Activity 002: File Format Comparison and Reverse Engineering Introduction

Activity ID: U11M3-ACT-002 Duration: 40 minutes Objective: Learners will export a processed scan mesh in multiple file formats, analyze the differences in data preservation and file size, and perform an introductory reverse engineering exercise by fitting geometric primitives to scan data.

Overview

This lab has two parts. In Part A, students export the same mesh in STL, OBJ, PLY, and 3MF formats and compare what data each format preserves or discards. In Part B, students perform a basic reverse engineering exercise — fitting planes, cylinders, and dimensions to a scanned mechanical part to understand the scan-to-CAD workflow.

Materials & Equipment Needed

• Computer workstation with MeshLab, CloudCompare, and a slicer (PrusaSlicer or Cura)
• Processed mesh from Activity 001 (with color/texture data if available)
• Simple mechanical part scan (provided by instructor — a part with identifiable geometric primitives: flat faces, cylindrical holes, fillets)
• CAD software with mesh import capability (Fusion 360 free license, or FreeCAD)
• File Format Comparison Worksheet (provided)
• Calculator
• USB drive for file transfer between applications

Instructions & Procedure

Part A: File Format Comparison (20 minutes)

Phase 1: Multi-Format Export (8 minutes)

1. Open your processed mesh from Activity 001 in MeshLab
2. Export in four formats, recording the settings used:

Export	Format	Settings
Export 1	STL Binary	Default
Export 2	STL ASCII	Default
Export 3	OBJ + MTL + texture	Include texture coordinates and material file
Export 4	PLY Binary	Include vertex colors, normals
Export 5	3MF	Include color/material if supported

1. Record file sizes for each export on your worksheet

Phase 2: Format Analysis (7 minutes)

1. For each exported file, determine what data was preserved:

Data Type	STL Binary	STL ASCII	OBJ	PLY	3MF
Vertex positions
Face normals
Vertex colors
Texture coordinates
Material properties
File size (MB)

1. Open the STL ASCII file in a text editor — examine the first 20 lines to understand the format structure
2. Compare visual quality: Open each format in its native viewer:
3. STL: Load in slicer software — does it slice correctly?
4. OBJ: Load in MeshLab — is the texture/color preserved?
5. PLY: Load in CloudCompare — are vertex colors and normals intact?

Phase 3: Format Selection Exercise (5 minutes)

For each scenario, select the best export format and justify your choice:

Scenario 1: Sending a scan to an FDM 3D printer for a functional replacement part Scenario 2: Archiving a museum artifact scan with full surface color documentation Scenario 3: Sharing a scan with a client who uses SolidWorks for engineering Scenario 4: Publishing a 3D model on a web-based viewer (Sketchfab) Scenario 5: Sending a colored scan to a full-color binder jet 3D printer

Part B: Introduction to Reverse Engineering (20 minutes)

Phase 1: Mesh Import into CAD (5 minutes)

1. Open Fusion 360 (or FreeCAD)
2. Import the mechanical part mesh (Insert > Insert Mesh in Fusion 360)
3. Orient the mesh to align with the coordinate system:
4. Identify the primary flat face — this becomes the XY reference plane
5. Identify the principal axis — align with X or Y
6. Use the Align tool to snap the mesh to the origin
7. Observe: the mesh is displayed as a fixed reference body — it cannot be edited as CAD geometry

Phase 2: Cross-Section Analysis (5 minutes)

1. Create a cross-section plane through the center of the part
2. Sketch the cross-section profile:
3. Identify straight edges (from flat faces) — note angles
4. Identify arcs (from cylindrical features or fillets) — estimate radii
5. Identify holes — estimate diameters
6. Measure key dimensions using the CAD software's measurement tools:
7. Overall length, width, height
8. Hole diameters
9. Wall thickness
10. Fillet radii
11. Record all measurements on your worksheet

Phase 3: Geometric Primitive Fitting (8 minutes)

1. Fit a plane to the largest flat face of the mesh:
2. In Fusion 360: use the Mesh Section tool or Sketch > Project to create a reference
3. Note: this identifies that the surface is planar and defines its orientation
4. Fit a cylinder to a cylindrical feature (hole or boss):
5. Measure the diameter from the cross-section
6. Create a sketch circle matching the measured diameter
7. Extrude to approximate the feature
8. Build a simplified parametric model:
9. Using your measurements, create a basic sketch with the overall profile
10. Extrude, add holes, and add fillets to approximate the scanned part
11. This does not need to be perfect — the goal is to understand the workflow
12. Compare your CAD model to the scan mesh:
13. Visually overlay the CAD body on the mesh
14. Identify areas where your parametric model deviates from the scan

Phase 4: Deviation Discussion (2 minutes)

1. Where did your CAD model match the scan well? Where did it deviate?
2. What features were hardest to parameterize?
3. How would professional reverse engineering software (Geomagic Design X) accelerate this process?

Discussion Points

1. Why is STL ASCII so much larger than STL Binary for the same mesh? What is the trade-off?
2. If you needed to archive a scan for 50 years, which format would you choose and why?
3. How did the reverse engineering exercise feel compared to designing a part from scratch?
4. What types of objects would be nearly impossible to reverse engineer from scan data?

Expected Outcomes

• Completed file format comparison table with sizes, data preservation, and format selection justifications
• Understanding of what each file format stores and discards
• A basic parametric CAD model approximating the scanned mechanical part
• Appreciation for the complexity and skill required in professional reverse engineering

Assessment Rubric

Criterion	4 (Excellent)	3 (Good)	2 (Fair)	1 (Needs Improvement)
Format Comparison	Complete, accurate comparison; understands data differences; correct format selections with strong justification	Good comparison with minor gaps; reasonable format selections	Basic comparison; incomplete data analysis; weak justification	Incomplete comparison or significant errors
Format Selection	All scenarios answered with correct format and multi-factor justification	Most scenarios correct with reasonable justification	Some correct selections but weak reasoning	Incorrect selections or missing scenarios
Reverse Engineering	Measurements taken accurately; parametric model captures main features; deviation analysis completed	Measurements reasonable; basic model created; deviation noted	Measurements attempted; model partially completed	Unable to complete reverse engineering exercise
Conceptual Understanding	Articulates why meshes differ from CAD; understands the full scan-to-CAD pipeline	Good understanding with minor conceptual gaps	Basic understanding of the workflow	Does not distinguish mesh from parametric CAD

Safety Considerations

• This is a computer-based activity with no direct physical hazards
• Maintain proper ergonomic workstation posture
• If transferring files via USB: scan for malware per facility IT policy before opening on lab computers
• If 3D printing any exported files: follow all applicable printer safety protocols

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Last Updated: 2026-03-19