Mechanical Rotor Stack Assembly Design Spec

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

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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.

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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.

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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.

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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).

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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:

1. Individual rotor thickness — currently ~15mm per rotor (Board A + 11.8mm internal gap + Board B). See design/Mechanical/Rotor/Design_Spec.md §2.
2. Inter-rotor connector gap — currently 10mm per rotor-to-rotor interface (§3.2 above).
3. STA/EXT enclosure thickness — the mechanical envelope of the Stator or Extension board at the low end of the mini-stack. Not yet defined.
4. 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.

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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)