Cryptex for Christmas  

Posted by Christopher W. Geis

First, pictures.

This year for my sister's clue present, I decided to take a second shot at making a cryptex
Fully assembled and locked cryptex, with the letters scrambled.  To open it, you have to solve the clue. (Or break thy cyptex)

With the key partially retracted.  A scroll was inside, with directions to where
the actual present was hidden.

With the key fully retracted.

How it works

The working principle of this cryptex is actually fairly simple. I settled on 10 sided polygons for the outside, as this provided good proportions on each face for the letters, and kept the number of letter plates I would need to make and glue on to a reasonable eighty.  Each ring is constructed of three layers of 1/4" (approx. 6mm) aspen board, cut to shape with a laser.  Two of the three layers have a fully circular cutout in the center, with notches to make the rings ratchet as they turn.  The third layer has a smaller radius coutout, with a cutout that matches the radius of the outer rings.

When these rings are assembled end to end, and the notches aligned the "key" can be inserted or withdrawn.  If you rotate one or more of the rings, the notches will not longer line up with one another, and the teeth of the key will prevent it from being withdrawn.  In order to withdraw the key and open the cryptex you need to re-align all the notches, by setting each ring so that is spells the correct word at the marked position.

The rest of the structure is the two end plates that are joined together by what I refer to as spines.  The open end plate is capped by the decorative cap on the key, and the closed end plate is covered by a matching end cap.

Examples of all the major components.


Back in 2010, my sister was living when my sister was living in London,and I was trying to think if there was some way I could continue our tradition of making clue presents, since I had to bring it to her across the ocean, and had no idea what the house she was living in was like.  It occurred to me that one way to do this would be to make a puzzle-box or a cryptex.  Then I figured that no, I didn't know how to make something like.  After all how would you go about making a cryptex?  The necessary internal mechanism would be far too complicated.

Then I realized this wan't really true.  So I set about making one, with the tools I had available to me at the time, namely an ancient, cast iron scroll saw and a dremel drill, plus a couple of files and a sanding block.  The result was  a functional, if somewhat rough cryptex.  I was proud of it, even if it did fall rather short of my imagination.
The foam was added to hold a sonic screwdriver
The scroll saw I used was a wonderful tool, and it opened up a lot of possibilities for projects for me over the years, but it was somewhat limited.  Even more limited were my woodworking skills, as you can see.  In the end, it took so long to get the functional parts of the cryptex done that most of the decorative bits I had planned got omitted, and even the lettering was done with a brown permanent marker.

What I really needed (other than more skill working with wood) was a laser cutter.  And now, thanks to Twin Cities Maker, I have access to one.

Making the Design

Parts sitting on the laser cutter waiting to have the protective
masking tape removed.
To design for they laser cutter, I've been using Google Sketchup, since I'm used to a CAD-style interface.  For some simpler designs, I tried working directly in Inkscape, but I just don't have enough familiarity with illustration programs for that to be a good solution.   (The base point for my circle is the lower left hand corner of the bounding rectangle?  How do I combine shapes and DELETE the lines I don't want?  Aaaaaaaaaugh!)  I'm sure Inkscape is a great program: I just don't understand it. (Yet)

Assembly of all the interior parts. In the actual cryptex, I have
ratchet springs on all four spines.
For this project the 3D nature of Sketchup was invaluable.  I drew up each component as a separate file, and then created sub-assemblies before creating on large assembly of all the parts.  I could then check the integration of each part, and go back and edit the individual parts as needed.

Another great aspect of Sketchup for this is the Push-Pull tool, which makes modeling laser cut parts quite easy.  Draw up a shape in 2D, then pull it out to the thickness of you material, and you've got your part.

At first I tried working by coloring my parts with dark edges, and light faces, to approximate the look of the laser cut parts.  This works great for relatively simple parts, as it give you a good preview of the final look.  For a complicated assembly like this, though, it just makes it impossible to see what is going on.

One thing I really wanted to do with this cryptex was to make the rings ratchet as they turned, so that when you got one of the letter plates lined up with the indicator, it would "snap" into place.  That way, as soon as you got the word figured out, you could open the cryptex, without  fiddling around, trying to get the notches exactly aligned.

First ratchet design.  No good.
 Hence the notches you can see in the ring assembly on the right.  A laser cutter makes this sort of thing easy.  It hardly even slows the cut down to add the notches to the inner circle, and they come out perfectly spaced and consistently cut.  In the end, it turns out I should have made them a bit bigger but the idea was sound.

Much better!
The ratchet spring was a little trickier, and I sketched up at least a dozen ideas of how I might do it, before settling on a design.  I cut a couple to see if they would work, and lo and behold, they were useless for what I wanted. They were weak, and easily broken, and worse, the seemed to curl after cutting so that when the face of the spring was pressed against the outer ring, the spike didn't even touch the surface, much less engage with the notches.  So I went back and redesigned them.  This design was much stronger, and seemed work much better.
Ratchet springs assembled on the spines.

In the end, the ratcheting action I wanted didn't quite work out, due to a fault in the design of the open end cap.  In order to engage with the rings all down the length of the cyrptex, the ratchet springs need to be up against the inner edge of the rings.  This only works if the spines that hold the ratchet springs are in the correct position.  Unfortunately, to maximize the size of the opening in the end of the cryptex, I made mortises in the open end cap as open slots, rather than having them closed off.  This meant that I had to rely on the glue to hold them in position, and due to time constrains ( I had only a couple of days to cut my parts and assemble them before Christmas)  I used super glue to hold everything together.  I didn't get all four spines in quite the right position before my glue dried, and the slight mis-alignment of the spines, coupled with the undersized notches in the rings means that only a couple of the rings ratchet properly. This at least provides a guide to light up the rest of the letters with, and the ratchet springs do still keep rings nicely aligned in a column, rather than "floating" several millimeters around the center line like they do in the original one I made.  Its a significant improvement, but I think I can better it if I were to tweak the design and make another one. An improved design for the ratchet springs could also make the assembly less dependent on the precise alignment of the spines.

Old and new, side by side.
The letters for this cryptex are laser engraved on a very thin pine veneer, and then each letter plate was cut out with the laser.  I could then glue a letter to each surface of the outer rings.

This entry was posted on December 28, 2015 at Monday, December 28, 2015 . You can follow any responses to this entry through the comments feed .


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