3D printing and CNC machining make parts in opposite ways — additive vs subtractive. Each has specific strengths. The economics shift dramatically with volume, geometry complexity, and material requirements. Picking the wrong process can cost 2-10× more than needed. This guide covers the decision logic.
Use 3D printing when any of these apply:
Use CNC machining when any of these apply:
At prototype quantities, 3D printing usually wins. At production, CNC usually wins. The crossover depends on:
| Part characteristic | 3D printing crossover qty |
|---|---|
| Small part, simple geometry, plastic | 5-20 units (CNC wins sooner) |
| Small part, complex geometry, plastic | 20-100 units |
| Medium part, complex geometry, plastic | 50-500 units |
| Small part, simple geometry, metal (DMLS) | 2-10 units (CNC wins very fast) |
| Small part, impossible geometry, metal | 1,000+ units (DMLS stays competitive) |
| Large part, any geometry | CNC wins at almost any volume |
Rule: the more complex the geometry, the higher the 3D printing crossover quantity. The simpler the geometry, the faster CNC becomes economical.
| Material | 3D print quality | CNC quality | Decision |
|---|---|---|---|
| Aluminum 6061 | Poor (porosity, anisotropic) | Excellent | CNC strongly preferred |
| Aluminum AlSi10Mg | Good (DMLS specific alloy) | n/a (different alloy) | 3D print for complex geometry |
| Stainless 316 | Good (DMLS) | Excellent | CNC unless geometry demands 3D |
| Titanium Ti64 | Excellent (DMLS matches wrought) | Excellent | Either works; CNC cheaper for simple |
| ABS, PP, PC | Good (FDM) | Excellent (CNC) | 3D for prototyping; CNC/mold for production |
| Nylon (PA12) | Excellent (SLS/MJF) | Good | 3D for complex, CNC for simple |
| PEEK, PEI (Ultem) | Good (specialty FDM) | Excellent | CNC preferred for structural |
| Elastomers (TPU) | Good (FDM, SLA) | Limited | 3D print or molding |
3D printing tolerances are looser than CNC:
| Technology | Typical tolerance | Surface Ra (as printed) |
|---|---|---|
| FDM | ±0.3 mm | 15-25 μm (visible layer lines) |
| SLA / DLP | ±0.1 mm | 2-5 μm (smooth) |
| SLS | ±0.2 mm | 10-15 μm (grainy) |
| MJF | ±0.15 mm | 8-12 μm (grainy but finer) |
| DMLS metal | ±0.1 mm | 8-20 μm (usually post-machined) |
| CNC milling | ±0.05 mm (standard) | 0.8-3.2 μm |
| CNC turning | ±0.025 mm | 0.8-1.6 μm |
For critical mating surfaces, ±0.1 mm or better, and Ra below 1.6 μm, CNC is the only viable option (or 3D printed parts with post-machined critical features).
FDM or SLA. Fast (3-7 days), cheap ($20-80 per part). Doesn't matter that CNC would be stronger — you're iterating design.
CNC typically. Needs mechanical rigidity and dimensional accuracy. FDM fixture could deflect; CNC aluminum fixture will last years.
DMLS (metal 3D print). Internal conformal channels are impossible with subtractive methods. Cost is justified by the geometry requirement.
CNC. At this volume, setup cost amortizes well, and CNC delivers better mechanical properties at lower per-part cost than 3D printing.
SLS or MJF. Around the crossover — if geometry is complex, 3D print wins; if boxy, CNC wins.
DMLS (metal 3D print) for complex patient-specific geometry + certification, or CNC titanium for standard stock components.
Email [email protected]. We offer both processes in the same facility — tell us quantity, geometry, and material and we'll suggest the optimal process and quote accordingly.
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