CNC Machining vs 3D Printing
CNC machining cuts a part out of certified wrought stock and holds tight tolerances; 3D printing builds it up layer by layer and charges nothing for geometric complexity.
- Rev
- 2026-08-11
- Candidates
- 2
Print it when you need the part this week, the geometry is complex, and quantity is under about ten. Machine it when the part has to hold a real tolerance, carry load in every direction, or be made from a certified alloy such as 6061, 7075, 304, or Ti-6Al-4V. Past roughly fifty parts, machining usually wins on price too.
CNC machining removes material from certified wrought stock with a rotating cutter or a turning tool, so the part inherits the properties of the bar it came from. 3D printing builds the part up in layers from filament, resin, or powder, so it inherits the properties of whatever the layer bond gives you.
Head-to-head
| Dimension | CNC machining | 3D printing |
|---|---|---|
| Typical tolerance | ±0.005 in (±0.13 mm) standard; ±0.001 in (±0.025 mm) precision, tighter on selected features | Service-bureau specs run about ±0.5% with a ±0.020 in (±0.5 mm) floor for FDM, ±0.3% with a ±0.012 in (±0.3 mm) floor for SLS and MJF, ±0.006 in (±0.15 mm) for SLA, and ±0.2% with roughly a ±0.004 in (±0.1 mm) floor for DMLS |
| Material structure | Isotropic wrought stock, with mill certs and lot traceability available | Layer-bonded and anisotropic; Z-direction strength sits below in-plane strength, worst in FDM |
| Materials | Any machinable metal or plastic sold in bar, plate, or tube | The vendor's catalog of resins, powders, and filaments |
| Surface finish | 125 µin Ra (3.2 µm) as-machined is the default; 32 µin (0.8 µm) with finishing passes | Layer stepping is visible: FDM 0.004–0.012 in (0.1–0.3 mm) layers, SLA 0.001–0.004 in (25–100 µm), SLS about 0.004 in (0.1 mm) |
| Tooling cost | None beyond cutters and fixturing | None |
| Setup cost | Real — CAM programming, workholding, first-article inspection. This is the fixed cost that amortizes | Near zero; slicing takes minutes |
| Per-part cost vs volume | Falls steadily as setup spreads across the lot | Essentially flat — part 1 and part 500 cost about the same |
| Lead time | 3–10 business days from a service bureau | 1–3 days |
| Geometry limits | The cutter must physically reach every surface; internal corners carry the tool radius; no enclosed cavities | Internal channels, lattices, conformal cooling, and undercuts cost nothing extra |
| Size | Common 3-axis envelopes reach roughly 48 × 24 × 24 in (1,200 × 600 × 600 mm) | Typical SLA and SLS build volumes run around 10 × 10 × 12 in (250 × 250 × 300 mm); large-format FDM goes bigger |
When to choose CNC machining
Choose machining the moment a drawing carries a fit. Bearing seats, dowel-pin holes, shaft journals, and sealing faces are called out with ISO 286 fits — see the ISO 286 fits and tolerances chart — and no common printing process holds an H7 bore without a machining operation afterward anyway. The same goes for threads: printed threads strip, so either machine and tap them using the tap drill chart or design in heat-set inserts.
Choose it for load-bearing metal parts. A machined 7075-T6 bracket has the tensile and yield strength listed in the material properties chart in every direction. A printed equivalent does not, and if the load has a component normal to the build plane the layer bond becomes the design allowable.
Choose it when documentation matters. Aerospace, medical, defense, and pressure work generally want a material certificate traceable to a heat lot, which is routine with bar stock and awkward with powder. Typical machined parts: motor mounts, manifold bodies, optical mounts, fixture plates, shafts on a CNC lathe, and contoured impellers on 5-axis equipment.
When to choose 3D printing
Choose printing when complexity is the point. Cooling channels that follow a contour, topology-optimized brackets, lattice-filled energy absorbers, and manifolds with curved runners are cheaper printed than machined — and often impossible machined, because a cutter cannot get inside a closed volume.
Choose it for iteration speed. Form-and-fit checks, ergonomics mockups, jigs, and soft-jaw prototypes are printed overnight, evaluated, and reprinted. Three design revisions in a week is normal in printing and unrealistic when a programming-and-setup cycle sits in front of every part.
Choose it for one-offs and short runs of low-stress parts: enclosures, cable guides, sensor mounts, tooling aids. And choose it for consolidated assemblies — printing five brackets as one piece removes fasteners, joints, and the machining setups that went with them. For metal, DMLS covers aluminum, stainless, and titanium; for tough nylon parts, SLS and MJF; for smooth cosmetic parts, SLA.
Cost comparison
| Quantity | Usually cheaper | Why |
|---|---|---|
| 1–5 | 3D printing | Machining setup and programming spread over too few parts |
| 5–50 | Crossover zone | Simple prismatic parts tip to machining early; intricate ones stay printed |
| 50–500 | CNC machining | Setup is amortized and cycle time, not fixed cost, sets the price |
| 500+ | Machining, or change process | For plastics evaluate injection molding; for metal, casting |
The mechanism behind the crossover is simple: machining has a real fixed cost and a low marginal cost, printing has almost no fixed cost and a marginal cost that never falls. Two variables move the crossover point. Geometric complexity moves it up, because complexity adds machining setups and fixtures while adding nothing to print time. Part size moves it down, because printing bills by material volume and build-plate hours, so a big solid block is expensive to print and cheap to mill.
Verdict
Start with the tolerance and the load path. If any feature is tighter than about ±0.010 in (±0.25 mm), or the part is loaded normal to where the layers would run, machine it — printed accuracy and layer anisotropy are the two things no amount of post-processing fixes cheaply. If the geometry contains internal channels, lattices, or consolidated features a cutter cannot reach, print it, because machining that shape costs several setups or is outright impossible. Between those poles let quantity decide: under about ten parts printing is faster and cheaper, over about fifty machining is cheaper, and in between the more complex part should be printed. Prototype in print and produce in machining is a good default, not a rule — verify that the printed prototype's fits are representative before you trust it.
Questions
6 questionsIs CNC machining more accurate than 3D printing?
Yes, by roughly an order of magnitude. Standard machining holds ±0.005 in (±0.13 mm) and precision work ±0.001 in (±0.025 mm), while typical printing specs are ±0.5% with a ±0.020 in (±0.5 mm) floor for FDM and ±0.3% with a ±0.012 in (±0.3 mm) floor for SLS and MJF. Printed parts that need a real fit get the critical feature machined after printing.
At what quantity does CNC machining become cheaper than 3D printing?
Commonly somewhere between 10 and 50 parts. Machining carries a fixed setup and programming cost with a low per-part cost after that, while printing has almost no setup and a per-part cost that does not fall with volume. Complex geometry pushes the crossover higher; large simple parts pull it lower.
Are 3D printed parts strong enough for functional use?
Often, but not in every direction. Layer-bonded parts are weaker in the build (Z) direction than in plane, most severely in FDM, so a printed part has to be oriented with the principal load in plane and derated for the layer bond. When the design allowables need to come straight off a material datasheet in all directions, machine from wrought stock.
Can you 3D print in aluminum or steel?
Yes — DMLS, SLM, and binder jetting produce metal parts in alloys such as AlSi10Mg, 316L, 17-4PH, and Ti-6Al-4V. Expect to machine critical features afterward, to heat treat, and to pay considerably more per part than machining the same shape from bar stock unless the geometry justifies it.
Which process is faster for a prototype?
Printing, typically 1–3 days against 3–10 business days for machining, because there is no CAM programming, workholding, or first-article step in front of it. The gap widens for intricate geometry and narrows for simple prismatic parts a shop can set up in one operation.
Can you machine a 3D printed part?
Yes, and it is a common hybrid. Print the complex body, leave stock on the features that need accuracy, then mill or turn the bores, faces, and threads. You get internal channels and consolidated geometry from printing while holding machined tolerances where the drawing requires them.
The candidates
Full process guides for both sides of this comparison.
Mechanical
CNC machining removes material from solid stock with a programmed cutting tool, producing accurate parts directly from a CAD model with no tooling.
Additive Manufacturing
Rapid prototyping builds a part layer by layer directly from a CAD model, with no tooling and no minimum order quantity.