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feat(mechanical): modular payload bay system (Issue #170) #174

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# Payload Bay BOM — Issue #170
**Agent:** sl-mechanical | **Date:** 2026-03-01
Modular dovetail payload rail system. Tool-free slide-and-click module swapping.
Cross-variant: SaltyLab, SaltyRover, SaltyTank (same rail profile).
---
## A. Rail Hardware
| # | Description | Spec | Qty (per robot) | Notes |
|---|-------------|------|-----------------|-------|
| R1 | Aluminium bar stock | 50×12 mm, 6061-T6, 200 mm length | 1–2 | Preferred over printed rail for 2 kg load rating. CNC mill or route dovetail slot per `payload_rail.dxf` profile. |
| R2 | M4×10 FHCS | Stainless, countersunk | 4–8 | Rail to adapter plate (or direct to deck); FHCS sits flush below rail bottom face |
| R3 | M4 heat-set insert | M4×5.7 L, Ø5.6 OD | 4–8 | Into deck adapter plate |
| R4 | Detent ball bearing | Ø6 mm, chrome steel (GCr15) | 2 per module | Module spring detent; standard bearing ball |
| R5 | Detent spring | Ø5.5 mm OD, 12 mm free length, ~2 N/mm | 2 per module | Lee Spring LC 055A 06 S or equivalent; behind ball in plunger |
| R6 | M4 thumbscrew (knurled) | M4×12, knurled head Ø14 mm | 1 per module | Safety lock; threads into M4 nut pressed into module side |
| R7 | M4 hex nut | DIN 934, stainless | 1 per module | Captured in module body for thumbscrew |
## B. Power + Data Connector
| # | Description | Spec | Qty (per rail) | Notes |
|---|-------------|------|----------------|-------|
| C1 | Pogo pin | P75-E2 style, Ø2 mm, 6 mm travel, rated 2 A | 4 | Rail-side spring contacts. AliExpress "P75-E2 pogo pin" or Mill-Max 0906 series. |
| C2 | Brass target pad | Ø4 × 1.5 mm disc | 4 per module | Module-side contact pads. Machine from Ø4 mm brass rod or order PCB pads. Press-fit with Loctite 603. |
| C3 | JST-XH 2.54 mm header | 4-pin, right-angle | 1 per rail | Rail-side connector to power harness |
| C4 | JST-XH housing + crimps | 4-pin female | 1 per robot | Wires from robot PSU (5 V, 12 V, GND, UART) |
| C5 | 20 AWG silicone wire | Red / black / yellow / white, 300 mm each | 4 | Rail connector to robot bus |
| C6 | Connector housing | `payload_connector_stl` | 1 | Press-fit into rail pocket |
### Pin Map
| Pin | Signal | Wire colour | Max current |
|-----|--------|-------------|-------------|
| 1 | GND | Black | Return |
| 2 | +5 V | Red | 2 A |
| 3 | +12 V | Yellow | 2 A |
| 4 | UART (3.3 V) | White | 0.5 A |
> **UART note**: Half-duplex (single wire). Module firmware connects to Jetson Orin NX UART2. Use RS-485 transceiver if module cable > 500 mm or multi-drop needed.
## C. Deck Adapters
| Part | File | Qty | Print | Mass est. |
|------|------|-----|-------|-----------|
| SaltyLab adapter | `payload_bay_rail.scad` `lab_adapter_stl` | 1 | PETG, 5 perims, 60% infill | ~30 g |
| SaltyRover adapter | `payload_bay_rail.scad` `rover_adapter_stl` | 1 | PETG, 5 perims, 60% infill | ~35 g |
| SaltyTank adapter | `payload_bay_rail.scad` `tank_adapter_stl` | 1 | PETG, 5 perims, 60% infill | ~35 g |
## D. Printed Parts
| Part | File | Qty | Print | Mass est. |
|------|------|-----|-------|-----------|
| Rail section (prototype) | `payload_bay_rail.scad` `rail_stl` | 1 | PETG, 5 perims, 60% infill, 0.2 mm layer | ~85 g |
| Connector housing | `payload_bay_rail.scad` `connector_stl` | 1 | PETG, 5 perims, 100% infill | ~4 g |
| Detent plunger | `payload_bay_rail.scad` `detent_plunger_stl` | 2 per module | PETG, 5 perims, 80% infill | ~2 g each |
| Module base (universal) | `payload_bay_modules.scad` `base_stl` | N | PETG, 5 perims, 60% infill | ~18 g |
| Cargo tray (200 mm) | `payload_bay_modules.scad` `cargo_tray_stl` | 1 | PETG, 4 perims, 30% infill | ~180 g |
| Camera boom | `payload_bay_modules.scad` `camera_boom_stl` | 1 | PETG, 5 perims, 50% infill | ~95 g |
| Cup holder | `payload_bay_modules.scad` `cup_holder_stl` | 1 | PETG, 4 perims, 25% infill | ~55 g |
---
## Dovetail Rail — CNC Specification
For aluminium production rail (preferred over printed for 2 kg rating):
```
Material: 6061-T6 aluminium
Stock: 50 mm × 12 mm flat bar, length to suit (200 mm, 300 mm, 400 mm)
Dovetail slot (top face, centred):
Slot open width at top: 37.2 mm
Slot width at bottom: 28.0 mm
Slot depth: 8.0 mm
Wall angle from vertical: 30.0° (60° included angle)
Surface finish: Ra 1.6 µm (smooth for low-friction sliding)
Detent dimples (slot floor):
Diameter: 4.9 mm (ball seats in)
Depth: 1.5 mm
Pitch: 50 mm
First dimple: 25 mm from each end
Safety-lock groove (both side faces, continuous):
Groove diameter: 4.5 mm
Depth: 1.5 mm
Z position: RAIL_T/2 - DOVE_H/2 = 8 mm from top face
(CNC with Ø4 mm ball-nose end mill, single pass at Z = -4 mm from top)
Mounting holes (bottom face, countersunk):
Diameter: 4.3 mm (M4 clearance)
C/sink: Ø8 mm × 82° (M4 FHCS)
Pitch: 50 mm
First hole: 25 mm from each end
Connector pocket (slot floor, centred in rail length):
Width: 26 mm (X), Depth: 8.4 mm (Y), Height: 7 mm (Z into slot floor)
Tolerance: +0.2 / 0 mm (press-fit housing)
DXF cross-section: export payload_rail.dxf for supplier drawing.
```
---
## Load Analysis
| Mode | Load | Safety factor | Method |
|------|------|---------------|--------|
| Static payload (detent only) | 0.5 kg | 2× | Ball detent retention force ~10 N |
| Static payload (thumbscrew locked) | 2.0 kg | 2× | Dovetail shear area ~800 mm² Al |
| Dynamic (robot motion, 2 m/s²) | 2.0 kg | 1.5× | Inertial force = 2 kg × 2 m/s² = 4 N; detent holds 10 N |
| Dovetail shear (PETG printed) | 1.2 kg | 1.5× | PETG tensile ~50 MPa; recommend Al rail for rated 2 kg |
> **⚠ For 2 kg payload: use machined aluminium rail. Printed PETG rail is prototype/light-duty only (<0.8 kg payload).**
---
## Module Developer Guide
### Adding a new module in 5 steps
1. **Copy the template** at the bottom of `payload_bay_modules.scad`.
2. **Set `MY_LEN`** — must be a multiple of 50 mm (detent pitch) for repeatable positioning.
3. **Call `_module_base(MY_LEN, n_detents)`** as the first statement in your module.
4. **Build payload geometry** starting at `Z = 0` (rail top face). Keep total height ≤ 200 mm for robot clearance under doorways.
5. **Verify connector alignment** — when module is slid to its operating position, the 4 target pads on the tongue bottom must align with `CONN_Y` on the rail (default: 100 mm from rail entry end). Adjust `conn_offset` if needed.
### Constraints
| Parameter | Limit |
|-----------|-------|
| Module length | Min 60 mm, max 400 mm |
| Module height above rail | Max 200 mm (clearance) |
| Payload mass | ≤ 2 kg (Al rail + thumbscrew locked) |
| Module width | Max 120 mm (robot shoulder clearance) |
| Connector draw | Max 2 A per power pin (5 V or 12 V) |
---
## Export Commands
```bash
# Rail DXF (for CNC / waterjet machining)
openscad payload_bay_rail.scad -D 'RENDER="rail_2d"' -o payload_rail_profile.dxf
# Rail STL (PETG prototype)
openscad payload_bay_rail.scad -D 'RENDER="rail_stl"' -o payload_rail_200mm.stl
# Rail accessories
openscad payload_bay_rail.scad -D 'RENDER="connector_stl"' -o payload_connector.stl
openscad payload_bay_rail.scad -D 'RENDER="detent_plunger_stl"' -o payload_detent_plunger.stl
# Deck adapters
openscad payload_bay_rail.scad -D 'RENDER="lab_adapter_stl"' -o payload_adapter_lab.stl
openscad payload_bay_rail.scad -D 'RENDER="rover_adapter_stl"' -o payload_adapter_rover.stl
openscad payload_bay_rail.scad -D 'RENDER="tank_adapter_stl"' -o payload_adapter_tank.stl
# Example modules
openscad payload_bay_modules.scad -D 'RENDER="cargo_tray_stl"' -o payload_cargo_tray.stl
openscad payload_bay_modules.scad -D 'RENDER="camera_boom_stl"' -o payload_camera_boom.stl
openscad payload_bay_modules.scad -D 'RENDER="cup_holder_stl"' -o payload_cup_holder.stl
openscad payload_bay_modules.scad -D 'RENDER="target_pad_2d"' -o payload_target_pad.dxf
```

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// ============================================================
// payload_bay_modules.scad — Payload Bay Module Template + Examples
// Issue: #170 Agent: sl-mechanical Date: 2026-03-01
// ============================================================
//
// ── HOW TO CREATE A NEW MODULE ──────────────────────────────────────────────
//
// 1. Copy the "MODULE TEMPLATE" section at the bottom of this file.
// 2. Set MODULE_L to your module's Y length (min 60 mm).
// 3. Add your payload geometry on top of the _module_base() call.
// 4. The _module_base() provides:
// • Male dovetail tongue (slides into rail slot)
// • Spring detent bore(s) (for Ø6 mm ball + spring plunger)
// • Connector target pads (4× Ø4 mm brass, matching rail pogo pins)
// • Safety-lock M4 thumbscrew bore (side of tongue)
// • Bottom-face flush with rail top (Z = 0 at rail top / module base)
// 5. Your payload geometry sits at Z ≥ 0 (above the rail top face).
// 6. Add a RENDER dispatch entry and export command.
//
// ── DOVETAIL TONGUE GEOMETRY ────────────────────────────────────────────────
//
// TONGUE_BOT = DOVE_SLOT_BOT - 2*DOVE_CLEAR = 28.0 - 0.6 = 27.4 mm
// TONGUE_TOP = DOVE_SLOT_TOP + 2*DOVE_CLEAR = 37.2 + 0.6 = 37.8 mm
// TONGUE_H = DOVE_H + 0.2 (slight extra depth, no binding at corners)
//
// Tongue runs full module length (-Y to +Y in module coords).
// Module body sits on top of tongue at Z = 0.
//
// ── CONNECTOR PADS ──────────────────────────────────────────────────────────
// 4× Ø4 mm brass discs press-fit into tongue bottom face.
// Pad positions: must align with rail connector at CONN_Y when module
// is slid to its intended position (any detent step).
// Default: pads centred in module length → module must be placed so its
// centre aligns with CONN_Y on rail. Or: set MODULE_CONN_OFFSET to shift.
//
// ── PAYLOAD RATING ──────────────────────────────────────────────────────────
// 2 kg rated payload when safety-lock thumbscrew is tightened.
// Detent-only (no thumbscrew): ~0.5 kg (impact / vibration condition).
//
// ── RENDERED EXAMPLES ────────────────────────────────────────────────────────
// Part 1 — cargo_tray() 200×100 mm cargo tray, 30 mm walls
// Part 2 — camera_boom() L-arm with sensor_rail-compatible head
// Part 3 — cup_holder() Ø80 mm tapered cup cradle
//
// RENDER options:
// "assembly" all 3 modules on ghost rail
// "base_stl" module base / tongue only (universal)
// "cargo_tray_stl" cargo tray module
// "camera_boom_stl" camera boom module
// "cup_holder_stl" cup holder module
// "target_pad_2d" DXF — Ø4 mm brass target pad profile
//
// Export:
// openscad payload_bay_modules.scad -D 'RENDER="base_stl"' -o payload_base.stl
// openscad payload_bay_modules.scad -D 'RENDER="cargo_tray_stl"' -o payload_cargo_tray.stl
// openscad payload_bay_modules.scad -D 'RENDER="camera_boom_stl"' -o payload_camera_boom.stl
// openscad payload_bay_modules.scad -D 'RENDER="cup_holder_stl"' -o payload_cup_holder.stl
// ============================================================
$fn = 64;
e = 0.01;
// ── Rail geometry constants (must match payload_bay_rail.scad) ────────────────
RAIL_W = 50.0;
RAIL_T = 12.0;
DOVE_ANGLE = 30.0;
DOVE_H = 8.0;
DOVE_SLOT_BOT = 28.0;
DOVE_SLOT_TOP = DOVE_SLOT_BOT + 2 * DOVE_H * tan(DOVE_ANGLE);
DOVE_CLEAR = 0.3;
DETENT_PITCH = 50.0;
DETENT_BALL_D = 6.0;
DETENT_SPG_OD = 6.2;
DETENT_SPG_L = 16.0;
CONN_PIN_SPC = 5.0;
CONN_N_PINS = 4;
CONN_HOUSING_D = 8.0;
// ── Module tongue (male dovetail) geometry ────────────────────────────────────
TONGUE_BOT = DOVE_SLOT_BOT - 2*DOVE_CLEAR; // 27.4 mm
TONGUE_TOP = DOVE_SLOT_TOP + 2*DOVE_CLEAR; // 37.8 mm
TONGUE_H = DOVE_H + 0.2; // 8.2 mm (slight extra depth)
// ── Connector target pad ──────────────────────────────────────────────────────
TARGET_PAD_D = 4.0; // brass pad OD (slightly larger than pogo Ø2 mm)
TARGET_PAD_T = 1.5; // brass pad thickness
TARGET_PAD_RECESS = 1.3; // press-fit recess depth (pad is 0.2 mm proud)
// 4 pads at CONN_PIN_SPC pitch, centred in module tongue
CONN_SPAN = (CONN_N_PINS - 1) * CONN_PIN_SPC; // 15 mm
// ── Module safety lock ────────────────────────────────────────────────────────
LOCK_BOLT_D = 4.3; // M4 thumbscrew bore through tongue side
LOCK_BOLT_Z = TONGUE_H/2; // Z of thumbscrew CL from tongue bottom
// Thumbscrew tightens against continuous groove in rail side.
// Fasteners
M3_D = 3.3;
M4_D = 4.3;
// ============================================================
// RENDER DISPATCH
// ============================================================
RENDER = "assembly";
if (RENDER == "assembly") assembly();
else if (RENDER == "base_stl") _module_base(80);
else if (RENDER == "cargo_tray_stl") cargo_tray();
else if (RENDER == "camera_boom_stl") camera_boom();
else if (RENDER == "cup_holder_stl") cup_holder();
else if (RENDER == "target_pad_2d") {
projection(cut = true) translate([0, 0, -0.5])
linear_extrude(1) circle(d = TARGET_PAD_D);
}
// ============================================================
// ASSEMBLY PREVIEW
// ============================================================
module assembly() {
// Ghost rail
%color("Silver", 0.25)
translate([0, 0, -RAIL_T])
cube([RAIL_W, 600, RAIL_T], center = true);
// Cargo tray at Y=50 (first detent position)
color("OliveDrab", 0.85)
translate([0, 50, 0])
cargo_tray();
// Cup holder at Y=300
color("RoyalBlue", 0.85)
translate([0, 300, 0])
cup_holder();
// Camera boom at Y=500
color("DarkSlateGray", 0.85)
translate([0, 500, 0])
camera_boom();
}
// ============================================================
// UNIVERSAL MODULE BASE (_module_base)
// ============================================================
// All modules use this as their foundation.
// Provides: male dovetail tongue, detent bore, connector pads, lock bore.
//
// module_len : module length in Y (≥ 60 mm)
// n_detents : how many ball detent bores to include (1 or 2)
// conn_offset: Y offset of connector pads from module centre (default 0)
//
// After calling _module_base(), add payload geometry at Z = 0 and above.
// The tongue occupies Z = -(TONGUE_H) to Z = 0.
// Rail top face is at Z = 0.
module _module_base(module_len, n_detents = 1, conn_offset = 0) {
ml = module_len;
difference() {
// ── Dovetail tongue (male) ────────────────────────────────────
translate([0, ml/2, -TONGUE_H/2])
linear_extrude(TONGUE_H, center = true, twist = 0,
convexity = 2)
_tongue_profile_2d();
// ── Spring detent bore(s) (up through tongue top face) ────────
// 1 detent: at module centre; 2 detents: at ±25 mm from centre
for (i = [0 : n_detents - 1]) {
dy = (n_detents == 1)
? ml/2
: ml/2 + (i - (n_detents-1)/2) * DETENT_PITCH;
translate([0, dy, -(TONGUE_H/2) - DETENT_SPG_L/2])
cylinder(d = DETENT_SPG_OD,
h = DETENT_SPG_L + TONGUE_H/2 + e);
}
// ── Connector target pad recesses (tongue bottom face) ────────
for (i = [0 : CONN_N_PINS - 1]) {
px = (i - (CONN_N_PINS-1)/2) * CONN_PIN_SPC;
translate([px,
ml/2 + conn_offset,
-TONGUE_H + e])
cylinder(d = TARGET_PAD_D + 0.1,
h = TARGET_PAD_RECESS + e);
}
// ── Safety-lock M4 thumbscrew bore (right side of tongue) ─────
// Bore exits tongue right face, tip bears on rail side groove
translate([TONGUE_TOP/2 + e, ml/2, LOCK_BOLT_Z - TONGUE_H])
rotate([0, 90, 0])
cylinder(d = LOCK_BOLT_D,
h = (TONGUE_TOP - TONGUE_BOT)/2 + 6 + e);
// ── Lead-in chamfer on entry end of tongue ────────────────────
// 2 mm chamfer on bottom corners of tongue at Y=0 end
translate([0, 0, -TONGUE_H])
rotate([45, 0, 0])
cube([TONGUE_TOP + 2*e, 4, 4], center = true);
}
}
// ── Tongue 2D cross-section (male dovetail, trapezoid wider at bottom) ────────
// Module tongue: narrower at top (entering slot), wider at bottom (interlocking).
// Note orientation: tongue points DOWN (-Z), so wider face is at bottom (-Z).
module _tongue_profile_2d() {
polygon([
[-TONGUE_TOP/2, 0], // top-left (at Z = 0, flush with rail top)
[ TONGUE_TOP/2, 0], // top-right
[ TONGUE_BOT/2, -TONGUE_H], // bottom-right (interlocking face)
[-TONGUE_BOT/2, -TONGUE_H], // bottom-left
]);
}
// ============================================================
// PART 1 — CARGO TRAY
// ============================================================
// 200×100 mm open tray for transporting small items.
// 30 mm walls, chamfered rim, 4× drainage holes.
// Two Velcro strip slots on tray floor for cargo retention.
// Module length: 200 mm (4 detent positions).
// Weight budget: tray printed ~180 g; payload up to 2 kg.
TRAY_L = 200.0; // module Y length
TRAY_W = 100.0; // tray interior width (X)
TRAY_WALL = 3.0; // tray wall thickness
TRAY_H = 30.0; // tray wall height above module base
TRAY_FLOOR_T = 3.0; // tray floor thickness
module cargo_tray() {
// Mount base on rail (2 detents at ±50 mm from module centre)
_module_base(TRAY_L, n_detents = 2);
difference() {
union() {
// ── Tray body on top of base ────────────────────────────
translate([-TRAY_W/2 - TRAY_WALL, 0,
0])
cube([TRAY_W + 2*TRAY_WALL,
TRAY_L,
TRAY_FLOOR_T + TRAY_H]);
// ── Corner gussets (stiffening for 2 kg load) ───────────
for (cx = [-1, 1]) for (cy = [0, 1])
hull() {
translate([cx * TRAY_W/2,
cy * (TRAY_L - TRAY_WALL),
0])
cube([TRAY_WALL + e, TRAY_WALL + e,
TRAY_FLOOR_T + TRAY_H * 0.6], center = true);
translate([cx * (TRAY_W/2 - 10),
cy * (TRAY_L - TRAY_WALL),
0])
cylinder(d = TRAY_WALL * 2, h = e);
}
}
// ── Tray interior cavity ─────────────────────────────────────
translate([-TRAY_W/2, TRAY_WALL,
TRAY_FLOOR_T])
cube([TRAY_W, TRAY_L - 2*TRAY_WALL,
TRAY_H + e]);
// ── Drainage holes (4× Ø8 mm in floor) ──────────────────────
for (dx = [-TRAY_W/4, TRAY_W/4])
for (dy = [TRAY_L/4, 3*TRAY_L/4])
translate([dx, dy, -e])
cylinder(d = 8, h = TRAY_FLOOR_T + 2*e);
// ── Velcro slot × 2 (25 mm wide grooves in floor) ────────────
for (dy = [TRAY_L/3, 2*TRAY_L/3])
translate([0, dy, TRAY_FLOOR_T - 1.5])
cube([TRAY_W - 10, 25, 2 + e], center = true);
// ── Rim chamfer (top inner edge, ergonomic) ───────────────────
translate([0, TRAY_L/2, TRAY_FLOOR_T + TRAY_H + e])
cube([TRAY_W + 2*TRAY_WALL + 2*e,
TRAY_L + 2*e, 4], center = true);
// ── Side slots for bungee cord retention (3× each long side) ─
for (sx = [-1, 1])
for (sy = [TRAY_L/4, TRAY_L/2, 3*TRAY_L/4])
translate([sx * (TRAY_W/2 + TRAY_WALL/2),
sy, TRAY_FLOOR_T + TRAY_H/2])
cube([TRAY_WALL + 2*e, 8, 6], center = true);
}
}
// ============================================================
// PART 2 — CAMERA BOOM
// ============================================================
// L-shaped arm: vertical mast + horizontal boom.
// Boom head accepts sensor_rail 2020 T-slot (RAIL_W/2 bolt pattern).
// Sensor head can be rotated 0/90/180° and locked with M4 bolt.
// Module length: 80 mm; arm rises 120 mm, boom extends 80 mm forward.
BOOM_MODULE_L = 80.0;
BOOM_MAST_H = 120.0; // mast height above rail top (Z)
BOOM_ARM_L = 80.0; // horizontal boom length (+Y forward)
BOOM_ARM_W = 20.0; // arm cross-section width
BOOM_ARM_T = 20.0; // arm cross-section height
BOOM_HEAD_W = 50.0; // sensor head width (matches 2020 rail flange)
BOOM_HEAD_H = 20.0; // sensor head plate height
BOOM_HEAD_T = 5.0; // sensor head plate thickness
module camera_boom() {
_module_base(BOOM_MODULE_L, n_detents = 1);
difference() {
union() {
// ── Mast (vertical column from module body) ──────────────
translate([-BOOM_ARM_W/2, BOOM_MODULE_L/2 - BOOM_ARM_T/2, 0])
cube([BOOM_ARM_W, BOOM_ARM_T, BOOM_MAST_H]);
// ── Horizontal boom (from mast top, extends +Y) ──────────
translate([-BOOM_ARM_W/2,
BOOM_MODULE_L/2 - BOOM_ARM_T/2,
BOOM_MAST_H - BOOM_ARM_W])
cube([BOOM_ARM_W, BOOM_ARM_L, BOOM_ARM_W]);
// ── Sensor head plate (at boom tip) ──────────────────────
translate([-BOOM_HEAD_W/2,
BOOM_MODULE_L/2 - BOOM_ARM_T/2 + BOOM_ARM_L,
BOOM_MAST_H - BOOM_ARM_W - BOOM_HEAD_H/2 + BOOM_ARM_W/2])
cube([BOOM_HEAD_W, BOOM_HEAD_T, BOOM_HEAD_H]);
// ── Junction gussets (mast + horizontal boom) ────────────
translate([-BOOM_ARM_W/2,
BOOM_MODULE_L/2 - BOOM_ARM_T/2,
BOOM_MAST_H - BOOM_ARM_W])
rotate([45, 0, 0])
cube([BOOM_ARM_W, BOOM_ARM_W * 0.7, BOOM_ARM_W * 0.7]);
}
// ── 2020 sensor-rail bolt pattern in head plate ───────────────
// 2× M5 slots (matches sensor_rail.scad tank_clamp slot geometry)
for (sz = [-BOOM_HEAD_H/4, BOOM_HEAD_H/4])
translate([0,
BOOM_MODULE_L/2 - BOOM_ARM_T/2 + BOOM_ARM_L + BOOM_HEAD_T + e,
BOOM_MAST_H - BOOM_ARM_W/2 + sz])
rotate([90, 0, 0])
hull() {
translate([-6, 0, 0])
cylinder(d = 5.3, h = BOOM_HEAD_T + 2*e);
translate([+6, 0, 0])
cylinder(d = 5.3, h = BOOM_HEAD_T + 2*e);
}
// ── Tilt angle slots (3 positions: 0°, ±15°) ─────────────────
for (ta = [-15, 0, 15]) {
translate([0,
BOOM_MODULE_L/2 - BOOM_ARM_T/2 + BOOM_ARM_L/2,
BOOM_MAST_H - BOOM_ARM_W/2])
rotate([ta, 0, 0])
translate([0, BOOM_ARM_L/2, 0])
cylinder(d = 4.3, h = BOOM_ARM_W + 2*e,
center = true);
}
// ── Cable tie slots in mast ───────────────────────────────────
for (cz = [BOOM_MAST_H * 0.3, BOOM_MAST_H * 0.6])
translate([0, BOOM_MODULE_L/2, cz])
cube([BOOM_ARM_W + 2*e, 4, 4], center = true);
// ── Lightening pockets in mast ────────────────────────────────
translate([0, BOOM_MODULE_L/2, BOOM_MAST_H * 0.4])
cube([BOOM_ARM_W - 6, BOOM_ARM_T - 6,
BOOM_MAST_H * 0.4], center = true);
}
}
// ============================================================
// PART 3 — CUP HOLDER
// ============================================================
// Tapered cup cradle for standard travel mugs / water bottles.
// Inner diameter: 80 mm at top, 68 mm at bottom (matches Ø70 mm typical mug).
// Flexible gripper ribs (cut slots) provide spring retention.
// Drain hole at bottom for condensation.
// Module length: 80 mm.
CUP_MODULE_L = 80.0;
CUP_INNER_TOP = 80.0; // inner bore OD at top
CUP_INNER_BOT = 68.0; // inner bore OD at bottom (taper for grip)
CUP_OUTER_T = 4.0; // wall thickness
CUP_H = 80.0; // cup holder height
CUP_GRIP_SLOTS = 8; // number of flex slots (spring grip)
CUP_SLOT_W = 2.5; // flex slot width
CUP_SLOT_H = 40.0; // flex slot height (from top)
module cup_holder() {
_module_base(CUP_MODULE_L, n_detents = 1);
difference() {
union() {
// ── Outer shell (tapered cylinder) ───────────────────────
cylinder(d1 = CUP_INNER_BOT + 2*CUP_OUTER_T,
d2 = CUP_INNER_TOP + 2*CUP_OUTER_T,
h = CUP_H);
// ── Base flange (connects to module body footprint) ───────
hull() {
cylinder(d = CUP_INNER_BOT + 2*CUP_OUTER_T + 4,
h = 4);
translate([0, CUP_MODULE_L/2, 0])
cube([RAIL_W, CUP_MODULE_L, 4], center = true);
}
}
// ── Inner bore (tapered cup cavity) ──────────────────────────
translate([0, 0, CUP_OUTER_T])
cylinder(d1 = CUP_INNER_BOT, d2 = CUP_INNER_TOP,
h = CUP_H + e);
// ── Base drain hole ──────────────────────────────────────────
translate([0, 0, -e])
cylinder(d = 12, h = CUP_OUTER_T + 2*e);
// ── Flex grip slots (from top down) ──────────────────────────
// Slots allow upper rim to flex inward and grip cup body
for (i = [0 : CUP_GRIP_SLOTS - 1]) {
angle = i * 360 / CUP_GRIP_SLOTS;
rotate([0, 0, angle])
translate([CUP_INNER_TOP/2 + CUP_OUTER_T/2, 0, CUP_H - CUP_SLOT_H])
cube([CUP_OUTER_T + 2*e, CUP_SLOT_W, CUP_SLOT_H + e],
center = true);
}
// ── Exterior branding recess (optional label area) ────────────
translate([CUP_INNER_BOT/2 + CUP_OUTER_T/2 - 0.5, 0, CUP_H/2])
rotate([0, 90, 0])
cube([25, 40, 1 + e], center = true);
}
}
// ============================================================
// ══ MODULE TEMPLATE ═══════════════════════════════════════════
// ══ Copy this block to create a new payload module ════════════
// ============================================================
//
// module my_new_module() {
// MY_LEN = 120.0; // module length — must be multiple of DETENT_PITCH (50 mm)
//
// // Always start with the base (provides tongue, pads, detent bore)
// _module_base(MY_LEN, n_detents = 2);
//
// // Add your payload geometry here.
// // Z = 0 is the rail top face / module mounting face.
// // Build upward from Z = 0.
//
// difference() {
// union() {
// // Example: a simple platform
// translate([-(RAIL_W + 10)/2, 0, 0])
// cube([RAIL_W + 10, MY_LEN, 10]);
//
// // Add your geometry...
// }
//
// // Add your cutouts...
// }
// }
//
// ── Don't forget to: ──────────────────────────────────────────
// 1. Add else if (RENDER == "my_module_stl") my_new_module();
// in the RENDER DISPATCH block above.
// 2. Add export command in BOM / README.
// 3. Test: verify tongue fits rail slot (should slide with 0.3 mm clearance).
// 4. Verify connector pad positions align with CONN_Y on rail.
// ============================================================

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// ============================================================
// payload_bay_rail.scad — Modular Payload Bay Rail System
// Issue: #170 Agent: sl-mechanical Date: 2026-03-01
// ============================================================
//
// Dovetail rail mounted on robot top deck. Payload modules slide on
// from either end and are retained by a spring-loaded ball detent plus
// an optional M4 thumbscrew safety lock.
//
// Dovetail geometry (60° included angle — balanced for print + load):
//
// ← RAIL_W (50 mm) →
// ┌──────────────────┐ ← rail top face (Z = RAIL_T)
// │ ╲ ╱ │
// │ ╲__________╱ │ ← dovetail slot (female, cut into top)
// │ (DOVE_SLOT) │
// └──────────────────┘ ← rail bottom face (Z = 0)
//
// DOVE_ANGLE = 30° from vertical (= 60° included).
// Slot width at top = DOVE_SLOT_TOP (open face)
// Slot width at bottom = DOVE_SLOT_BOT (inner base of slot)
// Slot depth = DOVE_H
//
// Module tongue (male dovetail) slides in with 0.3 mm clearance each side.
//
// Spring detent: Ø6 mm steel ball in module, spring behind, seats in
// Ø4.9 mm dimples drilled into rail slot bottom at DETENT_PITCH spacing.
// Provides tactile click-lock at each indexed position.
//
// Safety lock: M4 thumbscrew through module side, tightens against rail
// side wall. For vibration environments or >1 kg payload.
//
// Power+data connector: 4-pin pogo array in rail at fixed position.
// Pins: GND | +5 V | +12 V | UART (half-duplex)
// Module has matching brass target pads (Ø4 mm).
// Connector position: centred in rail length, at CONN_Y from one end.
//
// Cross-variant adapter plates (this file):
// lab_rail_adapter() — SaltyLab chassis top (Ø25 mm stem clear)
// rover_rail_adapter() — SaltyRover deck (M4 grid)
// tank_rail_adapter() — SaltyTank deck (M4 grid)
//
// Coordinate convention:
// Rail runs along Y axis. Cross-section in X-Z plane.
// Z = 0 at rail bottom face (= robot deck top face).
// Module slides in +Y direction.
//
// RENDER options:
// "assembly" rail + adapter + module ghost
// "rail_2d" DXF — dovetail cross-section (CNC/waterjet)
// "rail_stl" STL — printable rail section (PETG prototype)
// "connector_stl" STL — pogo connector housing insert
// "detent_plunger_stl" STL — spring detent plunger (print ×2 per module)
// "lab_adapter_stl" STL — SaltyLab deck adapter
// "rover_adapter_stl" STL — SaltyRover deck adapter
// "tank_adapter_stl" STL — SaltyTank deck adapter
//
// Export:
// openscad payload_bay_rail.scad -D 'RENDER="rail_2d"' -o payload_rail.dxf
// openscad payload_bay_rail.scad -D 'RENDER="rail_stl"' -o payload_rail.stl
// openscad payload_bay_rail.scad -D 'RENDER="connector_stl"' -o payload_connector.stl
// openscad payload_bay_rail.scad -D 'RENDER="detent_plunger_stl"' -o payload_detent.stl
// openscad payload_bay_rail.scad -D 'RENDER="lab_adapter_stl"' -o payload_lab_adapter.stl
// openscad payload_bay_rail.scad -D 'RENDER="rover_adapter_stl"' -o payload_rover_adapter.stl
// openscad payload_bay_rail.scad -D 'RENDER="tank_adapter_stl"' -o payload_tank_adapter.stl
// ============================================================
$fn = 64;
e = 0.01;
// ── Dovetail rail cross-section ───────────────────────────────────────────────
RAIL_W = 50.0; // rail total width (X)
RAIL_T = 12.0; // rail total height (Z)
RAIL_R = 2.0; // outer corner radius
RAIL_LEN = 200.0; // default rail section length (Y)
// Dovetail slot geometry
DOVE_ANGLE = 30.0; // degrees from vertical (60° included angle)
DOVE_H = 8.0; // slot depth (Z into rail from top)
DOVE_SLOT_BOT= 28.0; // slot width at bottom (inner)
// Derived: slot width at top = DOVE_SLOT_BOT + 2 * DOVE_H * tan(DOVE_ANGLE)
DOVE_SLOT_TOP= DOVE_SLOT_BOT + 2 * DOVE_H * tan(DOVE_ANGLE); // ≈ 37.2 mm
// Module tongue (male) clearance: 0.3 mm per side
DOVE_CLEAR = 0.3;
// → module tongue: bot_w = DOVE_SLOT_BOT - 2*DOVE_CLEAR, top_w = DOVE_SLOT_TOP + 2*DOVE_CLEAR
// ── Spring ball detent ────────────────────────────────────────────────────────
// Ø6 mm steel ball presses up through module tongue into dimples in rail slot.
DETENT_BALL_D = 6.0; // ball diameter
DETENT_HOLE_D = 4.9; // dimple bore in rail slot bottom (ball seats in)
DETENT_DEPTH = 1.5; // dimple depth (ball sinks in this far)
DETENT_PITCH = 50.0; // dimple spacing along rail (Y) — module index positions
DETENT_SPG_OD = 6.2; // plunger bore OD (ball + spring housing in module)
DETENT_SPG_L = 16.0; // spring pocket depth in module tongue
// ── Safety lock (M4 thumbscrew through module side into rail side groove) ─────
LOCK_GROOVE_D = 4.5; // groove in rail side wall (M4 thumbscrew tip seats in)
LOCK_GROOVE_DEPTH = 1.5; // groove depth into rail side
// Lock groove runs full rail length (continuous slot) for tool-free slide + lock anywhere
// ── 4-pin power+data connector ───────────────────────────────────────────────
// Pogo pin array mounted in rail body at CONN_Y from entry end.
// Pin map: 1=GND 2=+5V 3=+12V 4=UART
// Pogo: Ø2 mm spring contact (P75-E2 style), rated 2 A (power), 0.5 A (signal)
CONN_Y = RAIL_LEN / 2; // connector centred in rail section
CONN_PIN_D = 2.2; // pogo bore (2 mm pin + 0.2 mm clearance)
CONN_PIN_SPC = 5.0; // pin centre-to-centre spacing
CONN_N_PINS = 4; // GND / +5V / +12V / UART
CONN_HOUSING_W= CONN_N_PINS * CONN_PIN_SPC + 4; // housing width (X)
CONN_HOUSING_D= 8.0; // housing depth (Y, inside rail)
CONN_HOUSING_H= DOVE_H - 1.0; // housing height; sits inside slot (flush with slot floor)
// Connector pogo pins point upward into module pad targets.
// ── Deck mounting holes ───────────────────────────────────────────────────────
MOUNT_PITCH = 50.0; // M4 FHCS hole pitch along rail (countersunk from bottom)
MOUNT_INSET = 25.0; // first hole Y from rail end
MOUNT_D = 4.3; // M4 clearance
CSINK_D = 8.0; // M4 FHCS head diameter
// ── Cross-variant adapter plates ─────────────────────────────────────────────
ADAPT_T = 4.0; // adapter plate thickness
ADAPT_OVHG = 10.0; // adapter overhang past rail edge each side (flange width)
// SaltyLab deck: stem bore at centre
LAB_STEM_BORE = 26.0; // clear stem Ø25 mm
// SaltyRover deck: M4 bolt grid (spacing from rover_chassis_r2.scad)
ROVER_BOLT_SPC = 40.0;
// SaltyTank deck: M4 bolt grid (spacing from saltytank_chassis.scad)
TANK_BOLT_SPC = 40.0;
// Fasteners
M3_D = 3.3;
M4_D = 4.3;
// ============================================================
// RENDER DISPATCH
// ============================================================
RENDER = "assembly";
if (RENDER == "assembly") assembly();
else if (RENDER == "rail_2d")
projection(cut = true) translate([0, 0, -0.5])
linear_extrude(1) rail_profile_2d();
else if (RENDER == "rail_stl") rail_section(RAIL_LEN);
else if (RENDER == "connector_stl") connector_housing();
else if (RENDER == "detent_plunger_stl") detent_plunger();
else if (RENDER == "lab_adapter_stl") lab_rail_adapter();
else if (RENDER == "rover_adapter_stl") rover_rail_adapter();
else if (RENDER == "tank_adapter_stl") tank_rail_adapter();
// ============================================================
// ASSEMBLY PREVIEW
// ============================================================
module assembly() {
// Rail section
color("Silver", 0.85) rail_section(RAIL_LEN);
// Rover adapter under rail
color("SteelBlue", 0.70)
translate([0, 0, -ADAPT_T])
rover_rail_adapter();
// Ghost module sliding on
%color("OliveDrab", 0.3)
translate([0, 60, RAIL_T])
cube([RAIL_W + 10, 100, 40], center = true);
// Connector position marker
%color("Gold", 0.5)
translate([0, CONN_Y, RAIL_T - DOVE_H])
cube([CONN_HOUSING_W, CONN_HOUSING_D, CONN_HOUSING_H],
center = true);
// Detent dimple markers
for (dy = [MOUNT_INSET : DETENT_PITCH : RAIL_LEN - MOUNT_INSET])
%color("Red", 0.6)
translate([0, dy, RAIL_T - DOVE_H])
cylinder(d = DETENT_HOLE_D, h = DETENT_DEPTH);
}
// ============================================================
// RAIL CROSS-SECTION 2D (DXF export)
// ============================================================
// Outer profile minus dovetail slot.
// For CNC milling from 50×12 mm aluminium bar, or waterjet from plate.
// Also used for PETG prototype extrusion.
module rail_profile_2d() {
difference() {
// Outer rail cross-section (rounded rect)
minkowski() {
square([RAIL_W - 2*RAIL_R, RAIL_T - 2*RAIL_R],
center = true);
circle(r = RAIL_R);
}
// Dovetail slot (trapezoid, open at top)
translate([0, RAIL_T/2])
_dovetail_slot_2d();
}
}
// ── Dovetail slot 2D (trapezoid with open top) ───────────────────────────────
module _dovetail_slot_2d() {
// Trapezoid: wider at top (open face), narrower at bottom.
// Points listed clockwise from top-left:
polygon([
[-DOVE_SLOT_TOP/2, 0], // top-left
[ DOVE_SLOT_TOP/2, 0], // top-right
[ DOVE_SLOT_BOT/2, -DOVE_H], // bottom-right
[-DOVE_SLOT_BOT/2, -DOVE_H], // bottom-left
]);
}
// ── Dovetail slot for difference() operations (3D volume) ────────────────────
module _dovetail_slot_3d(length) {
translate([0, -e, RAIL_T])
linear_extrude(DOVE_H + e)
offset(delta = e)
_dovetail_slot_2d();
}
// ============================================================
// RAIL SECTION (3D — printable or aluminium)
// ============================================================
module rail_section(len = RAIL_LEN) {
difference() {
// ── Extruded profile ────────────────────────────────────────
linear_extrude(len)
rotate([0, 0, 90])
rail_profile_2d();
// ── Dovetail slot ────────────────────────────────────────────
translate([0, -e, RAIL_T])
rotate([0, 0, 0])
linear_extrude(len + 2*e)
rotate([0, 0, 90])
offset(delta = e)
_dovetail_slot_2d();
// ── Deck mounting holes (M4 FHCS, from bottom) ───────────────
for (my = [MOUNT_INSET : MOUNT_PITCH : len - MOUNT_INSET])
translate([0, my, -e])
cylinder(d = MOUNT_D, h = RAIL_T - DOVE_H + 2*e);
// Countersinks on bottom face
for (my = [MOUNT_INSET : MOUNT_PITCH : len - MOUNT_INSET])
translate([0, my, -e])
cylinder(d1 = CSINK_D, d2 = MOUNT_D,
h = (CSINK_D - MOUNT_D) / (2 * tan(41)));
// ── Spring detent dimples (slot bottom, at DETENT_PITCH) ──────
for (dy = [MOUNT_INSET : DETENT_PITCH : len - MOUNT_INSET])
translate([0, dy, RAIL_T - DOVE_H - e])
cylinder(d = DETENT_HOLE_D, h = DETENT_DEPTH + e);
// ── Safety-lock groove (continuous slot, both sides of rail) ──
// M4 thumbscrew tip seats anywhere along groove
for (sx = [-1, 1])
translate([sx * (RAIL_W/2 + e), -e,
RAIL_T - DOVE_H/2])
rotate([0, 90, 0])
hull() {
translate([0, 0, 0])
cylinder(d = LOCK_GROOVE_D, h = RAIL_W + 2*e);
}
// ── Connector housing pocket (at CONN_Y) ──────────────────────
translate([0, CONN_Y, RAIL_T - DOVE_H - e])
cube([CONN_HOUSING_W + 0.4,
CONN_HOUSING_D + 0.4,
CONN_HOUSING_H + e], center = true);
// ── Lightening slots (rail body between mounting holes) ────────
for (my = [MOUNT_INSET + 25 : MOUNT_PITCH : len - MOUNT_INSET - 25])
translate([0, my, RAIL_T/4])
cube([RAIL_W - 16, 20, RAIL_T/2 + 2*e], center = true);
}
}
// ============================================================
// CONNECTOR HOUSING (pogo-pin insert, press-fits into rail pocket)
// ============================================================
// 4× spring-loaded pogo pins (P75-E2, Ø2 mm, 6 mm travel).
// Printed housing press-fits into rail pocket; pins protrude up into module.
// Module has 4× Ø4 mm brass target pads at matching pitch.
//
// Pin map (left to right, looking at rail top from +Z):
// Pin 1: GND Pin 2: +5 V Pin 3: +12 V Pin 4: UART
module connector_housing() {
hw = CONN_HOUSING_W;
hd = CONN_HOUSING_D;
hh = CONN_HOUSING_H;
difference() {
// Housing body (press-fit into rail pocket)
cube([hw, hd, hh], center = true);
// 4× pogo pin bores (through housing, top to bottom)
for (i = [0 : CONN_N_PINS - 1]) {
px = (i - (CONN_N_PINS - 1) / 2) * CONN_PIN_SPC;
translate([px, 0, -hh/2 - e])
cylinder(d = CONN_PIN_D, h = hh + 2*e);
}
// Wire exit slot (bottom, routes into rail body)
translate([0, 0, -hh/2 - e])
cube([hw - 6, hd/2, hh/3 + e], center = true);
// Retention barbs (prevent housing pulling out of pocket)
for (sx = [-1, 1])
translate([sx * (hw/2 - 1), 0, hh/4])
rotate([0, sx * 15, 0])
cube([2, hd + 2*e, 2.5], center = true);
}
// Pin polarity label recess (on top face, GND side)
difference() {
translate([0, 0, 0]) cube([0, 0, 0]); // null
translate([-(CONN_N_PINS * CONN_PIN_SPC)/2 + 1, 0, hh/2 - 0.4])
cube([3, hd * 0.6, 0.5 + e], center = true);
}
}
// ============================================================
// DETENT PLUNGER (lives in module tongue; print 2× per module)
// ============================================================
// Press-fit into Ø6.2 mm bore in module tongue.
// Includes spring pocket; ball seated on top.
// Spring: Ø5.5 mm OD, 12 mm free length, ~2 N/mm (light detent).
// Ball: Ø6 mm steel (standard bearing ball, purchase).
module detent_plunger() {
bore_d = DETENT_BALL_D + 0.2; // 6.2 mm
body_od = DETENT_SPG_OD;
body_len = DETENT_SPG_L;
spg_d = 5.8; // spring OD
spg_pocket = 10.0; // spring pocket depth (bottom of housing)
difference() {
cylinder(d = body_od, h = body_len);
// Ball socket (top — partial sphere, retains ball)
translate([0, 0, body_len])
sphere(d = bore_d);
translate([0, 0, body_len - bore_d/4])
cylinder(d = bore_d, h = bore_d/2 + e);
// Spring pocket (bottom)
translate([0, 0, -e])
cylinder(d = spg_d + 0.3, h = spg_pocket + e);
// Retention lip (allows push-in but prevents pullout before spring seated)
translate([0, 0, spg_pocket])
cylinder(d1 = spg_d + 0.3, d2 = spg_d - 1,
h = 1.5);
}
}
// ============================================================
// CROSS-VARIANT DECK ADAPTER PLATES
// ============================================================
// Thin plates that bolt to the robot deck and provide M4 threaded
// studs (or through holes) for the rail mounting holes.
// All adapters: RAIL_LEN × (RAIL_W + 2×ADAPT_OVHG) footprint.
module _adapter_base() {
adapt_l = RAIL_LEN;
adapt_w = RAIL_W + 2*ADAPT_OVHG;
difference() {
// Plate
cube([adapt_w, adapt_l, ADAPT_T], center = true);
// Rail mounting holes (M4 FHCS up through adapter into rail bottom)
for (my = [MOUNT_INSET : MOUNT_PITCH : adapt_l - MOUNT_INSET])
translate([0, my - adapt_l/2, -ADAPT_T/2 - e])
cylinder(d = MOUNT_D, h = ADAPT_T + 2*e);
// Corner lightening
for (cx = [-1, 1]) for (cy = [-1, 1])
translate([cx * (adapt_w/2 - 12),
cy * (adapt_l/2 - 20), 0])
cylinder(d = 10, h = ADAPT_T + 2*e, center = true);
}
}
// SaltyLab adapter: clears Ø25 mm stem, 4× M4 to lab chassis ring
module lab_rail_adapter() {
difference() {
_adapter_base();
// Stem bore clearance (at centre of adapter)
cylinder(d = LAB_STEM_BORE, h = ADAPT_T + 2*e, center = true);
// 4× M4 mounting to lab chassis top ring (Ø44 mm bolt circle)
for (a = [45, 135, 225, 315])
translate([22*cos(a), 22*sin(a), -ADAPT_T/2 - e])
cylinder(d = M4_D, h = ADAPT_T + 2*e);
}
}
// SaltyRover adapter: 4× M4 to rover deck bolt grid
module rover_rail_adapter() {
adapt_l = RAIL_LEN;
adapt_w = RAIL_W + 2*ADAPT_OVHG;
difference() {
_adapter_base();
// 2 rows × 3 cols of M4 bolts into rover deck
for (rx = [-ROVER_BOLT_SPC/2, ROVER_BOLT_SPC/2])
for (ry = [-adapt_l/3, 0, adapt_l/3])
translate([rx, ry, -ADAPT_T/2 - e])
cylinder(d = M4_D, h = ADAPT_T + 2*e);
}
}
// SaltyTank adapter: M4 to tank deck; relieved for deck cable slots
module tank_rail_adapter() {
adapt_l = RAIL_LEN;
adapt_w = RAIL_W + 2*ADAPT_OVHG;
difference() {
_adapter_base();
// 2 rows × 3 cols of M4 bolts into tank deck
for (rx = [-TANK_BOLT_SPC/2, TANK_BOLT_SPC/2])
for (ry = [-adapt_l/3, 0, adapt_l/3])
translate([rx, ry, -ADAPT_T/2 - e])
cylinder(d = M4_D, h = ADAPT_T + 2*e);
// Deck cable slot clearance (tank deck has centre cable channel)
translate([0, 0, 0])
cube([10, adapt_l - 40, ADAPT_T + 2*e], center = true);
}
}