Wooden Centrifugal Puzzle

This is a wooden puzzle.  The two parts lock together and you give it to a friend, asking him to unlock and separate the two parts.  A friend in Germany made this one.  When he handed it to me to solve, I could not until he showed me the secret.  This Instructable will not only show how the puzzle is solved, but will also show how you can make these to give as gifts, especially at Christmas.

I have learned through a link by Make! Magazine that this is called a Duallock Cross Puzzle and it was invented by Nobuyuki Yoshigahara.

Here you see the two halves of the puzzle separated.  Each half has a center notch and a hole below it.  Internally, there are two holes in each with a piece of dowel pin that can move freely in each hole.  The dowel pins are each a bit longer than the center notch.  The center notch is the width of the wood's thickness on the parts.  I will use some 3/4 inch clear pine.

The person who solves the puzzle should see you assembling it.  He should know about the two dowel pins.  To assemble the parts, push the dowel pins back into their holes.  Invert one half of the puzzle and let the parts settle into the two notches. 

Tumble the assembly back and forth so the dowel pins move through the holes in the mating parts.  Hand it to a friend and ask him to solve the puzzle.  If he has seen this before, or is very smart, he will solve it right away.  If he has not seen it before, he will struggle with it until you show him how it is solved.

I hinted at the solution in the title of this Instructable.  Place the locked puzzle on a flat surface.  With your finger and thumb, spin the puzzle as rapidly as you can.  This creates a centrifugal force that flings the dowel pins outward from the center and releases the two pieces so one can easily be lifted from the other.

You can find these for sale on the Internet.  But, if you want to make your own, here are the steps.

I have cut a 1 x 6 piece of clear pine to a length of 7 inches.  The wood is actually 3/4 inch thick.  In the USA the one inch dimension refers to its thickness before it is planed to its finished thickness.  But, we still call it one inch lumber, or "one by...."  I am using a bandsaw because there is less wasted wood than there would be from a wide saw kerf.  I am ripping the wood to a little more than 3/4 inch because the marks left by the saw teeth will need to be removed.  In addition to some 3/4 inch lumber, short sections of 1/4 inch dowel rod and a little glue will be used.

I am using a sanding drum on my radial arm saw and a small table that raises the work level for more convenience in order to make the pieces exactly square and free of marks from the saw teeth.  The final dimension is not critical, but will be slightly less than 3/4 of an inch when completed.

The easiest way to make this puzzle is to glue two shorter pieces with a gap for the notch, gluing them to one of the longer pieces, and then repeating the process for the other long piece.  I am using the puzzle half my friend in Germany made in order to mark where to cut. 

Saw as squarely as possible.  Save the scrap piece from the middle.  It will be useful soon in a couple of ways. 

I am using my small riser table and a drill chuck on my radial arm saw to drill holes into the ends of the short pieces cut on the bandsaw.  Notice that I have clamped a piece of scrap to the top of the riser table.  This functions as a fence so I can slide the work into the drill bit.  Great care needs to be taken to insure that the holes are as near to centered on the ends as possible.  This is so the dowels will align with the holes in the rest of the puzzle and the puzzle pieces will work.  During setup I put a very small size drill bit into the chuck and used the scrap pieces to test how close to the center I was.  If you look closely at the photo, you can see I marked an "X" on the pieces.  The "X" goes down against the table surface when drilling.  If there is any error in centering, the error will compliment itself in all of the pieces, if the same orientation is preserved, and not work against itself to multiply as the project progresses. 

Drill the holes to a diameter of at least 9/32 inch and a depth of about 1 5/16 inches.  Check to be certain the dowels slide in and out of their holes freely and quickly by means of their own weight.  Make the holes a little larger, if necessary.

Update: I began to make more of these puzzles than I anticipated and wanted an easier way to drill these holes.  I made a special jig that also frees my saw for other things.

Cut 1/4 inch dowel pins 1 1/4 inch long.  You will need four pieces of dowel.  In the photo I have installed my sanding drum on my radial arm saw and have placed a dowel in a drill chuck.  When both are running it is easy to make a light chamfer on each end of each dowel.  This makes it easier for the dowels to slide in the puzzle's holes.

Insert a dowel pin into each hole.  Place the short pieces of 3/4 x 3/4 on the long pieces as shown with the "X" mark down against the surface of the long piece.  Use the little scrap block as a spacer to insure the notch will be wide enough to receive the other half of the puzzle.

Smear a light coating of a good wood glue on both surfaces where the pieces will make contact.  Clamp until the glue has set pretty well.  Then you can remove the clamps and scrape away any excess glue.  Scrape away any glue in the inside corners of the notches. 

Place one half of the puzzle over the other.  I drew a line freehand between the centers of the dowels.  The center of the hole will be on this line.  I marked the sides of the notch, too.  The center of the hole will be midway between the sides of the notch.  This hole will be 5/16 inch in diameter.  That is over-sized and will make up for slight inaccuracies. 

In order to be as accurate as possible, make a small dimple with the tip of the spinning bit and check to be sure it is on the center.

Chamfer both sides of the hole in each piece.

After gluing pieces together in step 12 the ends may not be even.  Trim a little from the ends.

Even after scraping away excess glue in step 12 there may be some uneven edges along the glue joint or some glue smears.  Use a sander, like the belt sander shown here, to remove irregularities.  Also use some sandpaper to round square edges on the two halves.  If you want to apply a finish, use an oil finish or a stain.  My German friend simply left his unfinished.  A varnish will make tolerances smaller and could get into the dowel holes where it might glue the dowels to the insides of the holes. 

You are now ready to enjoy your new puzzle and show it off to others.