prototyping-fabrication

Prototyping and Fabrication

Prototyping is the bridge between design intent and physical reality. A prototype is a testable embodiment of a design idea -- it answers questions that analysis alone cannot. This skill covers the full range of prototyping methods from cardboard mockups to CNC-machined metal parts, with emphasis on choosing the right fidelity level, the right fabrication method, and the right test plan for each stage of the design cycle.

Agent affinity: lovelace-e (materials and fabrication expertise), watt (mechanical systems and testing)

Concept IDs: engr-rapid-prototyping, engr-testing-methodology, engr-data-from-experiments, engr-failure-analysis

Prototyping Strategy

Fidelity Levels

| Level | Purpose | Typical materials | Time | Cost | |---|---|---|---|---| | Proof of concept | Does the idea work at all? | Cardboard, foam, tape, Arduino | Hours | $1-50 | | Form study | Does it look and feel right? | 3D-printed plastic, clay | Hours to days | $10-200 | | Functional prototype | Does it perform as designed? | 3D print, laser-cut, machined parts | Days to weeks | $100-5,000 | | Pre-production | Can it be manufactured? | Production materials, production-intent processes | Weeks to months | $1,000-50,000+ |

The cardinal rule. Build the lowest-fidelity prototype that answers the question. A cardboard mockup that proves a mechanism works in 30 minutes is worth more than a machined prototype that proves the same thing in 3 weeks. Save high-fidelity prototyping for questions that require it: material strength, precision fit, thermal behavior.

What Question Does This Prototype Answer?

Before building anything, write down the question:

• "Will the gear train fit in the housing?" (form study)
• "Can the mechanism produce the required force?" (functional prototype)
• "Does the user understand the interface?" (proof of concept + user test)
• "Will the part survive 10,000 cycles?" (functional prototype with production materials)

A prototype without a question is a hobby project. An engineering prototype has a test plan before the first cut is made.

CAD Fundamentals

Computer-Aided Design is the standard tool for defining geometry before fabrication.

CAD Workflow

1. Sketch in 2D: Define profiles with dimensions and constraints.
2. Feature in 3D: Extrude, revolve, sweep, loft, or shell the sketch into solid geometry.
3. Assembly: Mate parts together with constraints (coincident, concentric, parallel, distance).
4. Drawing: Generate 2D engineering drawings from the 3D model with dimensions and tolerances.
5. Export: STL for 3D printing, DXF for laser cutting, G-code for CNC, STEP/IGES for interchange.

Parametric vs. Direct Modeling

| Approach | How it works | Best for | |---|---|---| | Parametric | Features linked by a history tree; change a dimension and everything updates | Production parts with revision control | | Direct | Push/pull faces without history; faster for quick exploration | Proof-of-concept and form studies |

Common mistake. Over-constraining sketches. A fully constrained sketch turns black/green in most CAD systems. Under-constrained sketches move unexpectedly. Over-constrained sketches refuse to update. The goal is exactly constrained.

3D Printing

Fused Deposition Modeling (FDM)

| Parameter | Typical value | |---|---| | Layer height | 0.1 - 0.3 mm | | Materials | PLA, ABS, PETG, Nylon, TPU | | Accuracy | +/- 0.5 mm | | Strength | Moderate (anisotropic -- weak between layers) | | Cost per part | Low | | Build volume | 200x200x200 mm (desktop) to 500+ mm (large format) |

Best for: Proof of concept, form studies, jigs and fixtures, low-load functional parts.

Limitations: Anisotropic strength (layers are the weak point), visible layer lines, limited material properties compared to machined or injection-molded parts.

Stereolithography (SLA/DLP)

| Parameter | Typical value | |---|---| | Layer height | 0.025 - 0.1 mm | | Materials | Standard resin, engineering resin, flexible resin, castable resin | | Accuracy | +/- 0.1 mm | | Strength | Moderate to high (isotropic) | | Cost per part | Moderate | | Build volume | 130x80x150 mm (desktop) to 300+ mm (large format) |

Best for: High-detail form studies, snap-fit prototypes, investment casting patterns, dental/medical models.

Limitations: Post-curing required, resin handling (gloves, ventilation), UV sensitivity of uncured parts, higher material cost than FDM.

Selective Laser Sintering (SLS)

| Parameter | Typical value | |---|---| | Layer height | 0.1 mm | | Materials | Nylon (PA12, PA11), glass-filled nylon, TPU | | Accuracy | +/- 0.3 mm | | Strength | High (near isotropic) | | Cost per part | High | | Build volume | 250x250x300 mm typical |

Best for: Functional prototypes requiring durability, complex geometries (no support structures needed), living hinges, production-intent parts in nylon.

Limitations: Grainy surface finish, limited material palette, expensive machines (service bureaus common).

Choosing a Print Technology

| Question | FDM | SLA | SLS | |---|---|---|---| | Cost-sensitive? | Best | Moderate | Expensive | | Need fine detail? | No | Yes | Moderate | | Need strong parts? | Moderate | Moderate | Best | | Complex internal geometry? | Needs supports | Needs supports | No supports | | Large parts? | Yes | Limited | Moderate |

CNC Machining

CNC (Computer Numerical Control) removes material from a solid block to create the desired shape. It produces parts from real engineering materials (aluminum, steel, titanium, engineering plastics) with tight tolerances.

CNC Process Types

| Process | How it works | Typical tolerance | |---|---|---| | Milling | Rotating cutter, workpiece fixed | +/- 0.025 mm | | Turning (lathe) | Workpiece rotates, cutter fixed | +/- 0.013 mm | | Wire EDM | Electrical discharge cuts along a wire path | +/- 0.005 mm |

Design for CNC

• Avoid deep narrow pockets. Tool length/diameter ratio should not exceed 4:1 without special tooling.
• Use standard radii. Inside corners require a radius equal to the cutter radius. Specify standard cutter sizes.
• Minimize setups. Each time the part is repositioned introduces error. Design features accessible from as few orientations as possible.
• Specify tolerances only where needed. Tight tolerances cost money. Default tolerance (+/- 0.1 mm) is fine for most features; tighten only at functional interfaces.

Workshop Skills

Even in the age of digital fabrication, manual workshop skills are essential for rapid prototyping.

Core Competencies

| Skill | Tools | When used | |---|---|---| | Measuring | Calipers, micrometers, height gauges | Dimensional verification of all prototypes | | Cutting | Band saw, hacksaw, snips, utility knife | Rough shaping of stock material | | Filing and deburring | Files, deburring tools, sandpaper | Finishing edges and removing sharp burrs | | Drilling | Drill press, hand drill, step drill | Hole-making in any material | | Fastening | Taps, dies, wrenches, screwdrivers | Assembly of multi-part prototypes | | Soldering/brazing | Soldering iron, torch | Electrical connections, metal joining | | Adhesive bonding | Epoxy, cyanoacrylate, contact cement | Joining dissimilar materials |

Safety

Workshop safety is non-negotiable:

• Eye protection whenever cutting, drilling, grinding, or machining.
• Hearing protection with power tools above 85 dB.
• No loose clothing, jewelry, or long hair near rotating machinery.
• Know the emergency stop location on every machine before using it.
• Never work alone in a workshop with power tools.

Testing Prototypes

Test Planning

Before fabrication, write the test plan:

1. Objective: What question does the test answer?
2. Setup: How is the prototype mounted and loaded?
3. Instrumentation: What is measured (force, displacement, temperature, ti