Lorenz SZ (Cipher Attachment) Wheel Set

Parts List

Inventory of all parts needed for a full, mounted set of 12 wheels:

Printed Parts

• Thumb Wheel (12)
• Separator (6)
• Assembly Wrench (2)
• End Cap (2)
• Stand Bracket (2)
• for each wheel x (A, B, C, D, E, F, G, H, I, K, L, M):
  • Wheel x Internal Gear
  • Wheel x Spring
  • Wheel x Number Wheel
  • Wheel x Cam Top
  • Wheel x Cams and Sprue

Metal Parts

• M8 threaded rod 350mm
• M8 coupling nut 25mm (5)
• M8 half nut, 4mm thick (12)
• M8 dome nut (2)
• M2.5 x 6mm countersunk machine screws (36)

Other

• Wooden base (at least 225mm x 120mm)
• Suitable wood screws or self tapping screws (4)
• Number strips printed on paper

Which Files You Need

If you use Autodesk Fusion, and you want to generate your own slicer files, all you need is the F3D file.  The models are adjusted for each wheel by altering the WheelSteps parameter using Modify → Change Parameters.  Also the left-hand 6 wheels (A to F) have teeth to engage with blocking levers, so you should use the models in the Blocking Wheel Parts component for those, and the models in the Nonblocking Wheel Parts component for the others:

If you use PrusaSlicer, you can instead use the pre-built models in the 3MF files.  The Thumb Wheel file should be printed 12 times, the Stand Brackets file includes both brackets needed, then there are two files for each Wheel (these include the appropriate number of Assembly Wrenches, End Caps and Separators):

• Wheel x Main Parts
• Wheel x Cams

Lastly, if you use a different slicer, you can use the STEP or STL files, named as in the parts list above.

To label the wheels with the correct numbering, you should print a page of number strips.  Use the A4 or US Letter PDF file as appropriate for the paper size of your printer.

Colours

The real Lorenz SZ wheel components are made of silver-coloured metal, the only exception being the number label strips which are white on black.  For educational purposes, it may be helpful to print the components in different colours to highlight their different functions.  A suggested colour scheme:

I also chose to print the stand brackets with a colourless filament, and match the thumb wheel colours for each wheel.

Assembling Each Wheel

There are 501 cams in total across all 12 wheels, so lining up each one individually before gluing the wheel shell together would be extremely tedious.  To help speed this up, each wheel's cams are modelled with a “sprue” intended to hold them together while they are inserted into the wheel, and then broken off after assembly.  This is similar in concept to a “brim” in the slicer, except it is thicker and only on one side.  It is printed very close to the cams, but not overlapping with them, so that it partly fuses with them but can be broken off easily.

In preparation for gluing the Cam Top to the Number Wheel, it may be helpful to sand any bumps off the mating faces with medium-fine sandpaper.  Before gluing, insert the Cams into the cam top.  The cams have a flat side (modified to help bed adhesion while printing) and a rounded side.  Leave the cams attached to the Sprue and insert the rounded sides into the cam top.  The cam top and number wheel can then be glued together with a few drops of CA glue (superglue).  After the glue sets, break off the sprue, and the cams should be able to be moved independently.

Next, insert the Spring into the number wheel - the gap under the spring arms should face away from the cam top.  Then insert the Internal Gear into the spring, with the protruding part facing away from the cam top (you may well be tempted to insert the protruding part into the hole in the cam top, but that's the wrong way round).

Centre the internal gear within the spring, and then place the Thumb Wheel over the protruding part of the internal gear, with the toothed drive gear facing outwards.  Line up the screw holes and fasten the thumb wheel to the number wheel using three M2.5 countersunk screws.

Finally, cut out the appropriate Number Strip (refer to the number of steps for each wheel given in the first table above) and glue it to the circumference of the number wheel, lining up the numbers with the cam positions.  If using a wet glue like PVA, be careful not to stretch the damp paper when wrapping it around the wheel, otherwise the ends may not meet up correctly.  Note that the orientation of the wheels alternates, so the direction of the number strips should also alternate to match.  Wheels A, C, E, G, I and L have the thumb wheel on the left, and wheels B, D, F, H, K and M have it on the right.

Mounting the Full Wheel Set

The wheels attach to the threaded rod using M8 half nuts in the hexagonal cavities of the internal gears.  Lightly press in the nuts initially, as tightening the wheels will fully seat them later.  When placing the wheels on the rod, pay attention to the alternating orientation of the thumb wheels, as stated above.

Thread on a coupling nut near the centre of the rod, then, from the right-hand end, loosely spin on wheel G (thumb wheel on the left), a Separator, wheel H, another coupling nut, wheel I, a separator, wheel K, a coupling nut, wheel L, a separator, and wheel M (thumb wheel on the right). Finish with an End Cap and dome nut on the rod's right end.

Insert an Assembly Wrench into the internal gear of wheel M and tighten it and the end cap against the dome nut.  Continue tightening each component towards the right, using pliers to turn the coupling nuts, but don't overtighten.

Now from the left-hand side, thread and tighten the remaining components, continuing to alternate the orientation of the thumb wheels:  wheel F, separator, wheel E, coupling nut, wheel D, separator, wheel C, coupling nut, wheel B, separator, and wheel A.  Add an end cap and dome nut on the left end.

If there's not enough thread for the dome nut, loosen the right-side dome nut a couple of turns and retighten everything.  If there is too much thread to tighten the dome nut against the end cap, you can add a normal nut or washers between them, or shorten the rod using a file or hacksaw.

Attach the Stand Brackets by slotting them into the internal gears of wheels C and K.  Fit them as close to square as possible, then gently twist them so that they sit flat on the wooden base.  If needed, loosen adjacent components to allow enough slack to square them up.

Finally, mark the location of the screw holes on the wooden base, drill pilot holes and fix down the stand brackets with screws of a suitable size for your base.

If any of the wheels come loose later, you can use the two assembly wrenches to tighten the wheels on either side of a separator together, turning the wrenches in opposite directions.

Notes

The overall dimensions have been checked against real Lorenz SZ machines at Bletchley Park and the National Museum of Computing.  Some details of the model have been adjusted for ease of 3D printing (e.g. chamfers on the blocking teeth) and no attempt has been made to model the internal structure of the wheels.

This project has been developed in collaboration with Martin Gillow of Virtual Colossus, who kindly provided initial ideas and helpful feedback.  Enjoy his virtual simulation of the Lorenz SZ here.