Major parametric and structural design overhaul

Comprehensive geometry fixes, parametric calculations, and parts standardization were implemented for the LifeTrac v25 loader. Fastener rendering modules were added, and detailed engineering calculations for lifting capacity and structural adequacy are now included and output via echo statements. Documentation for custom fabricated parts and structural analysis was created, and the OpenSCAD model now supports parametric scaling and outputs pass/fail status for all major mechanical checks.

Author: dorkmo

LifeTrac-v25/mechanical_design/AI_DESIGN_PROGRESS.md

@@ -104,6 +104,189 @@ This document tracks the AI-assisted hardware design process for the LifeTrac v2
    - Create build workshops
    - Document lessons learned
 
+---
+
+### Session 2 - Major Design Refinement (2025-12-07)
+
+This session involved comprehensive geometry fixes, parametric calculations, parts standardization, fastener rendering, and structural engineering analysis.
+
+#### ✅ Geometry Fixes
+
+- **Wheel/Frame Overlap Resolution**
+  - Added `GROUND_CLEARANCE` (150mm) constant
+  - Created `FRAME_Z_OFFSET` to lift frame above ground
+  - Positioned wheels so bottom touches Z=0 (ground plane)
+  - Moved front tires back so front of tire aligns with front of machine body
+
+- **Cylinder Positioning**
+  - Fixed lift cylinder base mounts to attach on side walls between sandwich plates
+  - Repositioned bucket cylinders from arm tops to cross beam attachment
+  - Added `BUCKET_CYL_X_SPACING` to ensure clearance from inner panels
+
+- **Side Wall Orientation**
+  - Corrected triangular side panels to be vertical (YZ plane) not horizontal
+  - Fixed tall end at rear (Y=0) tapering to shorter front (Y=WHEEL_BASE)
+
+- **Arm Positioning**
+  - Created inline hollow square tubing using `difference()` for arms
+  - Positioned arms between inner and outer sandwich plates
+  - Arms pivot at high point on rear of side panels
+
+- **Cross Beam Alignment**
+  - Centered cross beams on X=0 (machine centerline)
+  - Added parametric arc slot cutouts in inner side wall panels for arm clearance
+
+- **Bucket Positioning**
+  - Shifted bucket up so bottom clears ground in all positions
+  - Fixed pivot point at bottom of QA plate with cylinders attaching at top
+  - Added `BOBCAT_QA_HEIGHT` offset for proper positioning
+
+#### ✅ Parametric Calculations
+
+- **Arm Length Calculation**
+  - Used trigonometry to calculate arm length from reach requirements
+  - `ARM_LENGTH = sqrt(ARM_PIVOT_Z² + MIN_HORIZONTAL_REACH²)`
+  - `ARM_GROUND_ANGLE = asin(ARM_PIVOT_Z / ARM_LENGTH)`
+  - Echo statements output calculated values for verification
+
+- **Hydraulic Cylinder Sizing**
+  - `lift_cyl_length(arm_angle)` - calculates cylinder length at any arm position
+  - `bucket_cyl_length(arm_angle, bucket_tilt)` - calculates bucket cylinder length
+  - Automatic stroke calculation with 15% safety margin
+  - Selection of standard stroke sizes (150, 200, 250... 600mm)
+
+- **Cross Beam Cutout Geometry**
+  - `cross_beam_local_pos(beam_dist, angle)` - transforms beam position to panel coords
+  - Trigonometric arc slot cutouts based on arm rotation range
+  - Accounts for `ARM_MIN_ANGLE` to `ARM_MAX_ANGLE` sweep
+
+#### ✅ Parts Standardization
+
+Created `CUSTOM_PARTS_LIST.md` documenting standardized fabricated parts:
+
+- **Type A: U-Channel Lug**
+  - Made from torch-cut square tubing (3"×3" or 4"×4")
+  - Top and bottom removed to create U-shape
+  - Hole drilled through both walls for clevis pin
+  - Used for all cylinder attachments
+
+- **Type B: Large Pivot Ring**
+  - CNC plasma cut from 3/4" plate steel
+  - Used for arm pivot reinforcement
+  - Welded to arm tube at pivot point
+
+- **Type C: Small Pivot Ring**
+  - CNC plasma cut from 1" plate steel
+  - Used for QA plate pivot bosses
+
+- **Round Bar Stock**
+  - 1.5" diameter for main pivots
+  - 1" diameter for clevis pins
+
+#### ✅ Fastener Rendering
+
+Added hex bolt heads and nuts throughout the assembly:
+
+- **New Modules Created**
+  - `hex_nut(bolt_dia)` - renders hex nut with proper sizing
+  - `hex_bolt_head(bolt_dia)` - renders hex bolt head
+  - `hex_bolt_assembly(bolt_dia, length, show_nut)` - complete bolt with nut
+  - `clevis_pin(pin_dia, length)` - clevis pin with cotter pin holes
+  - `u_channel_lug_with_pin()` - U-channel with clevis pin installed
+
+- **Fasteners Added At**
+  - Arm pivot pins (1.5" with hex nuts both ends)
+  - Bucket pivot pin (1.5" with hex nuts both ends)
+  - All hydraulic cylinder clevis connections
+  - Cylinder mounting lugs (clevis pins with cotter pins)
+  - Cross beam cylinder brackets
+  - QA plate cylinder attachments
+  - Standing deck mounting bolts (3/4" hex bolts)
+
+#### ✅ Lifting Capacity Calculations
+
+- **Hydraulic Force**
+  - Operating pressure: 3000 PSI
+  - Lift cylinder bore: 63.5mm (2.5")
+  - Calculates force per cylinder and total force
+
+- **Lever Arm Geometry Functions**
+  - `lift_cyl_moment_arm(arm_angle)` - perpendicular distance from pivot to cylinder force line
+  - `load_moment_arm(arm_angle)` - horizontal distance from pivot to bucket load
+  - `lift_capacity_kg(arm_angle)` - lifting capacity at any arm position
+
+- **Capacity Analysis**
+  - Calculates capacity at ground, horizontal, 45°, and max positions
+  - Rated capacity = minimum across all positions (conservative)
+  - Outputs mechanical advantage ratios
+
+#### ✅ Structural Analysis (Mechanics of Deformable Bodies)
+
+- **Material Properties Defined**
+  - A36 Steel: 250 MPa yield, 200 GPa modulus
+  - Grade 8 Bolts: 130 ksi yield, 90 ksi shear
+  - Safety factors: 2.0 static, 3.0 fatigue, 2.5 bolts
+
+- **Arm Bending Stress Analysis**
+  - Moment of inertia for hollow square tube: `I = (b⁴ - b_inner⁴) / 12`
+  - Section modulus: `S = I / c`
+  - Bending stress: `σ = M / S`
+  - Compares to allowable stress with safety factor
+
+- **Arm Deflection Analysis**
+  - Cantilever tip deflection: `δ = PL³ / (3EI)`
+  - Allowable deflection: L/180
+
+- **Pivot Pin Shear Analysis**
+  - Double shear: `τ = F / (2A)`
+  - Compares to allowable shear (0.6 × yield)
+
+- **Bearing Stress Analysis**
+  - Bearing stress on pin holes: `σ = F / (d × t × 2)`
+  - Allowable bearing: 1.5 × yield
+
+- **Clevis Pin Analysis**
+  - Grade 8 bolt shear strength
+  - Double shear at cylinder attachments
+
+- **Cross Beam Stress Analysis**
+  - Point load bending: `M = FL/4`
+  - 2"×2"×1/4" tube section properties
+
+- **Weld Stress Analysis**
+  - Pivot ring welds (fillet around tube perimeter)
+  - Cross beam welds (shear loading)
+  - E70XX electrode allowable: 145 MPa / SF
+
+- **Results Summary Output**
+  - Table with stress ratios for all components
+  - Pass/fail status for each check
+  - Overall structural assessment
+
+#### 📄 Documentation Created
+
+- `STRUCTURAL_ANALYSIS.md` - Comprehensive explanation of all engineering calculations with formulas and design modification recommendations
+
+#### 🔧 Technical Improvements
+
+- All calculations output to console via `echo()` statements
+- Parametric design allows easy scaling and optimization
+- Stress ratios indicate how close to limits (< 1.0 = safe)
+- Design modifications suggested if any check fails
+
+#### 📋 Key Constants Added
+
+| Constant | Value | Description |
+|----------|-------|-------------|
+| `GROUND_CLEARANCE` | 150mm | Frame bottom above ground |
+| `FRAME_Z_OFFSET` | 150mm | Vertical offset for frame |
+| `ARM_GROUND_ANGLE` | Calculated | Angle to reach ground |
+| `HYDRAULIC_PRESSURE_PSI` | 3000 | Operating pressure |
+| `STEEL_YIELD_STRENGTH_MPA` | 250 | A36 steel yield |
+| `SAFETY_FACTOR_STATIC` | 2.0 | Static load SF |
+
+---
+
 ## Design Decisions
 
 ### Material Choices
@@ -179,7 +362,7 @@ The design successfully meets all requirements specified in the original issue:
 ✅ Complete part numbering and descriptions  
 ✅ AI progress tracking document
 
-**Ready for:** Prototype fabrication and testing
+**Ready for:** Prototype fabrication and testing, with engineering calculations to verify structural adequacy
 
 ---
-Last Updated: 2025-12-06
+Last Updated: 2025-12-07

LifeTrac-v25/mechanical_design/CUSTOM_PARTS_LIST.md

@@ -0,0 +1,114 @@
+# LifeTrac v25 Custom Parts List
+
+This document identifies parts that are NOT standard CNC-cut plate steel or standard square/round tubing. These parts require custom fabrication using standardized methods.
+
+**Excluded from this list:** Hydraulic components (cylinders, motors, valves), wheels/tires, fasteners (bolts, pins), and purchased components.
+
+---
+
+## STANDARDIZED PART TYPES (Implemented in OpenSCAD)
+
+### TYPE A: U-Channel Lug (from Square Tubing)
+**Fabrication:** Torch-cut top and bottom of square tube to create U-shape with pivot hole through sides.
+
+| Location | Source Tube | Length | Hole Dia | Qty |
+|----------|-------------|--------|----------|-----|
+| Frame lift cyl base mount | 4"x4"x1/4" | 100mm | 27mm (1"+2) | 2 |
+| Arm lift cyl attachment | 3"x3"x1/4" | 80mm | 27mm (1"+2) | 2 |
+| Cross beam bucket cyl mount | 3"x3"x1/4" | 80mm | 21mm (3/4"+2) | 2 |
+| QA plate bucket cyl mount | 3"x3"x1/4" | 80mm | 21mm (3/4"+2) | 2 |
+
+**Total Type A Brackets: 8**
+
+---
+
+### TYPE B: Large Pivot Ring (CNC Plasma Cut)
+**Material:** 3/4" plate steel
+**Dimensions:** 106mm OD, 40mm ID
+
+| Location | Qty per Arm | Total |
+|----------|-------------|-------|
+| Arm pivot reinforcement (2 rings per arm, inner and outer) | 2 | 4 |
+
+**Total Type B Rings: 4**
+
+---
+
+### TYPE C: Small Pivot Ring (CNC Plasma Cut)  
+**Material:** 1" plate steel
+**Dimensions:** 70mm OD, 40mm ID
+
+| Location | Qty |
+|----------|-----|
+| QA plate arm connection flanges | 2 |
+
+**Total Type C Rings: 2**
+
+---
+
+### ROUND BAR STOCK (1.5" / 38mm diameter)
+**Material:** 4140 steel or equivalent
+
+| Part | Length | Qty |
+|------|--------|-----|
+| Arm pivot pin | ~160mm | 2 |
+| Bucket pivot pin | ~1000mm | 1 |
+| QA hook bar | ~1018mm | 1 |
+
+**Total Round Bar Pieces: 4**
+
+---
+
+### CNC PLATE PARTS
+
+| Part | Material | Dimensions | Qty |
+|------|----------|------------|-----|
+| QA arm flange plate | 1/2" plate | 80x100mm | 2 |
+| Standing deck plate | 1/4" plate | 700x400mm | 1 |
+
+---
+
+### SQUARE TUBE SECTIONS (Cut to Length)
+
+| Part | Source Tube | Length | Qty |
+|------|-------------|--------|-----|
+| Deck connection bracket | 3"x3"x1/4" | 100mm | 2 |
+| Deck vertical support | 2"x2"x1/4" | ~244mm | 2 |
+| Deck cross brace | 2"x2"x1/4" | 600mm | 1 |
+
+---
+
+## FABRICATION SUMMARY
+
+### Type A U-Channel Fabrication Steps:
+1. Cut square tube to specified length
+2. Mark cut lines for U-channel (remove top and bottom faces)
+3. Torch cut or plasma cut the marked areas
+4. Grind cut edges smooth
+5. Drill pivot hole through both remaining walls
+6. Deburr hole edges
+
+### Type B/C Ring Fabrication:
+1. Nest all rings on plate steel for efficient cutting
+2. CNC plasma or laser cut
+3. Deburr inner and outer edges
+
+---
+
+## PARTS COUNT SUMMARY
+
+| Part Type | Description | Quantity |
+|-----------|-------------|----------|
+| Type A (4"x4") | U-Channel Lug, 27mm hole | 2 |
+| Type A (3"x3") | U-Channel Lug, 27mm hole | 2 |
+| Type A (3"x3") | U-Channel Lug, 21mm hole | 4 |
+| Type B Ring | Large Pivot Ring (3/4" plate) | 4 |
+| Type C Ring | Small Pivot Ring (1" plate) | 2 |
+| Round Bar | 1.5" dia, various lengths | 4 |
+| CNC Plate | Flange plates | 2 |
+| CNC Plate | Deck plate | 1 |
+| Tube Section | 3"x3" bracket | 2 |
+| Tube Section | 2"x2" deck frame | 3 |
+
+**Total Custom Fabricated Parts: 26**
+**Unique Fabrication Types: 3** (U-Channel, CNC Ring, Cut Tube)