Picture of 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.
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Step 1: How the parts appear separated

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of Chamfer
Chamfer both sides of the hole in each piece.

Step 16: Trim the ends

Picture of 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

Picture of 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.
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ron.baer8 months ago

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!

jadronx2 years ago
my grandfather gave me one when i was a kid, it never gets old
jšindýlek3 years ago
Hi All !

This Puzzle is similiar with: "Hanayama Cast News"

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

dombeef4 years ago
Some one copied your instructable, here is a link: http://www.instructables.com/id/the-centrifugal-puzzle/
Phil B (author)  dombeef4 years ago
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 Phil B4 years ago
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 (author)  dombeef4 years ago
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.
justbennett4 years ago
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 (author)  justbennett4 years ago
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.
grunff4 years ago
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 (author)  grunff4 years ago
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 Phil B4 years ago
That's pretty neat, thanks for the explanation. Must look out for some similar attachments for my saw :-)
JJYork4 years ago
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.
Phil B (author)  JJYork4 years ago
Thank you for your report and your comment. I am glad it was clear enough and everything worked.
Rob O4 years ago
I made a few of these for Christmas gifts and they were well-received.

One change I made was to use a piece of brass tube (ace hardware, cut w pipe cutters) instead of wooden dowel. It looked very cool and worked just like the others which had the dowel.
Phil B (author)  Rob O4 years ago
Thank you for the report. I am glad they have been well-received. The brass tubing is a good idea. I would not have thought the brass tubing has enough mass to work, but the dowels are low in mass, too. The brass would be very attractive with dark wood like oak or walnut. I have made a few to have on-hand. Everytime I demonstrate it for someone I end up giving it away.
albf14 years ago
great instructable, just to say, the centrifugal force doesnt exist, it literally means "force of inwards". what you mean to say is centripetal force, the outwards effect of motion+gravity . =)
Sabata albf14 years ago
Sorry, you have it backward. Centrifugal means outward or away from a center or axis while centripetal means inward or toward a center or axis .
albf1 Sabata4 years ago
Centrifugal force is quite a valid concept.
Newton's Third law of motion says that for every action there is an equal and opposite reaction.
To move in a curve, you need a force towards the centre. This is a Centripetal force but there must be an equal and opposite force (say on the string of a conker as you swing it) which is directed away from the centre - a centrifugal force. BUT, as soon as you cut the string to remove the centripetal force, the centrifugal, reaction force will disappear. This is similar to the force upwards on your backside as you are sitting in the chair. The force disappears as soon as you get up - it can't throw you up into the air.

If you are ON a spinning object (a spinning frame of reference) you will experience two forces. The centrifugal force will 'feel' as if it is throwing you outwards and if you drop an object it will appear TO YOU to go outwards. A stationary observer will see this object leave your spinning frame on a tangent. It is not thrown out- it will keep moving in the direction in which it was moving when you let it go (in that observers frame). But to you, it has been thrown out and will travel in a curve!
Coriolis force is a wierd one; it is only experienced if you try to move and you will be 'thrown sideways' as you move towards or away from the axis of rotation. As you move away from the centre your tangential speed is less than that of the place you are moving to (it is moving with the same angular velocity as you but its radius is greater). You will have the impression of being pushed to one side as you move towards the place you want to go to. This is why, when air flows from high pressure areas to low pressure areas, on the Earth's surface, it goes in circles to cause hurricains, depressions and tornadoes.
Sit on a child's playground roundabout or spin on an office chair to experience both forces - it is the Coriolis force that is the really disturbing one.
I have a feeling that Physics teachers ban the concept of centrifugal force because they don't feel confident to give (or improve on) the above explanation.
'Real' physicists are quite happy with it. Tension is the common name for the centripetal force. according to many physics lecturers, the centrifugal force doesn't exist.
the force in question would therefore be gravity propelling the blocks away from each other by means of sudden loss of tension/centripetal force (the dowels).
hintss albf14 years ago
you don't expect me to do coordinate substitution in my head while strapped to a centrifuge, do you?

The xkcd reference totally needed to be made. Thank you
skype, with my non-geeky friend, I asked him if he read about the new ubuntu release (wasn't even thinking about xkcd, I just asked because I was running the new release, as I am now), and you can probably guess his response...
Anthony3124 years ago
If I gave this to one of my friends and asked them to solve it, they would just break it in half and say, "They're in two pieces now. Puzzle solved."
Phil B (author)  Anthony3124 years ago
Have you considered finding new friendss with fewer anger issues?
hintss Phil B4 years ago
people normally chase others while holding scissors ready to stab them, right?

anyway, I make an interesting character for a cartoon my friend is working on :D
Tell them to solve it WITHOUT breaking it.
Kryptonite4 years ago
I finished it! Sadly it was a rushed job (I had one hour) and it doesn't line up quite right. It'll take another day to fix. Thanks though! :D
Phil B (author)  Kryptonite4 years ago
I have been there and done that. You may still be able to salvage it. See my longer response to blaisepascal below in which I described drilliing in from an outside end to remove the dowel pin and enlarge the hole in which the dowel slides followed by closing up the hole with a larger dowel. You may also be able to enlarge the hole under the notch so the dowel pins fall into them more easily. I have built several of these several different ways in an attempt to make the results as consistent as possible. I believe what I described in this Instructable is probably the most reliable way and is likely the way my friend in Germany used. (I cannot ask him because he is no longer living.)
I've finished a true copy now! All the pins fit and when it's put together you literally can not see them. I replaced the wooden pins with metal rods, the heavier material along with the smooth sliding surface reduces the angle required for the rods to slide from ~70 degrees to ~30.

Brilliant Instructable, thanks so much! I love making little puzzles like this. :D
Phil B (author)  Kryptonite4 years ago
Thank you for the report. I am glad you got a good one that works well. I thought about using some metal rod. I have some 1/4 inch aluminum rod left over from something. I am hesitant to try steel because it could develop some rust coating from humidity over the years. And, here in the USA we have a couple of large home improvement stores that are in all larger cities (Home Depot and Lowe's). They regularly stock square dowels. I started buying and using 5/8 inch square dowels for the basic stock, rather than going to all of the work of making square stock. The holes I am drilling in the setup from step 9 are about 3/4 of a millimeter off center, but there is enough looseness in the holes that the round dowels still slide into position very well. I have now made and given away seven of these.
They're a brilliant little gift!

I used 8mm round mild steel rod in a 12 mm hole, it gives a little bit of lee way but not too much.
Hobbytik4 years ago
This puzzle is already produced and sold for many, many years now.
With a lot of different names. But why do you call it "wooden centrifugal puzzle"? Now you are giving away the principle of this puzzle. Puzzlers have to find out that by their own! That's happy puzzling!
Best regards,
Ad van der Schagt
The Netherlands
(Puzzle collector/designer voor about 20 years now)
Phil B (author)  Hobbytik4 years ago
I did not know what to call this puzzle. I found it with an Internet search and some key terms. What I found called it a "wooden centrifugal puzzle." That was before I learned it had been designed by Nobuyuki Yoshigahara who called it a Dual Lock puzzle.
fnirt4 years ago
Thanks for the instructable! I made one of these over the weekend and by the second day I was able to spin it on my palm and open it. This way you can show it to folks, turn slowly around (spin it while your back is turned) and do the reveal -- the whole room doesn't get the answer all at once!
Phil B (author)  fnirt4 years ago
Spinning the puzzle on your hands is a clever idea. Thank you for tryiing this and for reporting back.
qwerty9874 years ago
Phil B (author)  qwerty9874 years ago
srsantafe4 years ago
Thanks a lot, I realy like puzzles, and this one looks preaty cool
Phil B (author)  srsantafe4 years ago
Thank you for your comment. The idea of the puzzle is simple enough, although clever. The challenge comes in making the parts precisely enough that the puzzle works. It should not be that difficult, but little errors and inaccuracies easily creep into the process.
Jayefuu4 years ago
Congrats you're on make!

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