut out the finger curve.

Middle Boards are broken down into three parts :

1. Top Piece : Cut out the top bolt container space.
2. Center Piece :
   • Cut the top nut container about 4 mm of depth (fit the whole nut in) from the other side (facing the bottom piece).
   • Cut off/half way the coin holder. If you choose to cut off, it can hold thicker stuff like a wedding ring (and this is what I wanted).
   • Cut off bottom dummy nut container. I called it dummy because this has nothing to do with locking mechanism. It simply hold a nut that will "clink-clank" when we shake, pretending to be a locking part.
   • Drill a 5 mm hole at the chopstick holder. This chopstick simply prevent the center piece being pulled entirely out of the system.
   • Plug in the bamboo chopstick, around 2 mm pass the bottom surface and 2 mm over the top surface.
3. Bottom Piece : Cut out the bottom bolt container space.

Top Board

• Cut down the chopstick track half way down (about 2 mm of depth).
• Cut out the finger curve.
• Cut out the peep hole (small circle).
• Cut out the coin hole (large circle).

The Bolts

• Previously we have cut the bolts at 8 mm total length from top (approximately the head plus four threads). If you haven't cut it yet, now it is the time.
• File two opposite corners of the bolt to fit in the thickness of middle board. It will then have the shape of Thor's hammer :)

• Place the bottom board with finger curve on the left and nut track facing up.
• Put one nut in top nut container on center piece.
• Place the center piece on top of bottom board with the nut on the nut track and chopstick on chopstick track.
• Put the other nut in bottom dummy nut container.

• Put the top piece and bottom piece of middle board in place.
• Put a coin inside the coin holder.
• Test moving the center piece, make sure nothing block its movement along the nut track and the chopstick track.
• Glue the top piece and bottom piece on the bottom board.

• Smoothing the center piece with 200 grit sandpaper and then 400 grit to get a smooth movement.
• Put back the nuts and center piece in place and put the Thor's hammers in top and bottom bolt containers.
• Glue the top piece and bottom piece and put the top board onto them. Make sure your glue doesn't melt into the bolt containers or touch the center piece.
• Wait to dry completely and smooth all the edges with a file.

• A label will add psychological lock because people will stick with that orientation for most of the time, therefore keep them away from solving the puzzle.
• Put a coin in the coin holder. Ready?
• Slide the center piece bar inside (to the left). Click.
• Ouch! It will be auto-lock.

If you take a look back at the design, you will know how to unlock it in few steps. This kind of puzzle is pocket-friendly and set great challenges. When they shake it, they will predict what are inside the box and the dummy nut plays its role.

I am going to put a ring inside and hand it over to my Brother to solve it on his wedding day, December One :) Optionally you can apply varnish to get a glossy look.

Need a solution to this puzzle? Well, I will post it in comment section only if you ask for it ;)

Wait, what? You don't like woodworking? What about 3D printing? Well, I add another step attaching .STL files for 3D printing. You can also find the design on Tinkercad.com where I share it publicly. Go ahead to next step.

Here are the .STL files for 3D printing. There are 7 files but we need two of each Cubex2.STL and Tx2.STL so we get a total of 9 pieces. They are supposed to be Plug-n-Play, but please tinker my design on Tinkercad.com if you find anything that doesn't fit in for I haven't own a 3D printer yet.

I sent my design to a printing service to get them printed. As you can see it is now version 2.0 that means I have several things fixed from the previous version. Hope you enjoy my simple puzzle.

Bonus round is adding another level to the puzzle. Simply print ExtraLevelCenterMiddle.stl and ExtraLevelThinSquare.stl and replace the CenterMiddle piece and add the ThinSquare on top of the TopCube. You have to move the ThinSquare to the right side area to release the TopCube from its gutter below (by holding the puzzle bottom-up) before you can release the coin.

So much fun for both adults and kids. Can't wait for my Creality Ender 3 to arrive next week ^_^