CAD fundamentals, 3D printing (FDM/SLA/SLS), CNC machining, workshop skills, rapid prototyping methodology, and testing of physical prototypes. Covers fidelity levels, material selection for prototypes, dimensional tolerancing, assembly planning, and the iterate-test loop. Use when building prototypes, selecting fabrication methods, planning physical tests, or choosing between prototyping technologies.
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
| 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.
Before building anything, write down the question:
A prototype without a question is a hobby project. An engineering prototype has a test plan before the first cut is made.
Computer-Aided Design is the standard tool for defining geometry before fabrication.
| 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.
| 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.
| 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.
| 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).
| 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 (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.
| 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 |
Even in the age of digital fabrication, manual workshop skills are essential for rapid prototyping.
| 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 |
Workshop safety is non-negotiable:
Before fabrication, write the test plan:
| Test | What it reveals |
|---|---|
| Fit check | Do parts assemble correctly? |
| Load test | Does the structure carry the design load? |
| Cycle test | Does the mechanism survive repeated operation? |
| Drop test | Does the product survive impact? |
| Thermal test | Does performance change with temperature? |
| User test | Can the intended user operate it? |
When a prototype fails -- and it should, because that is how learning happens -- analyze the failure:
The mindset. A prototype that does not fail is either over-designed (wasted resources) or under-tested (missed knowledge). Failures in prototyping are cheap lessons that prevent failures in production.