3D printing has changed the prototype world; but when functional testing, series material and tight tolerances are required, CNC machining is still the standard. As the prototype-focused continuation of our CNC vs casting comparison, this article compares the two methods in a way that makes the decision easier. Our CNC prototype scope is on our prototype manufacturing page.
3D Printing Types and Their Real Capabilities
| Technology | Material | Typical tolerance | Surface | Strength | Limit |
|---|---|---|---|---|---|
| FDM/FFF | PLA, ABS, PETG, PA, PC, carbon-filled | ±0.2–0.5 mm | Visible layer lines | Cheap, fast, large parts | Weak Z strength, low precision |
| SLA/DLP | Photopolymer resins | ±0.05–0.15 mm | Very good | Detail, visual prototypes | Brittle, UV degradation, poor heat resistance |
| SLS | PA12, PA11, glass-filled PA | ±0.15–0.3 mm | Matt, slightly rough | Functional plastic, no supports needed | Dimensional stability, porosity |
| MJF | PA12, PA11, PP | ±0.1–0.3 mm | Better than SLS | Small-series plastic | Grey colour, limited material range |
| Metal SLM/DMLS | AlSi10Mg, 316L, Ti6Al4V, Inconel, tool steel | ±0.1–0.2 mm (before machining) | Ra 6–15 µm as built | Complex internal channels, lightweighting | Expensive, needs machining and heat treatment, anisotropy |
| CNC machining | All metals and engineering plastics (real stock material) | ±0.01–0.05 mm | Ra 0.4–1.6 µm | Series material, tolerance, surface | Internal channels and some geometries, tool access |
Decision by Purpose
- Form and fit verification: FDM or SLA is sufficient; cheap and available in 1–2 days.
- Functional testing (load, temperature, wear, leak-tightness): Series material is needed; CNC machining or metal printing + machining. In plastic parts SLS/MJF PA12 can be used for limited functional tests but does not behave the same as injection-moulded PA.
- Pre-series validation (PPAP, certification tests): The method closest to the series production method: CNC for machined parts, machined castings or a "casting-like" prototype from CNC for cast parts.
- Visual/marketing prototype: SLA + paint or CNC + coating; the surface is decisive.
- Complex internal geometry (conformal channels, lattice structures): Metal printing may be the only option; critical surfaces are machined afterwards.
The Tolerance and Surface Reality
Tolerance values given for 3D printing are usually "best case" values; as the part grows, the geometry becomes more complex and thermal distortion increases, deviation grows. If ±0.05 mm or below, hole diameter tolerance, a bearing seat or a sealing surface is required, the printed part must be machined; in this case the total cost of "printing + machining" often exceeds direct CNC. The difference in surface is also large: printed surfaces require blasting, vibratory polishing or machining; a CNC surface comes out directly at Ra 0.8. Our tolerance selection article advises questioning the real need.
The Material Equivalence Problem
Even if the printing material carries the same name as the series material, it is not the same: SLM AlSi10Mg shows different mechanical and corrosion behaviour from wrought 6082-T6; the fatigue strength and porosity of printed 316L differ from rolled 316L; FDM PA does not reach the crystallinity and strength of injection-moulded PA. If you want to transfer functional test results to the series part, making the prototype from series material by CNC is the safe route. This is critical especially in hydraulic, sealing and load-bearing parts.
Cost Curves
Approximate comparison for a simple metal part (60x40x20 mm aluminium bracket, 4 holes, 1 pocket):
| Quantity | FDM (PA-CF) | SLS PA12 | Metal SLM AlSi10Mg (incl. machining) | CNC 6082 |
|---|---|---|---|---|
| 1 | €15–30 | €40–70 | €250–450 | €90–160 |
| 5 | €12–25/pc | €30–55/pc | €200–350/pc | €40–70/pc |
| 20 | €10–20/pc | €22–40/pc | €180–300/pc | €18–30/pc |
| 100 | €9–18/pc | €18–30/pc | €170–280/pc | €9–15/pc |
For metal parts CNC is cheaper than metal printing even at 1 piece; for plastic parts 3D printing is more economical at 1–20 pieces, CNC at 50+ pieces (as the CNC plastic machining setup cost is spread). The crossover point varies with geometry: printing has the advantage in complex organic shapes, CNC in prismatic and rotational shapes. Our DFM article explains ways to reduce CNC cost.
Lead Time
FDM/SLA 1–2 days, SLS/MJF 3–5 days, metal printing 1–2 weeks (including heat treatment and machining), CNC prototype 3–10 days (if the material is in stock). Including courier, a CNC prototype can be delivered from Turkey to Europe within 1–2 weeks; a local 3D printing service is faster for visual prototypes. For functional metal prototypes CNC is both cheaper and faster than metal printing.
Hybrid Approaches
- Form by printing, function by CNC: Assembly check with FDM in the first round, functional test with CNC in the second; total cost and time are optimised.
- Metal printing + machining: Complex internal channels by printing, bearing seats and sealing surfaces by CNC; aerospace and heat exchanger applications.
- Printed fixture, CNC part: Clamping fixtures for small-series CNC are made cheaply by 3D printing; setup cost falls.
- Transition from CNC to series: Prototype by CNC, series by casting/injection moulding; the CNC prototype represents series tolerance and surface.
Decision Matrix
| Condition | Recommendation |
|---|---|
| Visual, assembly check, plastic | FDM/SLA |
| Functional plastic, 1–20 pieces, tolerance ±0.2 sufficient | SLS/MJF |
| Functional plastic, tight tolerance or POM/PEEK/PTFE | CNC (plastic machining) |
| Metal, any quantity, tolerance ±0.05 and below | CNC |
| Metal, complex internal channels, lightweighting | Metal printing + CNC machining |
| Pre-series validation, certified material | CNC (3.1-certified stock material) |
File Preparation: Different Expectations for the Two Methods
A 3D printing service usually asks only for STL/3MF and does not ask about tolerances; a CNC supplier wants to know the critical dimensions, surface expectation and material alongside the STEP file. When having a prototype made by CNC, sending a "critical dimension list" (which hole, which surface, which tolerance) even without a technical drawing both lowers the price and prevents unnecessary precision being applied. When organic geometries designed for printing are sent to CNC, tool access and minimum internal corner radius should be checked and the geometry simplified if necessary.
Frequently Asked Questions
Can metal 3D printing be cheaper than CNC?
Rarely; it can be economical only for geometries that cannot be made by CNC (internal channels, lattices) or titanium parts with a very high chip ratio. For standard parts CNC is cheaper even at 1 piece.
Can I do functional testing with a 3D-printed prototype?
Limited; the printing material is not equivalent to the series material. For load, leak and fatigue tests a CNC prototype from series material is recommended.
How many days does a CNC prototype take from Turkey?
Production 3–10 days + courier 2–4 days; total 1–2 weeks. It can be faster if the material is in stock.
Can I get a 3.1 material certificate for a prototype?
Yes; because stock material is used in CNC prototypes, a certificate is provided. Printing material has a powder certificate, but part properties depend on process parameters.
Conclusion
3D printing is unrivalled for visual and form verification, while CNC machining is the standard for functional testing, tight tolerances and series material; the most efficient projects use the two methods in sequence or together. Send the STEP files for your functional metal or engineering plastic prototypes via our quote form; for 1–20 pieces we give a price and delivery time within 24 hours, and we use the same program for the transition to series.
