# Impossible dovetail puzzle - my version



## Technical Ted (Sep 18, 2019)

Since I finished up installing the 3 phase motor and VFD on my Rhodes shaper, I was looking for a project to do on it and came up with this. A small size (1-5/16" cubed) impossible dovetail puzzle. I used this size since it was the only size I could make from the brass and aluminum material I had on hand. To get the double parallel dovetails to come out right I had to go with a steep dovetail angle. I used 75 degrees. This is where the shaper really shines because you can make any angle you want.

I've seen posts and videos where others have made these on milling machines, using a dovetail cutter, even if the cutter was homemade i.e. Clickspring, but I've never seen where someone has done it using a shaper to cut the dovetails. Oh, I'm sure I'm not the first, but I've just never seen it.

Anyways, using a dovetail cutter on a mill, especially with a DRO, is pretty straight forward since the dovetails are located using the DRO and the cutter cuts the dovetail to size. Even if you offset to take finishing passes with the cutter you can still use your DRO. This is not the case with the shaper, so I developed a plan. I used my Bridgeport to square up the stock and then very accurately, using my DRO, rough out the dovetail slots with an end mill being very careful with proper location and sizes so I could use the sides of these slots for measuring both the location and size of the dovetails when cutting them with my shaper. This was a very tedious operation on the shaper, but it proved to work and work well. Lots of measuring... using mics, depth mics, gauge pins, an anvil mic and dowel pins. 

I only shot for two thousands clearance so the joints were nice and tight. One issue I had was with the magnets. I ordered some N35 6mm diameter rare earths from eBay. These babies turned out to be too strong! I tried turning long, small diameter ends on the locking pin, but the magnets still held tight. I'm glad I thought to test this without assembling the two halves or I would have ended up scraping the pieces! I ended up drilling a through hole in the pin and driving in a tooth pick. Using that sticking out about 3/16" on each end did the trick. I will probably replace this with brass or aluminum rod but the tooth pick was a quick and dirty thing to try just to see if the concept would work and it did.

I didn't spend much time polishing this, because I'm sure it will be beat up by friends and family playing with it! Fun project!




I used a matching set of precision V blocks to hold the pieces at the required 45 degree angle. I rough aligned on center by finding the corners with an edge finder. This worked surprisingly well and I only had minimal tweaking to do while sweeping the four sides with my indicator. 




I made careful work of both the sizing and location for the slots since these were used with dowel pins to locate and size the dovetails on the shaper.




I used a precision ground 15 degree angle gauge to correctly set the required 75 degree angle on the work head. 




The finish product. I hated to have to break all the edges of the dovetails because they turned out so tight and it looked nice. But, safety ruled. I certainly didn't want anyone to win the prize of cutting their finger if they figure out how to open it. They were like razor blades when I got done on the shaper.







This was a great exercise in patience! The shaper is very, very, slow work. But, I had a lot of fun!

Ted


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## Flyinfool (Sep 19, 2019)

Nice job.
I was looking at the 1/8 Ø X 1/16 thick magnets for this very reason.
I am still looking to score a chunk of brass big enough to make things like this out of.


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## Technical Ted (Sep 19, 2019)

Flyinfool said:


> Nice job.
> I was looking at the 1/8 Ø X 1/16 thick magnets for this very reason.
> I am still looking to score a chunk of brass big enough to make things like this out of.



Yes, I think that size magnet would work better. Those little buggers are strong!

If you plan on using a standard 60 degree dovetail cutter in your mill you'll need a fairly good size hunk of stock. IIRC, maybe around 1-5/8" square?

Ted


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## Janderso (Sep 19, 2019)

Very well done. I have yet to use my Rhodes shaper


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## Latinrascalrg1 (Sep 19, 2019)

Ok so is the locking ped the magnet or just a steel piece the magnet pulls on to release the illusion?


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## Technical Ted (Sep 20, 2019)

There are two magnets; one in the aluminum piece, one in the brass piece. You basically toggle the steel locking pin back and forth to lock/unlock. The magnet that holds the pin in the unlock position probably is not needed, but all the designs I've seen use it so I added it as well. The pin is released from the magnets by taping the cube on the side you want the pin to move to. I just hit it against the palm of my hand. 

Ted


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## Cadillac STS (Sep 20, 2019)

The wooden dowel through the metal pin is so the pin doesn't stick to the magnet and not release right?


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## Technical Ted (Sep 20, 2019)

Yes, the magnets I used are so strong that even turning a small point/stem on the metal locking pin wouldn't allow the pin to be released by taping... So, I wanted to come up with a quick and easy way of testing a material that is non magnetic to maintain an air gap between the magnets and the pin. 

I had a tooth pick in my workshop and that was the easiest thing to use because I could simply tap it in a hole drilled through the locking pin and easily cut it to length. Too short? No problem, just drive in another tooth pick. Cheap, easy, and it held tightly in place. 

I am going to replace the tooth pick with a brass rod Loctited in now that I've come up with a good length. There was a fine line between its' being held too securely and not securely enough and I think I've found the Goldilocks's length.

Using a smaller or less powerful magnet would help. I used 6mm N35 rare earths. If I were to make another, which I most likely will at some point, I would buy some smaller ones. They're cheap on eBay. 

Ted


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## stioc (Sep 20, 2019)

Very cool!


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