saltylab-firmware/chassis/payload_bay_BOM.md

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).

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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

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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.

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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).

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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)

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Export Commands

# 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