A Real Scenario
A German automotive supplier sent us a gearbox housing CAD. The drawing called for “aluminium, high strength.” No alloy specified, no surface finish standard, no tolerance annotations on non-critical features.
They had already burned 4 weeks and €3,200 with a local shop that couldn’t hold flatness on the mounting face. The parts warped during machining.
We asked three questions before touching the file:
- Is this a fit-test prototype or a functional load test?
- What bolts into the threaded holes — steel fasteners or aluminium?
- What’s the assembly environment — outdoor, engine bay, clean room?
The answers changed everything. Instead of 6061 (their local shop’s default), we used 7075-T6. Instead of standard anodizing, we spec’d hard anodize with 50μm thickness for the mounting face. Instead of assuming ±0.1mm everywhere, we held ±0.02mm only on the dowel pin locations — saving machining time and cost.
The parts worked. First try. Five days from CAD to shipment.
This guide covers what that German team learned the hard way.
The 2 Alloys That Matter
Two grades cover 90% of prototyping needs. The trick is knowing which fits your test objectives.
6061 Aluminium — The Workhorse
Yield strength: 276 MPa. Excellent machinability. Takes standard anodizing (Type II) beautifully. Weldable. Corrosion-resistant enough for most indoor and covered outdoor applications.
Use 6061 when: you’re verifying fit and form, not pushing load limits. Brackets, enclosures, mounting plates, heat sinks, most consumer product housings.
Typical CNC tolerances achievable: ±0.05mm on features, ±0.02mm on bores. Surface finish as-machined: Ra 0.8-1.6μm.
Cost benchmark: A palm-sized 6061 part with 5 machined features typically runs $60-120 for a single prototype, including material.
7075 Aluminium — The Athlete
Yield strength: 503 MPa — nearly double 6061. Comparable to mild steel in strength, at one-third the weight. Harder to machine (requires sharp tooling, slower feed rates). Not weldable by conventional methods. Costs 30-50% more than 6061.
Use 7075 when: your prototype takes repeated impact, structural load, or will be tested to failure. Aerospace brackets, bicycle suspension links, drone frames, racing components, high-stress robotics joints.
Surface finish post-machining: Ra 0.8-1.6μm. Anodizing is possible but produces a slightly darker, less decorative finish than 6061.
Decision shortcut: If your part gets bolted on and stays there → 6061. If it takes impact, vibration, or load cycling → 7075. If you’re not sure, ask your manufacturer to review the FEA stress map with you.
CNC vs 3D Printing: A Practical Comparison
Both make aluminium parts. They solve different problems.
| Factor | CNC-Bearbeitung | 3D Printing (DMLS/SLM) |
|---|---|---|
| Surface finish | Ra 0.8-1.6μm | Ra 6-10μm (needs post-processing) |
| Tolerance | ±0.02-0.05mm | ±0.1-0.2mm |
| Material properties | Same as wrought | ~95% of wrought (slightly lower fatigue) |
| Cost (simple geometry) | $60-120 | $150-300 |
| Cost (complex geometry) | $200-500 | $150-300 |
| Threaded holes | Machined directly | Must be post-machined |
| Best for | Functional prototypes, production path | Complex organic shapes, topology-optimized |
Real scenario: A medical device startup needed 3 prototype surgical instrument handles. Complex ergonomic curves, but with M3 threaded inserts for assembly. They tried DMLS first — the thread areas needed secondary machining anyway, and the surface was too rough for sterilization testing. CNC from 6061 solved both problems in one process, at half the cost.
Rule of thumb: If your part has flat faces, holes, threads, or will eventually be CNC-machined in production → prototype with CNC. If it has organic, bone-like structures that can’t be machined → 3D print.
Quality Verification: 5 Things to Check Before Your Prototype Ships
Most overseas buyers accept “looks good” photos as inspection. Don’t.
1. CMM Dimensional Report
Request a Coordinate Measuring Machine report on features you’ve marked as critical. A real report shows: feature number matching your drawing callout, nominal dimension vs actual measured value, deviation in mm, upper/lower tolerance limits, and pass/fail per feature.
If your supplier cannot produce this, they are doing visual inspection — not measurement.
2. Surface Finish Data
“As machined” is not a specification. Neither is “smooth.” Specify Ra (roughness average) in μm or μin: Standard CNC finish: Ra 0.8-1.6μm. Fine finish (pre-anodize): Ra 0.4-0.8μm. Mirror polish: Ra <0.1μm.
Request a profilometer reading on one critical surface. It takes 30 seconds.
3. Material Certificate
For 7075 and other high-performance alloys, request the mill certificate (EN 10204 3.1 or equivalent). This traces the material back to the smelter and confirms chemical composition and mechanical properties. For 6061 from common stock, a certificate is less critical — but still worth requesting if your part is safety-related.
4. Photo Approval Before Shipping
Get three standardized angles: top-down overview, close-up of the most critical feature, and thread check (with a gauge inserted, if applicable). This catches 80% of issues: wrong surface finish, missed features, tool marks, obvious dimensional errors.
5. Thread Gauge Verification
The #1 overseas defect we see: wrong thread specification. M6 tapped when the drawing called for 1/4″-20 UNC. Or vice versa. Specify metric vs imperial explicitly. If your drawing is ambiguous, add a note: “ALL THREADS METRIC UNLESS MARKED OTHERWISE.”
China Sourcing Reality
Chinese aluminium prototyping costs 40-60% less than US/EU equivalents for comparable quality. Lead times run 3-7 days for standard parts vs 7-14 days domestically.
The trade-off is communication overhead, not quality. The shops that produce good work exist — you just need to find them before sending money. The five verification checks above are your filter.
One factory video call showing machines running, QC equipment, and material storage is worth 50 emails of back-and-forth.
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