The short answer buyers want is simple: for most production parts below a few hundred pieces, CNC machining wins on cost, tolerance, and material strength, while 3D printing wins for one-off prototypes and for enclosed geometry, internal channels, and lattice structures that no cutter can reach. The real decision is not "which technology is better" but "which one fits this part, this quantity, and this load." This guide compares the two on the numbers that actually move the outcome: cost crossover, as-printed tolerance, material anisotropy, and the post-processing most articles leave out.
At a Glance: CNC Machining vs 3D Printing
| Dimension | CNC machining | 3D printing (additive) |
|---|---|---|
| Process type | Subtractive (cut from solid stock) | Additive (built layer by layer) |
| Setup cost | Programming + fixture, a few hundred dollars | Near zero (file to machine) |
| Typical as-printed tolerance | ±0.01 to 0.05 mm (absolute) | ±0.1 to 0.5 mm (proportional to size) |
| Materials | Any machinable metal, plus engineering plastics | Nylon, resin, TPU, plus Al, steel, Ti powders |
| Mechanical consistency | Isotropic, same strength every direction | Anisotropic, weaker across layers (Z) |
| Minimum wall | ~1.2 mm (aluminum, stable) | ~0.5 mm (FDM/SLA, but fragile) |
| Hidden finishing | Light (deburr, anodize, bead blast) | Heavy (supports, cure, depowder, often CNC) |
| Best quantity | 1 to tens of thousands | 1 to ~100, and otherwise unreachable geometry |
| Lead time to first part | Days to two weeks (program + fixture) | One to three days for small parts |
The Cost Crossover Is Quantity, Not Technology
CNC is subtractive, so every part repeats the same cutting time and its per-part cost is a nearly flat line against quantity. 3D printing front-loads almost nothing, but its per-part cost also stays nearly flat, because material plus machine time barely moves with volume. The two lines cross at a break-even quantity, and that quantity is the real decision input.
In practice the crossover lands between 20 and 100 pieces, and it shifts with material, part size, and geometry. Below about 10 pieces, printing usually wins because it carries almost no setup. Between 20 and 100 pieces, machining starts to pull ahead on parts that are small, simple, or made from cheap stock, while printing can still win on large parts where CNC would throw away most of a billet. Above 100 pieces, CNC per-part cost keeps falling while printing cost stays put, so CNC normally wins by a wide margin.
| Cost element | CNC route | 3D printing route |
|---|---|---|
| Fixed cost | Programming + fixture (a few hundred dollars) | Near zero |
| Variable driver | Cutting time, material removed | Material feedstock + machine hours + finishing |
| Material efficiency | 40 to 70% becomes chips | High, only what the part needs |
| Finishing share | Low (deburr, anodize, bead blast) | 30 to 50% of delivered cost |
| Cost slope vs volume | Falls as setup amortizes | Flat |
Spindle time at a precision shop typically runs $40 to $90 per hour for 3-axis work and $90 to $180 for 5-axis; in lower-cost regions the same capability sits well under $30 per hour. Printing feedstock runs about $20 to $110 per kg for engineering polymers and $40 to $400 per kg for metal powders. The number that surprises buyers is the finishing line: across FDM, SLA, SLS, and metal printing, post-processing commonly eats 30 to 50% of the delivered cost, and on appearance parts it can match or exceed the printing itself. Always compare delivered-state quotes, not printing-only quotes.
Tolerance: The Proportional Versus Absolute Trap
This is the point most comparison articles get wrong. They list a single tolerance for each process and stop. The honest comparison has to separate absolute tolerance from proportional tolerance.
| Process | Typical as-printed tolerance | Envelope (100 to 300 mm part) |
|---|---|---|
| FDM / FFF | ±0.2 mm or ±0.2% of dimension | ±0.3 to 0.5 mm |
| SLA / DLP resin | ±0.05 to 0.15 mm | ±0.15 to 0.3 mm |
| SLS nylon (PA12) | ±0.1 to 0.25 mm or ±0.3% | ±0.3 to 0.5 mm |
| Metal LPBF (SLM/DMLS) | ±0.05 to 0.15 mm before machining | ±0.1 to 0.3 mm |
| CNC (plastic) | ±0.05 to 0.1 mm | ±0.05 to 0.15 mm |
| CNC (metal) | ±0.01 to 0.05 mm, ±0.005 mm on precision setups | ±0.02 to 0.1 mm |
A CNC center holds roughly ±0.025 mm on a critical bore whether that bore is on a 10 mm clip or a 300 mm plate, because the tolerance is absolute. Printed tolerances are usually proportional: SLS drifts about ±0.3% of the dimension, so a 300 mm part can wander ±0.9 mm. For a snap fit or a press fit, that difference is the gap between a part that assembles and one that jams. Reserve printing for clearance-class and visual-class tolerance, and machine the features that must hold a true position.
Material and Anisotropy: Strength You Cannot See in a Render
A render of a printed part looks identical to a machined one. The strength is not identical, and the difference is structural rather than cosmetic.
CNC parts are cut from solid stock, so their properties follow the native material and are essentially the same in every direction. Printed parts are bonded layer by layer, and the interlayer bond is the weak axis. In FDM and SLS nylon, tensile strength in the build direction is often only 60 to 80% of the in-plane value, and a printed snap fit can split along a layer line where a molded or machined one would survive thousands of cycles. Photopolymer resin is brittle in every direction. Metal prints avoid the directional weakness on strength but introduce a different problem, internal porosity, which is why load-bearing metal parts are stress relieved or hot isostatically pressed before they are trusted.
| Property concern | CNC machined | 3D printed |
|---|---|---|
| Directional strength | Isotropic (stock grain aside) | Z direction often 60 to 80% of XY |
| Internal porosity | None (solid stock) | Possible, needs HIP on metal |
| Long-term stability | Stable dimensions | Resin creep, nylon moisture uptake |
| Certified stock | Mill cert, EN 10204 3.1 available | Narrower, less proven long term |
The Green Part Trap: What "Printed" Really Means
The single biggest budgeting mistake buyers make is treating a printed part as finished. It is not. What comes off the machine is a green part that still needs work before it functions or ships.
SLA parts need washing and UV post-cure, or they stay soft and keep shrinking for days after printing. SLS and metal parts need depowdering, sometimes in internal channels you cannot see. Metal prints need stress relief or hot isostatic pressing to close porosity, then support removal in spots a tool barely reaches. After all that, the surfaces that actually mate, seal, or take a thread are rarely accurate enough as printed, so they get machined anyway. A printed bracket with a bored hole frequently ends up on a CNC center for that one hole, which means paying for both processes. Ask for a delivered-state quote that includes finishing, never a printing-only number.
Surface Finish and DFM Rules
Finish follows from the process. CNC can deliver Ra 0.8 to 3.2 directly off the cutter, and much finer after polishing or grinding. FDM shows visible layer lines and needs sanding to hide them. SLA is smooth but carries a stair-step and support-point marks. SLS has a grainy, powdery feel unless dyed or blasted.
Design rules differ too. Printing reaches internal channels, hollow lattice, and trapped voids that no cutter can enter, which is its real advantage. It also manages wall thickness down to about 0.5 mm, but those thin walls are fragile. CNC needs draft on deep features, accessible tool paths, and a minimum wall around 1.2 mm in aluminum to avoid chatter and distortion. The practical test is whether the complexity is enclosed, in which case printing wins, or open and reachable, in which case 5-axis CNC is competitive and usually cheaper at volume.
How the Part Gets Verified
Quality control differs in kind. CNC parts are measured on CMMs against the drawing, with first-article inspection and in-process checks that are well understood. Printed parts need the same dimensional checks plus something CNC rarely needs, verification of internal structure. Porosity inside a metal print is invisible from outside, so meaningful verification uses CT scanning or sectioning, not just a caliper on the outside. Support-mark scars and layer-line defects also need explicit inspection, because they sit exactly where a functional surface should be. If your specification demands documented, repeatable inspection, CNC is the lower-friction route.
When Each One Wins
Print it when
You need one to a few parts fast, the geometry has internal channels or lattice no cutter can reach, the loads are light, and visual or clearance tolerance is enough. Printing is also the cheapest way to validate an assembly before committing to tooling.
Machine it when
The part must carry load, hold a true position or press fit, use a certified alloy, stay stable for years, or run above the break-even quantity. CNC also wins on appearance parts that need a clean, isotropic surface without heavy finishing.
Do both when
The part combines an enclosed body with a few precision features. Print the near-net shape, then machine the bores, mounting faces, and threads. This hybrid is common for low-volume metal parts and for fixtures and jigs.
From the Shop Floor: What Buyers Miss
In the shop, the mistakes are predictable. Buyers spec a threaded hole and a press-fit bore on a printed nylon bracket, then discover the bore position drifts with the build and the part gets machined anyway, so the "cheap printing" route paid for printing and machining. They size a large FDM part from a render, forget the build time and the seam where two printed segments join, and watch lead time blow past a single CNC setup. They read a resin quote and miss that the part will creep and yellow months later. None of this shows up in a head-to-head feature table, and all of it decides whether the part actually works. The honest recommendation is rarely "print it" or "machine it" in the abstract. It is a delivered-state cost at your quantity, with the strength and tolerance the part truly needs.
Decision Framework by What the Part Must Do
| If the part must... | Route | Reason |
|---|---|---|
| Be one of 1 to 10, fast | 3D printing | Near-zero setup, days to part |
| Hold a press fit or true position | CNC machining | Absolute ±0.025 mm regardless of size |
| Carry real load or fatigue | CNC machining | Isotropic native material strength |
| Have internal channels or lattice | 3D printing | No cutter can reach enclosed geometry |
| Run above ~100 pieces | CNC machining | Flat printing cost loses to amortized machining |
| Use certified steel, Ti, or brass | CNC machining | Native stock with mill cert |
| Combine enclosed body with precision faces | Print plus CNC finish | Near-net shape plus machined critical features |
How FHY Helps You Choose
FHY runs CNC machining and prototype services under one roof, so process selection is an engineering question rather than a sales pitch for a single machine. Send the 3D model, target annual volume, and the tolerances that actually matter, and the review prices both routes on your real geometry, states the break-even quantity where the cheaper route flips, and flags the post-processing the printed route will need. The review is free, and the answer is the same whether or not you place the order: quantity first, geometry second, material third, with the trade-offs stated plainly.