# SaltyBot Bumper & Crash Protection — BOM + Assembly Notes
**Rev A — 2026-03-01 — sl-mechanical**

---

## 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 |

---

## 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. |

---

## 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.

---

## 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.

---

## 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.

---

## 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

---

## 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).
