What a CNC Machine Does

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  From a CAD file to a cut surface
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  What the number of axes actually changes
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  The cutting operations a CNC machine performs
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  Which materials fit, and where they fight back
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  How accurate a CNC machine really is
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  When a CNC machine is the wrong choice
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  How a CNC job runs from quote to shipment
• 8
  CNC machining compared with other processes

A CNC machine does one simple thing at its core: it takes a stream of numerical commands and turns them into precise movement of a tool relative to a workpiece. The operator does not steer the cutter by hand. The control reads G-code, calculates tool paths, and drives servo motors on each axis. Every pass is identical to the one before it.

The chain starts in CAD. A designer builds a solid model, then CAM software slices that model into tool paths. Those paths become G-code: coordinates, feed rates, spindle speeds, tool numbers, coolant commands. A post-processor converts the generic path into the dialect that the specific machine control understands.

Once loaded, the machine probes or touches off the stock, picks up tool offsets, and runs. On a 3-axis mill, the tool moves in X, Y and Z while the part stays still. On a 5-axis center, two rotary axes tilt either the tool or the table, so the cutter can approach a face at an angle instead of straight down.

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  Input3D CAD model plus material, tolerance and finish callouts
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  ProgrammingCAM tool paths, then post-processed G-code for the control
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  ExecutionServo-driven axes, tool changes, coolant, in-process probing
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  VerificationCMM or optical inspection against the drawing before shipment

Axis count is not a quality rating. It describes how many directions the machine can simultaneously control. A 3-axis machine cuts from one direction per setup, so a part with features on five faces needs multiple fixtures and re-datuming. Each setup adds stack-up error and handling time.

A 4-axis machine adds a rotary table, usually indexable around one axis. Shafts, bushings and cylindrical parts can be milled, drilled and slotted without unclamping. That alone often removes two or three setups from a job.

A simultaneous 5-axis center adds two rotary axes that move while cutting. The tool stays normal to a curved surface, which keeps the effective cutting speed and chip load consistent across a contoured wall. This is why impellers, turbine blades, deep pockets and complex mold cavities are cut on 5-axis machines rather than on a 3-axis with long reach tooling.

At GreatLight we run 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers. The largest travel reaches 4,000 × 400 × 150 mm, and a Ø400 mm rotary table handles round work. Setup choice follows geometry, not habit.

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  3-axisPrismatic parts, flat faces, plates, simple pockets
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  4-axisCylindrical and indexed parts, reduced re-fixturing
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  5-axisContoured surfaces, undercuts, deep cavities in one setup
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  Mill-turnTurned bodies with milled features, one chucking

Milling uses rotating multi-flute cutters. Face mills flatten a surface, end mills cut pockets and profiles, ball nose tools generate curved surfaces, and small diameter tools reach into corners. Milling suits flat-bottomed pockets, slots, threads and 3D contours.

Turning rotates the workpiece against a single-point insert. It is the fast route to cylindrical features: outside diameters, bores, shoulders, grooves and threads. A mill-turn center combines both so a part like a valve body or a motor housing does not travel between two machines and lose datum reference.

Drilling, reaming and tapping create holes. Drilled holes typically hold ±0.05 mm; reaming tightens that to around ±0.01 mm and improves bore finish. For holes that must be round and on position, boring after drilling is common on larger diameters.

Wire EDM and sinker EDM remove material with electrical sparks rather than a cutting edge. They cut hardened tool steel, thin walls and sharp internal corners that no end mill can reach. EDM is slower per cubic centimeter, so it is used for the features that milling cannot hold, not for bulk removal.

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  MillingPockets, profiles, faces, slots, 3D surfaces
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  TurningDiameters, bores, grooves, threads, faces
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  HolemakingDrill, ream, bore, tap, counterbore
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  EDMHardened steel, sharp corners, thin walls

Aluminum is the easiest common metal to cut. Grades 6061 and 7075 machine cleanly at high spindle speeds, and 6061-T6 covers most brackets, housings and fixtures. Plastics behave differently: POM and ABS cut well, but they move with heat, so light passes and sharp tooling matter more than raw speed.

Stainless 303 and 304 are routine. 316L and 17-4PH work-harden if the tool rubs instead of cutting, so feed per tooth must stay above a floor and coolant must reach the edge. Titanium Ti-6Al-4V and Inconel generate heat in the cut, not in the chip; they need lower surface speed, rigid setups and generous coolant.

Magnesium AZ31B and AZ91D cut fast but require chip control and fire-safe handling. Copper and brass are straightforward, and beryllium copper needs dust control. Carbon fibre machines as a composite: abrasive, dusty, and best cut with diamond-coated tooling and extraction.

GreatLight machines 6061, 2024, 5052, 5083, 6063, 6082, 7075, ADC12 aluminum; 303, 304, 316, 316L, 420, 430, 431, 440C and 17-4PH stainless; 1018, 1045, 4130, 4140, 4340 and A36 steel; C101, C110 and C36000 copper alloys; TA1, TA2, TC4 titanium and Inconel; plus ABS, PC, PMMA, POM, PA, PEEK, PP, HDPE and carbon fibre.

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  Easy to cut6061 aluminum, 303 stainless, brass, POM, ABS
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  Needs discipline316L, 17-4PH, 4140, PEEK
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  Hard on toolingTi-6Al-4V, Inconel, hardened tool steel
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  AbrasiveCarbon fibre, glass-filled plastics, magnesium chips

A general machining tolerance of ±0.005 mm (±0.0002 in) is achievable on critical features when the part, the fixture and the tool all cooperate. That number is not a blanket default. It applies to a specific dimension, measured in a controlled way, on a stable geometry.

Tolerance has to be earned by the whole stack. Thermal growth moves a part during a long cut. Tool deflection bends a small end mill in a deep pocket. Fixture clamping can distort a thin wall before the cutter even touches it. Deep holes drift. Long slender shafts chatter.

As a rule, the tighter the tolerance, the more operations, the slower the cycle and the higher the inspection cost. A feature held to ±0.005 mm may need a separate finishing pass, a temperature-soaked measurement, and a CMM report. If a ±0.05 mm callout does the job, specifying ±0.005 mm only adds cost.

Surface finish follows the same logic. Ra 1.6–3.2 μm is a normal as-machined finish. Ra 0.8–1.6 μm needs a controlled finishing pass. Ra 0.2–0.8 μm usually means fine stepovers, a dedicated finishing tool and often a secondary operation.

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  As machinedRa 1.6–3.2 μm, general tolerances
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  High finishRa 0.8–1.6 μm, finishing pass, tighter stepover
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  Fine finishRa 0.2–0.8 μm, dedicated tooling, secondary ops
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  Critical fit±0.005 mm, controlled temperature, CMM report

CNC machining removes material from solid stock. That makes it accurate and material-agnostic, and it also makes it wasteful on large volumes. If a part will be produced 50,000 times a year in a single geometry, casting or forging plus finish machining will usually cost less per part than cutting every cavity from billet.

Very thin, very large sheet parts belong to sheet metal fabrication. Hollow blow-molded bottles belong to molding. Fine lattice structures and internal channels that a tool cannot reach are better served by 3D printing or die casting, depending on material and volume.

There are also geometry limits. A cutter is a cylinder with a radius. Inside corners inherit that radius, so a square internal corner requires EDM or a relief detail. Deep, narrow pockets need long, thin tools that deflect. A hole 20 diameters deep is a drilling problem, not a milling one.

The practical test is simple. If the part needs tight tolerances, real material properties, a small to medium quantity, or a fast route to a working prototype, CNC fits. If it needs millions of identical thin shells, look at another process first.

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  Choose CNCTight tolerance, real material, 1 to 10,000+ parts
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  Choose castingHigh volume, complex internal cavities, thick sections
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  Choose sheet metalThin flat parts, bends, large enclosures
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  Choose EDMSharp internal corners, hardened steel, thin walls