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Design Review & Iteration Guide

Table of Contents

  1. Preview-Analyze-Iterate Workflow
  2. Visual Inspection Checklist
  3. Dimensional Verification
  4. Printability Analysis
  5. Common Issues and Fixes

1. Preview-Analyze-Iterate Workflow

After generating a model, ALWAYS run the preview-analyze-iterate loop before delivering the final STL.

Step 1: Generate Preview

# Install dependencies (first time only)
pip install trimesh pyrender Pillow

# Generate multi-view preview
python3 preview.py model.stl preview.png --views multi

Step 2: View the Preview

Use the view tool to look at the generated PNG. This lets you see the model from 4 angles (isometric, front, top, right) with dimensions.

Step 3: Analyze the Preview

Check against this visual inspection list:

Shape & Proportions

  • Does the overall shape match what was requested?
  • Are proportions reasonable (not too thin, not too bulky)?
  • Are all requested features visible (holes, slots, cutouts)?

Printability

  • Is there a flat bottom surface for bed adhesion?
  • Are there visible overhangs > 45 degrees that need supports?
  • Do thin features look thick enough to print (> 1.2mm)?
  • Are screw bosses and standoffs properly connected to walls?

Geometry Issues

  • Are there any obviously missing features?
  • Do boolean operations look clean (no floating geometry)?
  • Are fillets and chamfers applied to the right edges?
  • Does the interior look correctly hollowed out?

Dimensions

  • Check bounding box in the preview footer
  • Compare against user's stated requirements
  • Verify critical dimensions from the code match visual

Step 4: Fix Issues

If problems are found:

  1. Identify the specific issue from the preview
  2. Trace it to the relevant code section
  3. Fix the code
  4. Re-export STL
  5. Re-generate preview
  6. Verify the fix

Step 5: Deliver

Once the preview passes inspection:

  1. Export final STL (and optionally STEP)
  2. Generate final preview image
  3. Share both the STL and preview with the user
  4. Note any print recommendations (orientation, supports, infill)

2. Visual Inspection Checklist

Use this as a structured review after viewing the preview:

VISUAL REVIEW
=============
[ ] Overall shape matches request
[ ] All features present (holes, cutouts, mounts, etc.)
[ ] Proportions look correct
[ ] No floating or disconnected geometry
[ ] Flat bottom for printing
[ ] No extreme overhangs visible
[ ] Fillets/chamfers applied correctly
[ ] Wall thickness appears sufficient
[ ] Interior properly hollowed (if applicable)
[ ] Dimensions match requirements (check footer)

3. Dimensional Verification

Beyond visual inspection, verify dimensions programmatically:

import cadquery as cq

# After building the model:
bb = result.val().BoundingBox()
print(f"Width  (X): {bb.xlen:.1f} mm")
print(f"Depth  (Y): {bb.ylen:.1f} mm") 
print(f"Height (Z): {bb.zlen:.1f} mm")
print(f"Volume: {result.val().Volume():.0f} mm³")

# Check specific dimensions
assert abs(bb.xlen - expected_width) < 0.1, f"Width mismatch: {bb.xlen} vs {expected_width}"

Key Checks

  • Bounding box matches expected outer dimensions
  • Volume is reasonable (not suspiciously small or large)
  • Volume > 0 confirms the model is a valid solid
  • Compare volume with/without shell to verify wall thickness

4. Printability Analysis

Overhang Detection (Approximate)

import trimesh
import numpy as np

def check_overhangs(stl_path, max_angle=45):
    """Check for faces with overhang angle beyond threshold.

    Overhang angle is measured from the horizontal plane.
    A flat bottom (normal pointing straight down) = 0 deg overhang (safe).
    A face angled 50 deg from horizontal = needs supports if max_angle=45.

    Returns percentage of total faces that may need supports.
    """
    tm = trimesh.load(stl_path, force="mesh")
    normals = tm.face_normals

    # Only check downward-facing normals (potential overhangs)
    downward = normals[:, 2] < 0
    if not downward.any():
        return 0.0

    down = np.array([0, 0, -1])
    down_normals = normals[downward]

    # Angle between each normal and straight-down vector
    cos_angles = np.dot(down_normals, down) / (
        np.linalg.norm(down_normals, axis=1) + 1e-10
    )
    angles_from_down = np.degrees(np.arccos(np.clip(cos_angles, -1, 1)))

    # angle_from_down=0 means flat bottom (safe)
    # angle_from_down=90 means vertical face (safe)
    # Overhang angle from horizontal = 90 - angle_from_down
    overhang_from_horizontal = 90 - angles_from_down
    problem_faces = np.sum(overhang_from_horizontal > max_angle)

    return problem_faces / len(normals) * 100

pct = check_overhangs("model.stl")
if pct > 5:
    print(f"WARNING: {pct:.0f}% faces may need supports")
else:
    print(f"OK: Model looks printable without supports ({pct:.1f}% overhang)")

Thin Wall Detection

def estimate_min_wall(result):
    """Rough check - compare shelled vs non-shelled volume."""
    vol = result.val().Volume()
    bb = result.val().BoundingBox()
    outer_vol = bb.xlen * bb.ylen * bb.zlen
    fill_ratio = vol / outer_vol
    
    if fill_ratio < 0.15:
        print("⚠ Very thin walls - may be fragile")
    elif fill_ratio < 0.3:
        print("✓ Thin-walled design (typical for enclosures)")
    else:
        print("✓ Solid/thick-walled design")
    
    return fill_ratio

5. Common Issues and Fixes

Issue: Feature not visible in preview

Cause: Boolean operation failed silently Fix: Check that cutting bodies actually overlap the main body. Print intermediate volumes.

Issue: Model looks correct but walls are paper-thin

Cause: Shell thickness too small, or shell removed wrong faces Fix: Verify shell direction (negative = inward) and thickness >= 1.2mm

Issue: Standoffs appear disconnected

Cause: Union failed because standoffs don't touch the body Fix: Ensure standoff base cylinder intersects with the floor/wall of the body

Issue: Cutout in wrong position

Cause: Coordinate system confusion - CadQuery centers at origin by default Fix: Use centered=(True, True, False) and verify offsets from known reference points

Issue: Fillets look wrong or preview crashes

Cause: Fillet radius too large for the edge Fix: Reduce fillet radius. Rule of thumb: fillet radius < half the smallest adjacent face dimension

Issue: Preview shows correct shape but dimensions are wrong

Cause: Forgot to update a parameter or used wrong variable Fix: Add assertions comparing bounding box to expected dimensions