Week 4 – 3D Printing & Scanning
Assignment Brief
The challenge: design something that cannot be made subtractively—no milling, turning, drilling, or traditional machining could produce this geometry as a single piece.
Design: Orbis Core (Captive Ball Sphere)
Concept
A sphere trapped inside a hollow cage with orthogonal window openings. The ball is permanently captive: it can move freely inside but cannot escape because its diameter is larger than the window openings. This "parts within parts" geometry is only possible through additive manufacturing, where the ball and cage are printed simultaneously as separate bodies.
You cannot machine a captured sphere inside a cage without splitting the part. No drill bit can create an enclosed spherical cavity with a free-moving object inside. No lathe can turn internal geometry that's already enclosed. No mill can cut through to create the ball without also creating an exit path.
The only way to produce this is by printing the ball and cage simultaneously, where support material provides temporary scaffolding that's later removed to release the ball.
Design Iterations
Version 1 (White PLA, ~30mm):
- Outer cage: 32mm diameter
- Windows: 16mm diameter
- Inner ball: 14mm diameter
- Problem discovered: Ball diameter smaller than windows = ball could theoretically escape
- Print time: ~12 minutes with snug supports
- Learning: Dimensional constraints matter—supports were necessary at the pole to prevent sagging
Version 2 (White PLA, ~30mm):
- Corrected dimensions: ball diameter > window diameter
- Tested organic supports vs. snug supports
- Learning: Organic supports easier to remove but required more material
Version 3 (Black PLA, scaled up ~50mm):
- Outer cage: ~50mm diameter
- Windows: proportionally scaled (smaller than ball)
- Inner ball: successfully captive after support removal
- Print time: ~1.5 hours
- Success: Ball moves freely inside, rattles when shaken, cannot exit
CAD Process (Fusion 360)
- Created outer sphere (hollow shell using Shell command, 1mm wall thickness)
- Cut three orthogonal cylindrical windows (XY, YZ, XZ planes)
- Created inner sphere as separate body (not combined with cage)
- Added pole vent holes to reduce bridging requirements
- Verified clearance: ~1mm gap between ball and cage interior
Critical constraint: Inner ball diameter must be larger than window diameter but smaller than cage interior.
Design Files
Print Settings
Printer: Prusa Core One
Material: PLA (White and Black)
Layer height: 0.2mm
Perimeters: 3
Top/Bottom layers: 4
Infill: 15% gyroid
Supports: Organic (easier removal) or Snug (more precise)
Nozzle temp: 210°C (slightly lower to prevent fusion between ball and cage)
Bed temp: 60°C
Connection to Final Project (Orbis)
This captive ball concept relates to my final project—a tactile reflection dial—in two ways:
- Conceptual: Physical constraint as metaphor—the ball moves within defined boundaries, like reflection operates within personal/organizational scales
- Technical: Validates the print tolerances I'll need for the encoder housing and detent mechanism (clearance gaps, rotating parts, precision fits)
Key Learnings
- Dimensional relationships are critical: Ball diameter > window diameter > cage wall thickness
- Supports are necessary, not failure: The slicer understands overhangs better than theoretical "self-supporting" designs
- Iteration reveals constraints: First print taught me about the ball escape problem
- Clearance matters: 1mm gap prevents fusion while allowing movement
- Scale affects print time exponentially: 30mm sphere = 12 min, 50mm sphere = 1.5 hours
- Temperature tuning: Lowering nozzle temp by 5°C (210°C vs 215°C) reduced risk of ball fusing to cage during print
3D Scanning (Optional Component)
To be completed: Scan reference object using Polycam or lab scanner, process mesh, document workflow.
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