FDM vs CNC — when to print plastic, when to mill it.
FDM and CNC machining both produce plastic parts, but they reach the same goal from opposite directions. FDM is additive, building parts from filament layer by layer. CNC is subtractive, removing material from a solid billet of plastic stock. For prototype volumes under about 10 parts, FDM is almost always faster and cheaper. For tight-tolerance production plastics or polymer grades not available in 3D printing, CNC is still the right answer.
Detailed comparison
Property-by-property breakdown
| Factor | FDM 3D Printing | CNC Machining |
|---|---|---|
| Best tolerance | ±0.2mm | ±0.025mm |
| Complex geometry | Free | Multi-axis or multi-part setup |
| Typical lead time | 2–3 days | 5–10 days |
| Setup cost | $0 | $100–$500 per part |
| Material options | 6 thermoplastics (PLA, PETG, ABS, ASA, PC CF, TPU) | 40+ machinable plastics (+ all metals) |
| Surface finish | Visible layer lines | Smooth as-machined |
| Cost per part at 1 unit | $ | $$$ |
| Cost per part at 100 units | $$ | $$ |
| Best for | Rapid iteration, complex geometry, cheap prototypes | Tight tolerances, hard polymers, machined finishes |
Our recommendation
Choose FDM when geometry is complex (undercuts, lattices, internal channels), when volume is low, or when turnaround is the priority. Choose CNC when tolerances are tighter than ±0.1mm, when the material must be POM, PEEK, Ultem, or HDPE, or when surface finish has to be machined-smooth.
FAQ
Frequently asked questions
Is FDM cheaper than CNC for plastic parts?
For 1–10 parts, FDM is almost always cheaper. CNC setup costs $100–$500+ per part (fixturing, tool paths). FDM has zero setup cost — upload a file and print. At 100+ identical parts, CNC can become competitive as setup amortizes.
What tolerances can FDM achieve vs CNC?
FDM holds ±0.2–0.5mm tolerances. CNC machining holds ±0.025mm — roughly 10x tighter. For precision-critical parts, CNC is the better process. For functional prototypes and iterative designs, FDM is sufficient.
What materials can CNC machine that FDM cannot print?
CNC can machine 40+ plastics including PEEK, POM, Ultem, HDPE, and all metals. FDM is limited to 6 thermoplastics (PLA, PETG, ABS, ASA, PC CF, TPU). If your material spec requires a specific polymer grade, CNC may be required.
Can FDM handle complex geometry better than CNC?
Yes. FDM prints internal channels, lattice structures, undercuts, and organic curves that would require multi-axis CNC setups or be impossible to machine. Complex geometry is free in 3D printing.
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CNC machining vs 3D printing — subtractive vs additive.
Choose CNC when tolerances are tight (under ±0.05mm), when the geometry is prismatic (blocks, plates, shafts that a mill can easily reach), or when you need a polymer grade like polyacetal, PEEK, or Ultem that is not available in 3D printing. Choose 3D printing when geometry is complex (undercuts, lattices, organic curves, internal channels), when part volume is low, or when turnaround matters more than absolute tolerance.
FDM vs SLA — which 3D printing technology is right for your part?
Choose FDM when cost and speed matter most, or when you need tough engineering thermoplastics (PETG, ASA, PC CF) at larger build volumes. Choose SLA when surface finish, dimensional accuracy, or fine detail resolution is the priority — visual prototypes, snap-fit validation, and presentation models.
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