Mechanical Rotor Stack Assembly Design Spec - thepinkmile/Enigma-NG GitHub Wiki
Rotor Stack Assembly — Mechanical Design Specification
Status: Draft Project: Enigma-NG Author: Izzyonstage & GitHub Copilot Version: v0.1.0 Associated Hardware Revision: Rev A Last Updated: 2026-05-07
1. Overview
The Rotor Stack Assembly defines the mechanical and dimensional properties of a mini-stack: the fundamental operational unit of the Enigma-NG rotor system. A mini-stack consists of five individual rotor modules stacked along a shared central shaft, bounded at each end by an enclosure board (STA/EXT at the low end, EXT/REF at the high end). Multiple mini-stacks are chained through Extension boards to form the full 30-rotor machine.
This document captures:
- Mini-stack grouping and boundary definition
- Inter-rotor PCB-to-PCB separation (connector-driven, not a free mechanical variable)
- Dimensional constraints the inter-rotor gap must satisfy
- Why total mini-stack depth remains TBD pending STA/EXT/REF enclosure details
This document does not define the individual rotor module. For the individual rotor assembly (shroud, bearings, encoder slots, internal headers), see
design/Mechanical/Rotor/Design_Spec.md.
2. Mini-Stack Definition
| Parameter | Value | Notes |
|---|---|---|
| Rotors per mini-stack | 5 | Fixed; mechanical carry propagates within the group |
| Low-end boundary board | STA (first group) or EXT (subsequent groups) | Carries ERF8 female sockets for Rotor 1 ERM8 male headers |
| High-end boundary board | EXT (intermediate groups) or REF (final group) | Carries ERM8 male headers for the final rotor's ERF8 sockets, or the passive Reflector |
| Total mini-stack depth | TBD | Cannot be defined until STA, EXT, and REF enclosure thicknesses are confirmed (see §5) |
The carry mechanism is mechanical within a mini-stack. At each Extension boundary, the mechanical
carry event is converted to an ACTUATE_REQUEST signal that regenerates a servo step for the next
mini-stack. See design/Mechanical/Rotor_Actuation_Assembly/Design_Spec.md §7 for the carry
mechanism detail.
3. Inter-Rotor Connector Stack Height
3.1 Connector Selection and Stack Height
Adjacent rotor boards are connected via the Samtec ERM8/ERF8 Edge Rate® vertical mezzanine connector pair. Each rotor's Board B carries ERM8 male headers (J1–J3); the adjacent rotor's Board A carries ERF8 female sockets (J4–J6).
The board-to-board separation between adjacent rotor PCBs is entirely determined by the connector
lead style (the -XX.X- suffix in the part number). The relationship is additive:
Board-to-board gap = ERM8 lead style height + ERF8 lead style height
This is confirmed by Samtec's own specification: the stated stack height range for the ERM8/ERF8 family is 7mm–18mm, where 7mm = minimum ERM8 lead style (2mm) + minimum ERF8 lead style (5mm), and 18mm = maximum ERM8 (9mm) + maximum ERF8 available (9mm).
3.2 Current Design
| Parameter | Value | Part Numbers |
|---|---|---|
| ERM8 lead style | 5mm (-05.0) | ERM8-005-05.0-S-DV-K-TR |
| ERF8 lead style | 5mm (-05.0) | ERF8-005-05.0-S-DV-K-TR |
| Total PCB-to-PCB gap | 10mm | Confirmed from datasheets (DEC-059) |
The current -05.0/-05.0 selection represents the minimum inter-rotor gap achievable for this connector pairing. It was selected to minimise the overall rotor stack footprint pending mechanical design validation.
This 10mm inter-rotor gap has not yet been validated against the full list of mechanical clearance requirements in §4. If mechanical testing reveals the gap is insufficient, a DEFECT-DEC must be raised (see §3.3).
3.3 Available Stack Height Options
The following lead-style variants exist in the Samtec ERM8/ERF8 family. Increasing the stack height
requires changing one or both connector MPNs; this must be formally recorded as a DEFECT-DEC
(amendment decision entry in design/Design_Log/index.md) before any part number changes are made.
| ERM8 Lead Style | ERF8 Lead Style | Total Stack Height |
|---|---|---|
| -02.0 (2mm) | -05.0 (5mm) | 7mm |
| -03.0 (3mm) | -05.0 (5mm) | 8mm |
| -05.0 (5mm) ← current | -05.0 (5mm) ← current | 10mm |
| -05.0 (5mm) | -07.0 (7mm) | 12mm |
| -08.0 (8mm) | -05.0 (5mm) | 13mm |
| -05.0 (5mm) | -09.0 (9mm) | 14mm |
| -08.0 (8mm) | -07.0 (7mm) | 15mm |
| -09.0 (9mm) | -07.0 (7mm) | 16mm |
| -08.0 (8mm) | -09.0 (9mm) | 17mm |
| -09.0 (9mm) | -09.0 (9mm) | 18mm |
Note: ERF8 -10.0 (10mm) is listed in the Samtec datasheet but is marked as not tooled and is therefore not available for ordering. The practical maximum is 18mm.
Reducing below 10mm (e.g., using ERM8 -02.0 or -03.0 with ERF8 -05.0) is geometrically possible but has not been evaluated for signal integrity or mechanical fit, and would also require a DEFECT-DEC.
4. Inter-Rotor Gap Clearance Requirements
The 10mm PCB-to-PCB gap between adjacent rotors must simultaneously satisfy all of the following clearance requirements. These are listed as design constraints; values are TBD pending detailed mechanical design:
| Requirement | Description | Status |
|---|---|---|
| Shroud flange/cover thickness | The rotor shroud flanges extend over both PCB flat faces. The combined axial depth of the Board B shroud cover (of the lower rotor) and the Board A shroud dish rim (of the upper rotor) must fit within the 10mm gap. | TBD |
| Rotation gear wheel clearance | The stepping ratchet/gear wheel on each rotor shroud outer face must clear the gap between adjacent rotors without interference. | TBD |
| Mechanical case clearance | Any structural frame or casing material between adjacent rotor positions must not reduce the effective gap below the shroud + gear wheel requirements above. | TBD |
| Position encoder sensor standoff | The retention bar and actuation detent mechanism must be positioned so they do not intrude into the radial sensor zone (capacitive electrode radius r = 44mm on each PCB face). | TBD — see design/Mechanical/Rotor/Design_Spec.md §5 for sensor geometry |
If the sum of the above clearances cannot be satisfied within 10mm, the stack height must be increased by raising a DEFECT-DEC (§3.3).
5. Total Mini-Stack Depth
The total depth (axial length along the central shaft) of a complete mini-stack cannot be accurately defined at this stage. It depends on:
- Individual rotor thickness — currently ~15mm per rotor (Board A + 11.8mm internal gap +
Board B). See
design/Mechanical/Rotor/Design_Spec.md §2. - Inter-rotor connector gap — currently 10mm per rotor-to-rotor interface (§3.2 above).
- STA/EXT enclosure thickness — the mechanical envelope of the Stator or Extension board at the low end of the mini-stack. Not yet defined.
- EXT/REF enclosure thickness — the mechanical envelope of the Extension or Reflector board at the high end of the mini-stack. Not yet defined.
Indicative PCB-only calculation (no enclosures):
5 rotors × ~15mm + 4 inter-rotor gaps × 10mm = 75mm + 40mm = ~115mm per mini-stack
This figure is illustrative only and excludes enclosures, retention hardware, and tolerance stack-up. It must not be used as a design constraint until formally verified.
6. Cross-References
| Document | Description |
|---|---|
design/Mechanical/Rotor/Design_Spec.md |
Individual rotor module: shroud, bearings, encoder slots, internal header assembly, component height constraints |
design/Mechanical/Rotor_Actuation_Assembly/Design_Spec.md |
Depression bar, carry mechanism, and 5-rotor group stepping |
design/Mechanical/Complete_System_Assembly/Design_Spec.md |
Full machine assembly sequence; Extension boundary integration |
design/Electronics/Rotor/Design_Spec.md |
Rotor electrical spec: ERM8/ERF8 connector pinout (J1–J6), BOM |
design/Datasheets/erm8-xxx-xx.x-xxx-dv-xxxx-xx-mkt-datasheet.md |
Samtec ERM8 lead style definitions and dimensions |
design/Datasheets/erf8-xxx-xx.x-xxx-dv-xxxx-xx-mkt-datasheet.md |
Samtec ERF8 lead style definitions and dimensions |
design/Design_Log/DEC-059_erm8erf8-stacking-height-confirmed-rotor-stack-assembly-specification-created.md |
DEC-059 (ERM8/ERF8 -05.0 stack height confirmation) |