Tube Puzzle

This little puzzle was pretty fun to make and allowed me to use up some of the magnets from my last project. The goal of the puzzle is to remove the center piece from the tube. You will quickly notice that there isn't much to the puzzle - no buttons, latches or any other obvious locking mechanism. The pictures shown in the this step don't give away much. If you go to any further step, you will see the solution.

I made two puzzle versions: a wood one and a plastic 3D printed one. The wood version was pretty straight forward but required patience when measuring the hole locations since alignment is critical to the puzzle working. I also needed to fine tune the fit with a little hand sanding. As expected, the 3D print worked without any modifications. The files for that version are included in a later step.

Tools/Materials (Wood Version)

Tools:

Table Saw
Router
Drill Press
3/8" Forstner Bit
3/16" Drill Bit
Wood Clamps
Ruler or Calipers
Pencil

Materials:

Wood: 1.5" Wide x 2" Long x .75" Thick
Wood Glue
Epoxy
Magnets (5mm Diameter x 2mm Thick)
Steel Ball (3/8")

Drawings

You will be building to this drawing. The parts are color coded for easy reference.

Tube

The tube was built in two parts. I started by cutting down a 3/4" thick piece of wood to 1 1/2" wide. Note that I used a pine 2x4 for my puzzle. If available, I would suggest using something nicer/stronger since the solution requires repeated abuse.

Although I only needed two pieces that were 2" long, I cut a strip that was about 16" long. This allowed me to have extra pieces in case I messed something up along the way.

From there, I used a router table and a 1/2" bit to cut the 3/8" deep slot in the workpiece. I slowly moved the fence out while rotated the workpiece 180 degrees back and fourth in order to dial in the 3/4" wide slot.

Once I was happy with the slot, I cut the strip into 2" long sections. You will need two of these pieces for the puzzle.

Hole

You will need a blind counter-bore hole in one of the two pieces. The hole should be centered on the slot side of the workpiece. Take your time at this step since alignment is critical for the puzzle to work.

Once you have the hole marked, use a 3/8" Forstner bit to drill a hole to the depth shown on the drawing. I suggest using the steel ball as a gauge at this point. Drill short then do a fit check. Drill deeper until the ball sits exactly half way in the hole.

Next, use a 3/16" drill bit and drill the magnet hole per the drawing. Note that magnet diameter is 5mm which is approximately 0.197 inches. Since I wanted a press fit, the holes 3/16" (.188") worked perfectly. When drilling these holes, make sure to not break through the bottom of your workpiece.

Tube Assy

At this point, you should have two completed pieces. Do a fit check to see if everything lines up correctly. If all good, glue/press the magnet into the small hole. Clean up in burs or rough spots in the slot with sandpaper. Next, glue the two sections together. Add some identifier (pencil mark, indentation, etc.) to the tube side with the hole. Since the puzzle is symetrical, this mark will give you a clue on unlocking it.

Since nobody (especially me) is perfect, measure the actual X and Y of your tube. I was off by ~ 0.05" in one direction but that's ok. You will use these dimensions for the center section.

Center

Cut your center section to the measured tube size. I cut mine on the high end and hand sanded to get a tight fit. Later, I decided to paint the center piece. To guarantee fit, I sanded off another .003" from each side.

Drill the holes the same way as described previously. For this hole, the depth needs to be greater than the ball diameter. As stated previously, take time measuring these holes! Lastly, insert the magnet into the bottom of the hole. If needed, use a small dowel to push the magnet into the hole.

Clean Up

Sand pieces and paint/stain as desired. I used oil on the tube and dark red paint for the center piece. If you added paint, make sure the ball still drops into the hole nicely on the center section.

Ball

Drop the ball into the center piece. If you made it correctly, the ball should rest slightly below the surface of the block.

Lock the Puzzle

This is the scary part. Place the center section / ball into the tube with the tube/center holes in the same orientation. At this point, the center section should freely move in/out of the tube.

To lock the puzzle, you must smack the puzzle on a hard surface (see video) so that the ball moves from the center section to the tube. Recall that in this position, the ball is at its midpoint. This causes the center section to be locked inside the tube (see pictures).

Unlocking the Puzzle

Push the tube back and fourth. It should be locked. You can shake, toss, or spin it but it should stay locked. The solution is the same as the lock. Recall that I mentioned earlier about marking one of the tube sides. This is the reference for unlocking. With that side up, smack the puzzle against a hard surface. That hit will allow the ball to fall back in the center piece.