CNC Machining Processes

November 7, 2026

CNC machining makes a part by cutting material away from a solid block, plate or bar, with every move of the machine controlled by a program. CNC stands for computer numerical control. It's the default way to make accurate metal parts in quantities from one to thousands, and most other processes, from molding to 3D printing, end with a machined surface somewhere. The main machining processes are described below, with what each is good at and what decides the price.

From CAD model to finished part

CAD: the 3D model, plus a drawing with tolerances, material and finish.
CAM programming: a programmer chooses the tools, the order of operations, and the speeds and feeds, and the CAM software turns them into the G-code the machine runs.
Setup: the stock is clamped in a vise, a chuck or a custom fixture, the tools are loaded and measured, and the machine is told exactly where the part is.
Cutting: roughing passes remove most of the material quickly, then finishing passes bring each feature to size.
Deburring and inspection: sharp edges are broken and the part is checked against the drawing.

Programming and setup cost about the same for one part as for a hundred, which is why the price per part drops so quickly as the quantity goes up.

Milling

In milling, a rotating cutter removes material while the part and the tool move relative to each other along the X, Y and Z axes. End mills cut pockets, slots, steps and contours; face mills flatten large surfaces; drills, reamers and taps make holes and threads. Milling makes brackets, plates, housings, manifolds and most other blocky parts.

On a 3-axis mill the tool always points down, so each side of the part that has features needs its own setup. Inside corners keep the radius of the tool that cut them, and deep, narrow pockets need long tools that have to cut slowly. Those two facts explain a lot of machining cost; see CNC design features that add cost.

CNC machined aluminum parts laid out on a shop table

4-axis, 3+2 and 5-axis machining

Adding rotary axes lets the machine reach more of the part in one setup.

4-axis machines add a rotary table, so features around a part, like holes around a shaft, can be cut without re-clamping.
3+2 machining, also called positional 5-axis, tilts the part to a fixed angle and then cuts with the usual three axes. Five sides can be reached in one setup.
Simultaneous 5-axis moves all five axes at once, keeping the tool at the best angle to the surface. It is how impellers, turbine blades, ports and complex molds are machined.

Fewer setups means features stay accurate to each other, because the part never moves between them, and shorter tools can be used, which improves the finish. The trade-off is a higher machine rate and more programming time.

Turning

In turning, the part spins in a lathe's chuck while a stationary tool cuts it. Lathes make round parts: shafts, pins, bushings, spacers, threaded parts and anything with a turned profile. For round parts, turning is faster and cheaper than milling.

Mill-turn and live tooling

Lathes with live tooling have powered tools in the turret that can mill flats, slots and cross holes while the part is still in the chuck. Mill-turn machines go further, with a full milling spindle and often a second spindle, so a complex part can come off finished on all sides in one operation.

Swiss-type turning

A Swiss-type lathe feeds bar stock through a guide bushing and cuts right next to it, so long, thin parts don't bend away from the tool. It's the usual choice for small-diameter parts made in high volume, such as medical screws, pins and connector contacts.

EDM

Electrical discharge machining (EDM) removes metal with controlled sparks in a bath of dielectric fluid. There's no cutting force and hardness doesn't matter, so EDM cuts hardened tool steel and carbide as easily as soft steel.

Wire EDM cuts through-profiles with a thin brass wire, often 0.25 mm in diameter. It makes punches and die sections, gears and splines, and inside corners far sharper than an end mill can, holding tolerances of a few microns.
Sinker EDM burns a shaped electrode, usually graphite or copper, into the part. It is used for mold cavities, deep narrow ribs and sharp corners at the bottom of blind pockets.

EDM is slow, so it's saved for places where cutting tools can't reach or can't cut.

Grinding

Grinding uses an abrasive wheel to remove small amounts of material very accurately. Surface grinding produces flat, parallel faces; cylindrical grinding finishes shafts, pins and bearing journals. It's often the last step after heat treatment, holding tolerances of a few microns and finishes of Ra 0.4 µm or better.

Tolerances and GD&T

A tolerance is how far a dimension may vary from its nominal size. Dimensions without their own tolerance usually fall back to a general standard noted on the drawing, often ISO 2768 medium, which allows ±0.2 mm on features from 6 to 30 mm long and ±0.3 mm from 30 to 120 mm.

Around ±0.05 to ±0.1 mm is ordinary machining.
±0.01 to ±0.025 mm on a critical feature is routine but costs more: slower finishing passes, temperature control and inspection time.
A few microns usually means grinding, wire EDM or lapping.

GD&T (geometric dimensioning and tolerancing) adds controls that plain ± tolerances can't express: flatness, perpendicularity, concentricity and the true position of holes, all measured from datums that match how the part sits in its assembly. Used well, it tells the machinist which features matter and how they'll be checked. The rule that saves the most money is simple: put tight tolerances only on the features whose function needs them.

Surface finish

Surface roughness is usually given as Ra, the average height of the surface's peaks and valleys, in micrometers. A standard machined finish is about Ra 3.2 µm, with light tool marks. Ra 1.6 µm takes finishing passes, and Ra 0.8 µm or better, typical for sealing faces and bearing fits, takes slower finishing or grinding. Bead blasting gives an even matte look, and coatings change the surface again; see surface finishes for metal parts.

Inspection

Calipers, micrometers, bore gauges and go/no-go gauges handle most checks. A CMM (coordinate measuring machine) touches the part with a probe at many points and compares them with the model or drawing, which is how flatness, position and profile are measured. In production, a first article inspection checks every dimension on the first part before the run continues. If you need an inspection report on critical dimensions, ask for it with your quote. See quality.

CMM touch probes measuring a machined part

Materials

Aluminum machines fastest and is usually the cheapest metal to machine. Carbon and alloy steels take longer, stainless steel longer still because it work-hardens, and titanium is slowest because heat builds up at the tool instead of leaving with the chips. Engineering plastics cut quickly but need sharp tools and care with heat and clamping pressure. For choosing between them, see how to choose a metal for a machined part.

What drives the cost

Setup and programming, spread across the batch.
Machine time, set by how much material comes off, the material itself, the complexity and the finish.
Number of setups. Every time the part is flipped or re-clamped, cost and the chance of error go up.
Tolerances and inspection. Tight tolerances slow the cutting and add measuring time.
Material: the price of the stock and how easily it cuts.
Finishing, such as anodizing, plating or heat treatment.
Quantity. The first part carries the setup; the rest mostly cost machine time.

For a faster, more accurate quote, see what to include in a quote request. When you're ready, send the drawing through our CNC machining page.

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