Impossible Dovetail Cube

Technical Ted

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I played around with drawing one up using a 60 degree dovetail cutter (because that's what I have) and to get everything to come out right the cube size was bigger than any brass and aluminum stock I have on hand. So, now that I have my shaper I'm going at it again because I can make any angle/size dovetail I want.

I've got some 1-7/8" dia. bar so the biggest square I can make is around 1-5/16". With a 75 degree dovetail I can get things to come out alright. The challenging part is cutting the dovetails to the correct size, angles and location so they all line up, since there are three of them and they all have to be right for nice tight fits. I've got a plan I'm going to try using dowels to measure size and placement. Should be a fun project.

From playing around with the design in Fusion 360, you'll need to size things depending on your method of cutting the dovetail and, if you use a mill cutter, the size/angle of the cutter. In the video you'll see they use a pretty steep angled cutter.

Draw it out in CAD or paper and see what you come up with.

Have fun!
Ted
 

martik777

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Thanks
.
I think I'll try it on some hardwood first. Let me know how you make out

There are some on Thiniverse for 3D printing:
 

Flyinfool

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The cutter used in the vid was a 30° included angle. I did a short search of my favorite sources and did not find one.
 

T Bredehoft

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If I remember correctly (i's been about 55 years,) I used a piece of about 2 1/4" mahogany, cut in to two pieces equal length. I bandsawed the cavity in one piece the dovetail in the other. Both parallel the sides. Then glued it together. There was some fitting involved, but that 's not part of this discussion. Once the glue was cured (overnight) I turned the tablesaw blade to 45º and made two cuts, returned the blade to 0 and cut the other two sides. It looks just like your example except it's 6" long.
 

rgray

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Here is an old thread on this.
 

martik777

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Thanks, it did not show up on my search. I'm surprised Clickspring did not use the ball bearing and magnet on his cube.
 

RJSakowski

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I decided to lay one out in SolidWorks. I chose a face angle of 60º. which requires a cut angle of 67.7923º. A 60º cut angle would yield a face angle of 50.77º. A cut angle of 76º (common woodworking dovetail angle) would yield a face angle of 70.58º.

For those interested in making one, the relationship between the cut angle A and the face angle B is tan(A) = sqrt(2)*tan(B).
. Dovetail Cube.JPG
 

Technical Ted

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I decided to lay one out in SolidWorks. I chose a face angle of 60º. which requires a cut angle of 67.7923º. A 60º cut angle would yield a face angle of 50.77º. A cut angle of 76º (common woodworking dovetail angle) would yield a face angle of 70.58º.

For those interested in making one, the relationship between the cut angle A and the face angle B is tan(A) = sqrt(2)*tan(B).
I realize I'm not the brightest colored crayon in the 64 box of Crayolas, but you lost me... what is this cut angle? In my simple terms, a 60 degree dovetail has a 60 degree angle. Please elaborate.

Thanks,
Ted
 

RJSakowski

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I realize I'm not the brightest colored crayon in the 64 box of Crayolas, but you lost me... what is this cut angle? In my simple terms, a 60 degree dovetail has a 60 degree angle. Please elaborate.

Thanks,
Ted
The cut angle is the angle of the actual cutter. Since the path of the dovetail cutter is at a 45º to the faces of the cube, the apparent angle on the faces is less than the cut angle. BTW, cut angle is a descriptive term that I coined, not necessarily the correct term.
Here are two views of the base. The first is from a plane perpendicular to the cut path and the second is from a plane parallel to a face. The difference between the actual and apparent angles is clear.
Dovetail Base, view 1.JPG Dovetail Base, view 2.JPG
 

Technical Ted

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BTW, cut angle is a descriptive term that I coined, not necessarily the correct term.
OK, that clears things up. When I drew it up in Fusion 360, I made a custom view of the cube at a 45 degree angle so the actual cuts I needed to make on the mill (roughing) and shaper (finishing) were straight on since this is the way I had them oriented on my machines. I set and used 75 degrees for everything.

Didn't mean to hijack the thread, but this is all good info for anyone wanting to make one of these.

See my recent post here: https://www.hobby-machinist.com/threads/impossible-dovetail-puzzle-my-version.79744/

Thanks for the clarification,
Ted
 

Technical Ted

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Just to add to the content, the way I drew it up was the same as I had the pieces lined up in my machines... at a 45 degree angle. You can either turn your vise (or however you are holding them) to 45 degrees or use V blocks like I did. There are probably other ways of holding them and, of course, if you are using a CNC you can hold them any way you want with the proper programming.

Dovetail View.jpg

Using Fusion 360 I created a construction plane at 45 degrees and drew everything up in that view. In the view above, the center is looking head on to one of the corners. Everything is inline and no need for additional calculations for machining.

Also, I kept the two partial dovetails on the outside edges of this view at 1/2 the dovetail width. That makes everything come out right and look proportional.

Ted
 
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