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.

Step 1: How the Parts Appear Separated

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.

Step 2: Readying the Puzzle to Be Solved

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. 

Step 3: Tumble

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.

Step 4: The Solution

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.

Step 5: To Make Your Puzzle

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.

Step 6: Make the Pieces Exactly Square

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.

Step 7: Mark and Cut Two of the Pieces

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. 

Step 8: Saw and Save the Scrap

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

Step 9: Mark and Drill Holes Into the Ends of the Short Pieces

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.

Step 10: Cut Dowels and Chamfer the Ends

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.

Step 11: Prepare to Glue

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.

Step 12: Glue and Clamp for Drying

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. 

Step 13: Prepare to Drill Through the Long Piece

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. 

Step 14: Drill the Holes

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.

Step 15: Chamfer

Chamfer both sides of the hole in each piece.

Step 16: Trim the Ends

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

Step 17: Finish Sanding

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.

Comments

[Erwin-J]

Hi Phil,
Nice instructable, I see it has been on here for quite a while now.
I came upon it while looking for '2 piece wooden puzzle centrifugal'.
The reason I was looking for that is that I have bought such a puzzle about 25 yrs ago on a Czech market while on holiday. I recently found it back in some 'junk-drawer' in my attic after forgetting about it for many years. Now I found it back and had be fiddling with it for a while I figured it would be nice to try and make one myself.
But as I am not quite sure how it was constructed exactly I was looking for some kind of instruction like yours.
But reading your instructable I realise my puzzle, although it looks identical on the outside, even if the 2 parts are taken apart, is a bit more elaborate, and that was just the thing I would have liked to see worked out. (I don't want to take my puzzle apart to actually check how it is done exactly)
The thing is, your puzzle can be opened simply by rotating it on a table, and just take the two halves apart.
In my puzzle there is an extra locking mechanism that allows only the top 2 dowels to move to the outside when you spin it. You then have to flip the puzzle on the table on its other side (carefully rotating it over the center of the longest part if you look on top of it), spin it once more, so the 2 dowels, that were at the bottom, but now at the top, can also be moved to the outside. If you did it correctly all 4 dowels are now on the 'outside' and the 2 halves can be taken apart.
This makes it very much harder to 'accidentally' find the solution and adds to the mystory.
Even if you know you have to flip it , but rotate in the wrong direction, it will not open because the first 2 dowels you spun to the outside in the first move will fall back in the centre. But I do have a pretty good idea how it is done, using a couple extra dowels, I just made a sketch of a section of one part, have a look. So what do you think, new challenge? ;-)

Regards,

Erwin.

[Phil B]

Erwin,

Thank you for your comment. I had a discussion with another person who commented about a puzzle that had to be correct side up to open. At the far inside end of the dowel channel was a short vertical shaft with a 1/4 inch ball bearing in it. When one side was up, the ball bearing fell out of the way and the puzzle opened normally. Turned over, the ball bearings kept the dowels from moving far enough out of the way and the luzzle remained locked. Is that possibly what you have in the Czech puzzle?

[Erwin-J]

Hi Phil,

I had thought of that, because if you replace the short vertical dowels in my sketch by ball bearings, that would definately have the same effect, and because of weight and smoothness far less likely to seize in the wooden channel.
But just going on the sound it makes when you make the small dowel/ball bearing move I'd say it is wood too, hence I thought of a short dowel. I tried a (strong!) magnet on it, and no effect. Might still be a plastic or non ferro metal ball though.

In my puzzle both parts are equipped with this extra gimmick, so it is not a matter of putting it on the right side, but you really have to spin the puzzle twice, once on each side and when you flip the puzzle after the first spin you must take care the bottom dowels can't slide back to the center, or you have to start all over.

This puzzle is made in some (semi)hardwood so the bored channels are quite smooth. Despite its small size (parts are 4" each) it works flawlessly.
Although it's simple to understand once you've seen it, a genius construction!

[H-Wesley]

So I saw this Instructable about a week ago, so I decided to make it for my grandpa for Christmas. I did a few things that might be interesting to you. First, I used metal pins. Quarter inch steel, which I thought (and was right), that they would be heavier and therefore easier to be pushed outwards by Centrifugal force. I also didn't want wooden pins to stick on the holes and not slide when they needed to. I also used Maple, which, reading through the comments seemed like I wasn't the only person to use a different kind of wood. When I drilled the first set of holes, I didn't get them perfectly centered on the sticks, which made it much more difficult to drill the holes in the sides later on. Any tips for making sure the sticks are perfectly square before drilling?

[Phil B]

You have identified the frequent problem I have had making these puzzles. I tried a variety of approaches. The grain in the wood seems to make the hole go off center and drift. Finally, I made a jig and wrote it up as an Instructable.

[H-Wesley]

I used my 8" Drill Press to drill the holes, but I didn't get the center. I rotated the table to the top was facing to the side, then I set our fence, and C-clamped the wood into place. The Idea was to get the same hole in each piece, which I did achieve, even if they weren't centered. Have you had any of the problems I mentioned above, with the wooden pins sticking in their holes?

[Phil B]

From the Instructable I linked, I drilled the holes with a bit a little over sized. The bit was 19/64 inch and the dowel was 1/4 inch (16/64). I did not have problems with dowels sticking. I did position the pieces with the cross piece in place and a dowel running from one side through to the piece on the other side. That way the holes all met. The holes needed to be on center as much as possible so the holes would still work if the cross piece was reversed end for end. I hope that helps.

[ron.baer]

I am just starting to get into woodworking, carving and restoring things, mostly my father and grandfather's wood things, mostly bookshelves and coffee tables. Once I am done with my latest resto project I am going to be making one of these for MY grandchildren, and quite a few of the other wooden puzzle/game items I"ve found here! Cheers, ron!

[jadronx]

my grandfather gave me one when i was a kid, it never gets old

[jšindýlek]

Hi All !

This Puzzle is similiar with: "Hanayama Cast News"

like this: http://hlavolamy.heureka.cz/hlavolam-hanayama-cast-news/

C.

[dombeef]

Some one copied your instructable, here is a link: https://www.instructables.com/id/the-centrifugal-puzzle/

[Phil B]

Thank you for the message. I saw the new Instructable on the wooden centrifugal puzzle that you linked. His design and construction approach is different from the Instructable I did. The situation reminds me of the numerous laptop supports, knex crossbows, wifi cantennas, and other frequently repeated projects at Instructables. Each of us has the right to present a variation in design that might suit someone else better. My only surprise was that my earlier Instructable did not show in the "Related Instructables" on the right side of the screen. Oh, well...

[dombeef]

Should you ask atleast for credit? It is clearly like your design, and the person is not commenting to what I said(it looks like Phil B.'s design, etc) , yet he comments to other people way after I posted the comment.

[Phil B]

I was actually copying one given to me by a German friend about ten years ago. Eventually, they all go back to a Japanese man mentioned in the Introduction. The part I worked out was a procedure for making them reliably with the tools I have. I see a couple of people commented that there is an earlier Instructable on the same puzzle. I do not know if he was aware of that or not. Several times I have published something that is very similar to another Instructable by someone else. I link that other Instructable and then explain why and how mine is different from the earlier publication.

[justbennett]

Upon first viewing, I did not realize that these pieces were formed by gluing. While I was trying to figure out how you drilled the holes inside the notch (I'm no woodworker!) I came up with a different idea for the design of this puzzle.

I thought the holes could be drilled from the outside ends of each piece. Then you would replace the appropriate amount of wood with a dowel glued in place. Make sense?

It's only a slight difference in the result and, if you can hide the dowel on the end grain it would really add to the mystery. Thanks for sharing!

[Phil B]

I did experiment with what you suggest. There is a tendency for the wood to have different densities within it and these cause the drill bit to drift. That means the holes do not align precisely enough for the dowel pins to move back and forth as well as they need to. I found the less drilling the better. Drilling from the outside ends increases the probability of the bit drifting off center. Thank you for your comment.

[grunff]

Great instructable, thanks for writing it up. But even more impressive is your ingenious use of a radial arm saw as a sander and a drill! Would you care to share how you attached the sanding drum mandrel and the drill chuck to the saw's spindle?

[Phil B]

Thank you for looking and thank you for your comment. I am not as clever as you suppose. The back end of the motor shaft on Craftsman radial arm saw came from the factory with a 1/2 inch x 20 thread spindle about half of an inch long. It is made to accept a 1/2 x 20 thd. drill chuck. Sears also sold a 3 inch long sanding drum 2 inches in diameter designed to attach to the same thread specifications.

[grunff]

That's pretty neat, thanks for the explanation. Must look out for some similar attachments for my saw :-)

[JJYork]

I enjoyed making your Instructable. Your directions are clear and easy to follow. My puzzles worked the first time. I did use metal pins (stainless) that others suggested and I also used instant glue so I didn't have to worry about sanding and later changing my dimensions.