Three Rotor Cipher Demonstrator (Enigma style teaching model)

Press a lever, watch the rotors step, and follow a letter through the wiring with your finger. This is a rotor cipher you can see inside.

What it is

This is a simplified teaching model of a three rotor cipher machine in the style of the Enigma I. It works like a paper Enigma in 3D. Three rotors, a fixed reflector and a fixed entry plate sit side by side on one 5 mm rod. Each rotor uses the historical wiring of Enigma rotors I, II and III with turnover notches at Q, E and V, and the reflector uses the historical reflector B wiring.

Every rotor rim is engraved with its wiring. In each row the right letter is the contact you arrive at and the left letter, past the arrow, is where that wire goes. You trace a letter from right to left through the three rotors, bounce off the reflector and come back from left to right to the answer. The trace card explains the steps and has a lampboard style grid to mark your results.

Each rotor has an alphabet ring that you can lock to the wiring core at any of 26 offsets with a small pin, so ring settings work just as they did on the real machine. The turnover notch is on the ring, so it moves with the ring setting too.

The lever drives three pawls. The right rotor steps on every press, the middle rotor steps when the right rotor reaches its notch, and the left rotor steps when the middle rotor reaches its notch. Because of the way the pawls sit, the middle rotor also steps a second time on the next press: the famous double step of the real machine. The rotors step before you trace, as on the original. Printed springs give each rotor a positive click, and a printed spring returns the lever.

It pairs with the free cipher games at dexmlabs.app, so you can check your traced letters on screen. A plain Python reference script with test vectors is included too. With rotors I II III, reflector B, rings AAA and start AAA, five presses on A give BDZGO, the standard check value.

What it is not: there is no plugboard, no keyboard, no lamps and no electrical contacts. The rotor order and the reflector are fixed. It is a model for learning how the wiring and the stepping work, not a replica.

A little history: the Enigma was a rotor cipher machine used in the Second World War, and the codebreakers at Bletchley Park, building on Polish work, read much of its traffic. This model is my own simplified design for teaching, uses only public historical facts, and is not affiliated with any museum or company. "Enigma" is used only as a historical description.

What you get

* ✔ 3 rotor cores with the wiring of rotors I, II and III engraved on the rims

* ✔ 3 alphabet rings with turnover notches at Q, E and V, settable at any of 26 offsets

* ✔ Fixed reflector B and fixed entry plate, both with engraved letter rims

* ✔ Lever with three printed flexure pawls and the true double step

* ✔ Base with printed detent springs, index bar with pointers at the window row

* ✔ Trace card with a lampboard style grid and the tracing steps

* ✔ Fit coupon, optional printed axle

* ✔ README with print settings, assembly and a worked example

* ✔ CONVENTIONS.md, SOURCES.md and a plain Python reference script with test vectors

* ✔ OpenSCAD source for every part

What you need

* About 330 g of filament (estimate) in up to three colours

* One 5 mm round rod, 176 mm long, or print the included axle

* 4 M3 x 12 countersunk screws and 1 M3 x 16 screw

* Acrylic paint for the engraved letters (optional, but it makes tracing much easier)

Print settings

* PLA for most parts. PETG is better for the base, the pawl carrier and the lever arm, because they carry the springs.

* 0.4 mm nozzle, 0.2 mm layers, 3 walls, 15% infill

* No supports. Every file is already in its print orientation.

* Print the fit coupon first to check the rod holes, the pin holes and the spring gaps on your printer.

* Every part fits a 220 x 220 mm bed. Estimated total print time is about 20 hours.

Assembly

1. Pin each alphabet ring to its core at the ring setting you want.

2. Hold the reflector, rotors I, II and III and the entry plate between the arms of the pawl carrier and push the rod through.

3. Fit the lever arm with its stop pin in the entry plate slot and screw it to the carrier.

4. Set the whole unit on the base and screw the two stands down from below.

5. Drop in the index bar and stand the trace card in its groove. Full notes are in the README.

How to encipher a letter

1. Roll each rotor towards you until the start letters show at the pointers, for example A A A.

2. Press the lever. The rotors step first, just like the real machine.

3. Find your letter on the entry plate and trace it right to left through the rotors, using the right letter and the left letter of each row.

4. At the reflector, read the partner letter and come back left to right.

5. The row you arrive at on the entry plate is your cipher letter. Mark it on the card.

To decipher, set the same start letters and trace the cipher letters. The plain text comes back out.

Please note: this is a simplified teaching model. The files have been checked carefully in software for fit, clearances and stepping, but I have not made a test print of this version, and the pictures are renders. The springs and pawls are small printed flexures, the bottom rows of the rotors face the base so you lift the front to read them, and changing a ring setting means taking the rotor off the rod.

If you print one, I'd love to see it. Post a make with a photo, good or bad, and it helps me improve the next version.

Happy to tweak the design on request.