Introduction
CNC machining at a glance
| Attribute | Typical Range |
|---|---|
| Process type | Subtractive (material removed from stock) |
| General tolerance (no callout) | ISO 2768-m, roughly ±0.1 to ±0.2 mm on mid-size features |
| Tighter achievable tolerance | ±0.025 mm (±0.001 in) on turned diameters; ±0.05 mm (±0.002 in) on milled features |
| Precision-feature tolerance | Down to ±0.01 mm (±0.0004 in) with the right process and inspection |
| As-machined surface finish | Ra 1.6 to 3.2 μm (63 to 125 μin) typical |
| Common materials | Aluminum, steel, stainless, titanium, brass, copper, ABS, nylon, PEEK, POM |
| Best-fit volumes | 1 to ~10,000+ parts, prototype through production |
Values above are typical industry capability for general machined parts, not a guarantee for a specific geometry. Achievable tolerance depends on the feature, material, workholding, and inspection method.
What is CNC Machining
In CNC machining, a CAD model of the part is converted into machine instructions (G-code and M-code), and the machine executes those instructions to move a cutting tool relative to the workpiece with high positional accuracy. G-code controls motion, position, and feed; M-code handles auxiliary functions like tool changes, spindle on/off, and coolant. Because the toolpath is defined in software, the second part is identical to the hundredth, which is what makes CNC suitable for both prototyping and production.
The important distinction for a buyer is subtractive versus additive. CNC removes material, so it starts from solid stock and cuts away everything that isn't the part. That gives excellent material properties (you keep the full strength of wrought or cast stock) and tight tolerances, at the cost of some material waste and geometric limits: a tool has to physically reach every surface it cuts.
The main CNC machining processes
CNC is an umbrella term. The processes below differ in how the tool and workpiece move, and each one holds different tolerances and suits different geometries. Most real parts use more than one.
CNC Milling
The workpiece is held stationary (or on a rotating table) and a rotating multi-flute cutter removes material. Milling produces prismatic shapes: housings, brackets, plates, manifolds, slots, pockets, and 3D contours. It's the most versatile CNC process and handles the widest range of geometries.
Milling machines are described by their number of axes:
- 3-axis: the tool moves in X, Y, and Z. Best for flat faces, pockets, drilling, and features accessible from a single direction. Lowest programming and setup cost. Multi-sided parts are handled by re-fixturing the part in several setups.
- 4-axis: adds rotation about the X-axis (the A-axis), letting the machine reach four sides of a part in one setup and cut features at an angle to the primary axes.
- 5-axis: adds a second rotational axis. In "3+2" (indexed) machining the two rotary axes position the part at a compound angle and then hold still while cutting; in full continuous 5-axis, all five axes move at once. Both let a complex part be finished in fewer setups, which reduces stack-up error between features and enables swept, organic surfaces like turbine blades and impellers.
More axes is not automatically better. For a part that can be fully dimensioned in flat 2D views, 3-axis machining in a few indexed setups often produces the same result as 5-axis at lower cost. Axis count is a manufacturability and cost decision, not a quality ranking. Typical milling holds about ±0.05mm (±0.002in) on general features.
CNC turning (lathe work)
The workpiece rotates on a spindle while a single-point tool moves along it. Turning is the efficient choice for cylindrical and rotationally symmetric parts: shafts, pins, bushings, fittings, threaded components, and connectors. Because the part spins against a rigid tool, turning generally holds tighter diameters than milling holds general features, on the order of ±0.025mm (±0.001in). Many turning centers now include live tooling, so they can mill flats, drill cross-holes, and cut features off-axis without moving the part to a separate mill.
CNC drilling, boring, and reaming
These are hole-making operations, often performed on the same milling or turning machine rather than a dedicated one. Drilling creates the initial hole; boring enlarges and trues it for position and roundness; reaming finishes it to a precise diameter and better surface. Reaming and boring reach tighter tolerances than drilling alone, roughly ±0.01mm (±0.0004in) on well-controlled features, which is why precise bores for bearings and dowels are reamed or bored rather than left as-drilled.
CNC grinding
An abrasive wheel removes very small amounts of material to reach the tightest tolerances and smoothest finishes CNC can produce, roughly ±0.005mm (±0.0002in) or better. Grinding is a secondary or finishing operation used on hardened materials and on features like bearing journals and sealing faces where a milled or turned finish isn't fine enough.
EDM (electrical discharge machining)
EDM removes material with controlled electrical sparks between an electrode and the workpiece, submerged in dielectric fluid. It cuts only electrically conductive materials, but it does so regardless of hardness and with no cutting force, which makes it the process of choice for sharp internal corners, thin ribs, deep narrow slots, and hardened tool steel that would be difficult to mill. Wire EDM uses a traveling wire to cut profiles all the way through; sinker EDM burns a shaped cavity, as in injection mold tooling.
Non-machining CNC cutting: laser, plasma, and waterjet
These share CNC motion control but cut flat stock rather than machining 3D features:
- Laser cutting: a focused beam cuts sheet metal and some plastics with a narrow kerf and clean edges. Fast and accurate on thinner material.
- Plasma cutting: an ionized gas arc cuts electrically conductive metal, faster than laser on thick plate but with a wider heat-affected zone.
- Waterjet cutting: a high-pressure abrasive water stream cuts almost any material with no heat, so it won't distort heat-sensitive metals or delaminate composites.
These are usually the right starting point for flat parts that will be bent or welded, not for 3D machined components.
Process selection cheat sheet
| Process | Best For | Typical Tolerance |
|---|---|---|
| Milling (3/4/5-axis) | Prismatic parts, pockets, contours, housings | ±0.05 mm (±0.002 in) |
| Turning | Round parts: shafts, pins, fittings | ±0.025 mm (±0.001 in) on diameters |
| Drilling / Boring / Reaming | Precise holes and bores | Down to ±0.01 mm (±0.0004 in) |
| Grinding | Hardened parts, ultra-fine finish, bearing faces | ±0.005 mm (±0.0002 in) or better |
| EDM (Wire / Sinker) | Hard metals, sharp internal corners, tooling | Very tight, geometry-dependent |
| Laser / Plasma / Waterjet | Flat stock, sheet-metal blanks | Depends on material and thickness |
Tolerances and surface finish: what CNC actually holds
This is where a lot of general CNC guides stay vague, and where a specification either saves money or wastes it.
General tolerances and ISO 2768
When a drawing doesn't call out a specific tolerance on a feature, a general tolerance standard fills the gap. The most common is ISO 2768-1, which defines four classes for linear and angular dimensions: fine (f), medium (m), coarse (c), and very coarse (v). Medium (m) is the usual default for machined metal parts. ISO 2768-2 covers general geometric tolerances (flatness, straightness, and similar) in classes H, K, and L. Defaulting a whole drawing to ISO 2768-m, then calling out tighter tolerances only on the few features that need them, is the standard way to control cost.
Achievable tolerance by operation
Tighter is not free. Moving a feature from a standard to a precision tolerance can roughly double its cost, and tolerance cost rises exponentially, not linearly, as you approach the limits of a process. As a practical rule of thumb for well-controlled features:
- Milling: about ±0.05mm (±0.002in)
- Turning: about ±0.025mm (±0.001in) on diameters
- Reaming / boring: about ±0.01mm (±0.0004in)
- Grinding: about ±0.005mm (±0.0002in) or better
Final capability always depends on machine condition, tooling, workholding, material behavior, feature geometry, and how the feature is inspected. Long, slender turned parts and deep, thin-walled milled features are harder to hold than the numbers above suggest.
Surface finish (Ra) is a separate spec
Tolerance controls how big a feature is; surface roughness (Ra) controls how smooth it is. A part can be dead-on size and still too rough to seal, or mirror-smooth and out of tolerance. They are chosen separately. As-machined CNC surfaces are typically Ra 1.6 to 3.2 μm (63 to 125 μin). Finer finishes come from finishing passes, grinding, or post-processing. Call out a specific Ra only on faces where function depends on it, such as sealing surfaces, bearing seats, and sliding contacts, because a blanket fine-finish requirement raises cost everywhere.
When to reach for GD&T
ISO 2768 controls size, not the relationship between features. When form, orientation, or location matters, for example the flatness of a sealing face, the perpendicularity of a boss to a bore, or the true position of a bolt circle, use geometric dimensioning and tolerancing per ASME Y14.5 (or ISO 8015 / ISO 286 for fits). GD&T applied only to functional interfaces communicates intent precisely without over-tightening the rest of the part.
Materials for CNC machining
CNC works across a wide range of metals and engineering plastics. Material choice drives machinability, cost, achievable tolerance, and finish as much as the process does.
Common metals
| Material | Notable Properties | Typical Uses |
|---|---|---|
| Aluminum (6061, 7075) | Light, easy to machine, good strength-to-weight | Housings, brackets, aerospace, prototypes |
| Stainless Steel (303, 304, 316) | Corrosion-resistant, strong | Medical, food, marine, fixtures |
| Steel (mild, alloy, tool) | Strong, tough, hardenable | Structural parts, shafts, tooling |
| Titanium (Ti-6Al-4V) | High strength-to-weight, corrosion- and heat-resistant, biocompatible | Aerospace, medical implants |
| Brass / Copper | Good machinability, conductivity | Fittings, electrical, decorative |
Common plastics
Plastics such as ABS (tough, general-purpose), nylon (wear- and impact-resistant), POM/Delrin (stiff, low-friction, dimensionally stable), PEEK (high-temperature, chemical-resistant), and PTFE (low-friction, inert) machine well for enclosures, gears, bearings, insulators, and chemically demanding parts. Carbon-fiber-reinforced composites offer high stiffness at low weight for aerospace and high-performance applications.
Softer, free-machining materials like aluminum and brass hold tight tolerances more easily and cost less to cut than titanium or hardened steel, which wear tooling faster and machine more slowly. If two candidate materials both meet the functional requirement, the more machinable one usually lowers cost.
Benefits and limitations of CNC machining
CNC earns its place in most engineering supply chains, but it isn't the right answer for every part. Weigh it honestly.
Benefits
- Tight, repeatable tolerances. Software-defined toolpaths make part number one and part number one thousand effectively identical.
- Broad material range. The same process handles aluminum, hardened steel, titanium, and engineering plastics with a tooling and parameter change.
- Full material properties. Cutting from solid wrought or cast stock preserves the strength of the base material, unlike some additive methods.
- Fast for prototyping and low-to-mid volume. No dedicated tooling is required, so first parts can ship quickly and design changes are cheap to implement.
- Excellent as-machined finish. Many parts need no secondary finishing at all.
Limitations
- Material waste. Being subtractive, CNC turns some of the stock into chips. For very high volumes of a simple plastic part, molding wastes less material per unit.
- Geometry constraints. A tool has to reach every cut surface, so deep narrow cavities, true internal undercuts, and fully enclosed features can be difficult or impossible without EDM or splitting the part.
- Per-unit cost at very high volume. Above a certain quantity, a molded or cast part is usually cheaper per unit than a machined one, because CNC cost scales largely with machine time per part.
- Setup and programming overhead. Complex multi-axis parts require skilled programming and fixturing, which adds fixed cost that a one-off part has to absorb.
A quick way to sanity-check fit: CNC is usually the right call for metal parts needing tight tolerances at prototype to mid production volumes. For very high volumes of a simpler plastic part, compare against injection molding; for a small run of plastic parts before committing to tooling, compare against urethane casting.
Applications of CNC machining by industry
CNC shows up wherever parts need to be accurate, repeatable, and made from real engineering materials.
These are categorized by their main function:Aerospace
Structural brackets, housings, engine and turbine components, and landing-gear parts, often in aluminum and titanium, where tight tolerances and material traceability are non-negotiable. Complex flight-critical geometries frequently drive 5-axis work. AS9100D-certified partners are available in Clarwe's network for programs with a hard flow-down requirement; confirm this at quote time so the job is matched accordingly. See aerospace machining.
Automotive
Prototype and production parts including housings, brackets, pistons, manifolds, and custom or motorsport components. CNC covers both fast design iteration and repeatable mid-volume production. See automotive manufacturing.
Medical devices
Instruments, housings, and implants, commonly in stainless steel, titanium, and PEEK, where biocompatibility, fine finishes, and tight tolerances matter. For device work governed by ISO 13485:2016, that requirement can be matched to a certified partner in the network; state it at quote time. See medical device manufacturing.
Electronics and consumer products
Enclosures, heat sinks, connectors, brackets, and precision housings where fit and finish are visible to the end user. CNC delivers the cosmetic surfaces and repeatable dimensions these products need. See consumer products.
How to choose a CNC machining service
- Process and axis coverage. Confirm they can do the specific operations your part needs, milling, turning, EDM, multi-axis work, so the whole part is handled in one place.
- Tolerance and inspection capability. Ask what they hold on your critical features and how they verify it. First-article and CMM inspection matter for tight-tolerance and regulated work. See QA & QC.
- Material breadth. Confirm they machine your specified material and grade, not just a near-equivalent.
- Certifications matched to your program. ISO 9001 as a baseline, plus AS9100D or ISO 13485 matched to the job where your program requires it.
- Transparent quoting. A clear breakdown of material and machining cost, and DFM feedback that flags features driving cost before you commit, is worth more than a headline price.
FAQs about CNC Machining
What tolerances can CNC machining hold?
General machined features without a specific callout typically follow ISO 2768-m. With process control, turning holds about ±0.025mm (±0.001in) on diameters and milling about ±0.05mm (±0.002in) on general features. Precision features can reach ±0.01mm (±0.0004in), and ground features tighter still. Actual capability depends on the feature, material, and inspection method.
What is the difference between CNC milling and CNC turning?
In milling, the workpiece is held still (or on a rotating table) and a rotating cutter removes material, which suits prismatic parts like housings and brackets. In turning, the workpiece rotates against a stationary tool, which suits round parts like shafts and fittings. Many parts use both, and modern turning centers with live tooling can do some milling in the same setup.
Is CNC machining better than 3D printing?
They solve different problems. CNC is subtractive and gives tight tolerances, fine finishes, and full material strength from metals and engineering plastics. 3D printing is additive and excels at complex internal geometries and undercuts that a cutting tool can't reach. CNC is generally preferred where dimensional accuracy, surface finish, and material properties are critical.
How much does CNC machining cost?
Cost is driven mainly by machine time, which depends on part complexity, material machinability, tolerances, finish, and quantity. Tighter tolerances and finer finishes raise cost non-linearly, so specifying them only where they're functionally needed is the main way to control price. For a specific figure, upload your CAD file for a quote.
What materials can be CNC machined?
A wide range of metals (aluminum, steel, stainless steel, titanium, brass, copper) and engineering plastics (ABS, nylon, POM, PEEK, PTFE), plus some composites. Material affects machinability, achievable tolerance, finish, and cost, so it's worth confirming a supplier machines your specific grade.