The question comes up regularly: “Can we mill that instead?” Often, yes – but whether it makes sense depends on quantity, geometry and budget. Here’s an honest, practice-based comparison.
What CNC Can Do That 3D Printing Can’t
CNC machining is the superior process when it comes to:
- Metal parts (steel, aluminum, brass)
- Very tight tolerances under ±0.05 mm
- Surface finishes like Ra 0.8 or better
- Parts that must withstand high continuous loads or temperatures above 150 °C
For these cases, 3D printing is not an alternative.
What 3D Printing Can Do That’s Hard for CNC
FFF 3D printing is more economical or technically superior when:
Complex geometries are required. Undercuts, internal channels, honeycomb structures – anything that requires multiple setups or special tooling when milling costs nothing extra in 3D printing.
Small quantities under ~20 parts. No setup costs, no fixtures, no minimum quantity. A single part costs the same as five.
Functional prototypes under time pressure. From CAD model to a physical part in 24–48 hours – CNC machinists often need weeks of lead time.
Lightweighting through topology optimization. Structures like lattice infill or optimized wall thicknesses are barely achievable when milling.
Cost Threshold: When Does CNC Become Cheaper?
As a rough rule of thumb:
| Quantity | Recommendation |
|---|---|
| 1–20 parts | 3D printing usually cheaper |
| 20–100 parts | Depends on geometry and material |
| 100+ parts | CNC or injection molding often more economical |
For high-performance polymers like PETG-HF or ASA, the cost gap to CNC machining for plastics narrows further, because milling PETG is time-consuming and expensive.
Tolerances in FFF Printing
Realistic expected values on calibrated systems:
- XY plane: ±0.1–0.2 mm
- Z-axis: ±0.2–0.3 mm
- Bore diameter: ±0.1 mm (measuring and, if needed, reaming afterward is recommended)
For fits, we recommend marking critical dimensions in the STEP model. We account for this during slicing.
Hybrid Approach
In practice, we often work hybrid: the base housing comes from the 3D printer, while individual bearing seats or threaded bushings are press-fit or drilled afterward. This combines the geometric freedom of 3D printing with the precision of mechanical post-processing.
Conclusion
3D printing is no replacement for CNC – but for plastic parts in small quantities with complex geometry, it’s usually faster, cheaper and more flexible. If you’re unsure, describe the part to us – we’ll give you an honest assessment of which process makes sense for your case.