Introduction: 3D Printed Cryptex With a Twist

Repeatedly over the last few years I have looked into cryptex. They are a fun way to lock up small items inside a combination locking cylinder. The way they work have always intrigued me. My mechanical mind is always drawn to reverse engineer things of this nature. Since getting a 3D printer making a cryptex has always been on my list of projects. There are many examples of how these work and how to make one all over the internet. Two examples I really liked is by SavageRodent on Thingiverse and pjensen here on Instructables. Although I looked at many styles of crytex, these two were what I jumped off from to create my own.

I will continue with my thoughts on how I designed each part and the unique features that I included. I hope that you can use this to help create your own design. If you desire, I have included the .STL files for each part and you can skip to step 7 where I start the prints and assemble the cryptex.

Step 1: Design Features

I am always looking at how I can improve on any design and make it my own. When you look at a basic cryptex design, you have a series of rings with letters, numbers or both as the combination. These rings interfere with a straight row of pins/lugs that prevent the cryptex from being opened without the correct fixed combination.

The following features are a mix of others and my own designs:

  • Programmable combination:
    • By adding an outer ring with a key that is inserted into a key way on the inner ring, the combination can be changed at any time to keep the cryptex secure.
  • All 3D printed construction:
    • To keep the rings from sliding off the cylinder a locking ring is added to the end that could easily be attached with fasteners or hardware. To keep all non 3D printed parts out of this project, a locking ring with lugs is used that can be easily removed to change the combination but secure when the cryptex is locked.
  • Indexing rings:
    • Watching numerous videos on Youtube, one common thing that is seen when entering the combination is the need to hold all the rings you have entered in one hand while continuing with the other so as to not move adjacent rings through the friction between them. With the addition of flexing tabs on the inside of the rings, the rings will register to flutes in the outer cylinder holding them in the desired position as the remaining combination is entered.
  • Helical pin channel (There's the twist!)
    • All cryptex that I have seen have one or more straight row(s) of pins/lugs. By entering the combination in, you align a straight channel for the pins/lugs to move through unlocking the cryptex. To shake things up, turn that straight line into a helical path opening it like a screw. Those attempting to solve the cryptex will not expect it.

Step 2: Outer Cylinder

My main goal with the following steps is show my thought process into designing each part. I will included images of each step for the different parts. They are meant more as a guide to my process and not to be copied exactly. I used Solidworks to create the 3D models. There are many other CAD software's that will work just as well.

To start, pick a part to establish a size. This will determine the sizes for the other parts. It could be the inside diameter of the chamber where you will place your items to be locked up or it could be the overall diameter of the cryptex.

For me I started with the overall diameter which is the end cap of the outer cylinder. This is where little details and artistic style can be used. The end cap can be fancy like the one from The Davinci Code or it can be simple like mine with a flat bottom. This is mainly for ease of printing. I did not want to many supports in the prints. Add an arrow to indicate where the combination should be entered. Because this cryptex has the helical path for the pins, the combination will need to follow this same path. You will notice in one of the images I rounded over the detail, which there are 26 of, just behind the arrow. This will help indicate the start of the combo. You will see where I take this later.

Moving to the cylinder, size it to leave enough room for your rings to sit flush with the diameter of the end cap. on the surface of the cylinder there will be the flutes for indexing the ringers. Size the depth of the flute to allow enough friction to hold the rings in place and still rotate them when needed with ease.

Now for the channel the pins slide through. This helical path needs to follow the same path the letters make as they move one position counter-clockwise around the ring. My ring includes the 26 letters of the alphabet which is one letter ever 13.85 degrees or simply 360/26. Each ring is 1/2" thick with a total of 8 rings. The helix rotates about 110.77 degrees over the length of 4". This ends up being a pitch of 13 with one full rotation. This helix will be used with all the parts.

Lastly 2 holes will be placed at the end for the lock ring to engage with. location of these holes might be easier after you have designed the lock ring.

Step 3: Inner Ring

The inside diameter of this ring will be determined by the size of the outer cylinder. What I have found for a tight enough fit that will still allow the ring to rotate freely is an increase of .02" for the clearance needed. Add a shoulder for the outer ring to sit against and acts as a spacer between the outer rings. The ring will need to be thick enough to cut a space for the pin to ride through as you rotate the ring. The height and width of this space will be determined by the size of pin you would like to use.

Using the same helical path from before, add the pin channel that allows the cryptex to be unlocked. Where the outer ring slides over the inner ring a number of key ways will need to be added to be able to position the desired letter over the pin channel (program). The profile of these rings just needs to match the key that will go on the outer ring. The number of key ways will be how ever many letters, numbers or characters you choice.

The most important feature on this ring, which is my favorite of all the parts, is the indexing finger. This finger will need to be long and thin enough to flex in and out of the flutes on the outer cylinder. For the size of my ring I found that the length of 45 degrees of the inner diameter and .2" thick gives me just enough spring. A minimum of 2 will be need because at some point 1 finger will fall into the pin channel making the ring loose without the other finger(s).

Step 4: Outer Ring

This part is fairly straight forward. Size this ring with the same clearance for the inside diameter and matching the outside diameter of the end cap on the outer cylinder. Add flats to the outside of the ring matching the number of letters, numbers or characters you are using. This makes cutting the characters into the ring easier later.

Centered under the first position, add the key that will interface with the key ways on the inner ring.

Adding the characters to each face can be tedious. I just centered text with my desired font and used this profile to cut into the ring. It does not have to be really deep to stand out. To finish the ring off, cut shallow grove to separate the characters.

Step 5: Lock Ring

This ring will match the same profile of the inner ring. Two similar flexing fingers will be cut in. On the end of these fingers a lug will be added set back from the small side of the ring. I found that the lugs do not need to be too tall. To make pushing the lock ring over the end of the outer cylinder add a chamfer on the side of the large end of the ring.

Again, add the same pin channel as you did in previous parts using the helical path.

Step 6: Inner Cylinder

The end cap for the inner cylinder is going to be identical to the outer cylinder with a pocket cut out for the lock ring to slide into and one very important change. The placement of the arrow will remain inline with the other to make others think the combo needs to be entered in a straight line like normal, but the real indicator of where the combo ends will be identified by the one rounded detail like on the outer cylinder. This one will be 7 of those details counter clockwise to be inline with the true end of the combo. Just something to trip people up.

Moving on to the cylinder itself, size it to fit inside the outer cylinder with the .02" clearance. The final step is adding the pins in the same helical path as always. This can be a little tricky and I'm sure there are many ways to accomplish this. My way is to create a plane perpendicular to each pin. Getting the first plane at the right angle is crucial. The pins need to be placed in relation to the arrow on the end cap so that when the cryptex is closed the 2 arrows line up. After that, create the other planes using the previous plane, angled the same as the characters are apart. Make sure that the pins then land in the spaces cut away in each ring to allow the ring to spin.

That finished the design.

Step 7: Printing

The 3D printer that I use is a Creality CR-10 and I use CURA to slice the models.

I plan on leaving the parts unpainted. I printed all the parts black except for the outer rings. Those were printed in white.

The following settings will be used for all parts except as noted:

  • layer heigth: 0.1mm
  • Infill: 25%
  • Extruder Temp: 200.0 degrees Celsius
  • Build Plate Temp: 65.0 degrees Celsius
  • Flow: 100%
  • Build Plate Adhesion: Brim

Supports will be needed for all parts except the outer ring

Orient parts to minimize supports.

I suggest, if possible, printing multiples of the rings in one print with the option of printing one at a time. This will save time starting prints over and over.

Step 8: Post Processing & Assemble

With all parts printed, remove the brims and supports where needed. Special care needs to be taken with the pins on the inner cylinder not to break them off with the supports. There will be a thin line of supports in the tiny gap of the fingers on the lock ring. Use some kind of thin metal to break the supports out from the gap. I ended up using one of those flexible metal rulers with the cork on the back. It pushed out the supports nicely.

Do a quick test fit of the rings on the outer cylinder. Make sure to have the inner and outer rings together. Depending on the quality of the finished print the width of the rings might be slightly to big and you will not be able to fit the lock ring on. To fix this, tape or glue a large sheet of sandpaper to a table or piece of wood and sand down the sides until the two rings are flush. Repeat this process until the lugs of the lock ring can fall into the holes without really forcing it to much. Don't be hasty and remove to much at once. The rings will be too loose in the end. Before placing the lock ring in place for the final time, make sure to program your code by positioning the desired letter above the pin channel on each ring.

Make sure that with the correct code entered, the inner cylinder slides all the way into place and each ring can spin freely. If you find any resistance you make need to file down one or more if the pins.

Thank you for your interest in my instructable. I would be honored if you feel that this cryptex is worthy to vote for me in the Epilog X contest.

Thank you again and I would love to see your prints.

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