saltylab-firmware/chassis/bumper_BOM.md

SaltyBot Bumper & Crash Protection — BOM + Assembly Notes

Rev A — 2026-03-01 — sl-mechanical

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System Overview

Three independent protection layers:

Layer	File	Protects
Sensor head roll cage	bumper_frame.scad	RPLIDAR, RealSense, 4× IMX219
Base plate bumper ring	base_bumper.scad	FC, motors, wiring, wheels
Stem impact sleeves	stem_protector.scad	25 mm stem tube along length

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Design Constraints Met

Constraint	How addressed
RPLIDAR 360° FOV	Roll cage posts at 45°/135°/225°/315° (between cameras). At RPLIDAR scan-plane height (~105 mm from cage collar), posts are ~78 mm from stem CL — well outside RPLIDAR body (35 mm radius). Crown ring sits at Z=148 mm, above RPLIDAR top.
Camera views	Posts avoid all four camera directions (0°/90°/180°/270°).
FC status LEDs	Base bumper clips below plate edge — does not cover plate top face or sides. LEDs remain visible.
Removable	All parts clip or clamp; no permanent fasteners into robot structure.
No supports	All parts print flat-face-down (collar halves, posts, crown ring, brackets, sleeves).
Lightweight	PLA frame + TPU pads only where needed; crown ring is hollow, brackets are thin-wall.

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Part A — Sensor Head Roll Cage (bumper_frame.scad)

Printed parts

#	RENDER	Qty	Material	Settings	Notes
1	collar_front	1	PETG	5 perims, 40% infill	Flat-face-down
2	collar_rear	1	PETG	5 perims, 40% infill	Flat-face-down; mirror of front
3	post	4	PETG or PLA	4 perims, 25% infill	Lay flat on narrow face
4	crown	1	PETG or PLA	4 perims, 30% infill	Flat (natural print orientation)
5	tpu_pad	1	TPU 95A	3 perims, 20% infill, 25 mm/s	Snap-on crown impact ring

Fasteners

#	Spec	Qty	Use
6	M4×25 SHCS	2	Cage collar clamping bolts
7	M4 hex nut	2	Captured in collar rear half
8	M4×10 set screw	1	Height/rotation lock (front collar)
9	M4×20 SHCS	8	Post-to-collar (2 per post, through boss)
10	M4×16 SHCS	8	Post-to-crown (2 per post, through crown top)
11	M4 hex nut	8	Under collar for post bolts
12	M4 flat washer	16	All M4 bolt heads

Cage installation — height reference

Z from cage collar base:
  Z =   0 mm  — cage collar bottom (clamp to stem here)
  Z =  24 mm  — cage collar top
  Z =  54 mm  — sensor-head collar bottom (≈30 mm above cage collar)
  Z =  90 mm  — sensor-head collar top + platform
  Z = 100 mm  — RPLIDAR base (above anti-vibration ring)
  Z = 130 mm  — RPLIDAR top
  Z = 148 mm  — cage crown ring (12 mm clear of RPLIDAR)

Install the cage collar ~30 mm BELOW the sensor-head collar on the same 25 mm stem. Slide collar down stem, position posts, attach crown ring, then tighten collar clamp bolts.

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Part B — Base Plate Bumper (base_bumper.scad)

Printed parts

#	RENDER	Qty	Material	Notes
13	corner	4	PETG	Print upright (Z = tall dimension); 4 perims, 40% infill
14	side	4	PETG	Same orientation; 4 perims, 40% infill
15	tpu_cap	~14	TPU 95A	40 mm segments; install end-to-end along all rails

Rails (straight sections between clips)

Rails can be 3D-printed from SCAD profile or procured:

#	Description	Qty	Size	Alternative
16	Front rail	1	8×50×270 mm PLA/PETG extrusion	25×8 mm aluminium flat bar
17	Rear rail	1	8×50×270 mm	same
18	Left side rail	1	8×50×240 mm	same
19	Right side rail	1	8×50×240 mm	same

DXF export: RENDER="rail_2d" gives the 8×50 mm cross-section. Extrude to required length in CAM software.

Fasteners

#	Spec	Qty	Use
20	M4×16 SHCS	16	Rail to corner bracket (2 heights × 2 rails per corner = 4 per corner × 4)
21	M4×16 SHCS	8	Rail to side bracket (2 per side bracket × 4)
22	M3×8 set screw	16	Bracket lock to plate edge (2 per clip × 8 clips)
23	M4 hex nut	24	All M4 rail bolts

Installation — clip orientation

TOP VIEW of base plate corner:

          plate edge (Y direction)
               │
    ───────────┤
               │◄── corner_bracket arm sits here, M3 set screw bites plate edge
    plate      │
    ───────────┘
               ↑
    plate edge (X direction)

The clip slot engages the BOTTOM face of the plate. Slide bracket onto corner from below; tighten M3 set screws finger-tight → hand-tight.

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Part C — Stem Protector (stem_protector.scad)

Printed parts

#	RENDER	Qty	Material	Notes
24	front	3–4	TPU 95A	Flat-face-down; qty = no. of stem zones to protect
25	rear	3–4	TPU 95A	Mirror; snap together around stem

Recommended placement (25 mm stem, ~1000 mm total stem)

Position	Z from base plate	Purpose
Sleeve 1	50–130 mm	Lowest zone — hit first in a side-fall
Sleeve 2	300–380 mm	Mid stem (near battery carousel)
Sleeve 3	600–680 mm	Upper stem
Sleeve 4	850–930 mm	Below sensor head collar

No fasteners required

Sleeves snap closed around stem. For extra security on smooth aluminium stem: wrap one loop of 25 mm Velcro strap over each sleeve before snapping closed.

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Tools Required for Assembly

• M2.5 / M3 / M4 hex drivers
• Torque wrench: M4 cage-collar bolts 2 N·m, M3 set screws 0.8 N·m
• Thread locker (Loctite 243 blue) on cage crown M4 bolts

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Mass Estimate

Part	Material	Est. mass
Roll cage collar (×2 halves)	PETG	~60 g
Roll cage posts (×4)	PETG	~80 g
Crown ring	PETG	~45 g
TPU crown pad	TPU	~15 g
Base bumper (×4 corners + ×4 sides)	PETG	~160 g
Base bumper TPU caps	TPU	~40 g
Stem sleeves (×8 halves)	TPU	~60 g
Fasteners	SS	~40 g
Total		~500 g

Balance note: All protection hardware is symmetric about the robot's fore-aft axis. Roll cage is symmetric about all axes. Net CG shift is negligible (<5 mm vertical, <2 mm lateral).