How CNC Machine Works

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  Key takeaways
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  How CNC Machine Works: From Block to Finished Part
• 3
  What a G-code Program Actually Tells the Machine
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  Workholding and Datum Choice Drive Real Accuracy
• 5
  Tolerance, Surface Finish, and What They Cost
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  How CNC Machine Works: The 7 Steps of a Job
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  Choosing the Right Setup for the Part

CNC stands for computer numerical control. A machine tool moves a cutting tool along coordinates that a computer gives it. Nothing is cast or molded. A solid block of aluminium, steel, or plastic goes in, and a smaller finished part comes out. The operator does not turn handwheels; the controller reads a program and drives the axes.

Three motions do most of the work. The spindle rotates the cutter. The linear axes move the workpiece or the tool in X, Y, and Z. On multiaxis machines, one or two rotary axes tilt either the tool or the part. Add coolant and chip evacuation, and you have a complete cutting system.

The important consequence for engineers is this: geometry is limited by tool access, not by a mold. Undercuts, deep pockets, and compound angles that a casting cannot release are normal work for a 5-axis machine. The trade-off is cycle time. Every pocket is cut line by line, so material removal rate drives cost.

A part that is easy to model is not always easy to machine. Long, thin tools deflect. Deep holes need peck drilling. Hard materials push tool wear up and feed rates down. Understanding how CNC machine works means knowing where those limits sit before you finalize the drawing.

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  Additive vs subtractiveCNC removes material; 3D printing adds it layer by layer. CNC wins on density, surface finish, and tight tolerances.
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  Milling vs turningMilling uses a rotating cutter against a stationary part. Turning rotates the part against a stationary tool.
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  3-axis vs 5-axis3-axis cuts one face per setup. 5-axis reaches five faces in one setup with less re-fixturing error.

A G-code file is a list of blocks. Each block has a line number, a motion command, coordinates, and feed or speed values. G00 moves fast to a position without cutting. G01 feeds in a straight line at the programmed rate. G02 and G03 cut arcs. M-codes handle spindle on, coolant on, and tool changes.

Feeds and speeds come from the material and the cutter. A 10 mm carbide end mill in 6061-T6 aluminium might run at 8,000 rpm and 2,500 mm/min. The same cutter in 316 stainless drops to around 1,200 rpm and 300 mm/min. Get this wrong and you either burn the tool or chatter the wall.

Modern controllers let the CAM programmer set feed per tooth, not just feed per minute. Feed per tooth is the chip thickness each cutting edge takes. Too small a chip rubs the edge and work-hardens stainless. Too large a chip overloads the flute. The sweet spot depends on cutter diameter and material.

You do not need to write G-code by hand to judge a quote. But reading the first twenty lines tells you the setup: which tool is loaded, where the work offset sits, and whether the programmer is roughing conservatively. That is often enough to explain a long cycle time.

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  G00Rapid positioning move, no cutting.
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  G01Linear cut at the programmed feed rate.
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  G43Applies tool length compensation so Z heights stay correct.

The stiffest machine in the shop still cuts a bad part if the block moves. Workholding options run from a vise and soft jaws to a vacuum plate, a 3-jaw chuck, or a custom fixture. For thin-walled aluminium parts, soft jaws machined to the part profile spread the clamping load and stop the wall from bowing.

Datum selection is the design decision that most affects the result. A datum is the surface the programmer uses as zero. Pick a machined face, not a raw saw-cut edge, and keep the datum consistent from operation one through operation five. When the datum shifts, every downstream feature shifts with it.

Multiple setups introduce stacking error. Each re-clamp can add 0.02 to 0.05 mm of positional drift. That is why a part with bores on four sides is often cheaper on a 5-axis machine than on three separate 3-axis setups. Fewer setups means fewer chances to lose the position.

Thermal growth matters on long runs. A spindle warms up over the first hour, and a 500 mm steel part can grow with it. Shops that hold ±0.005 mm run warm-up cycles and check the first article after the spindle has stabilized, not on a cold machine.

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  Rigid beats cleverA simple vise on a solid block outperforms a flexible fixture on a thin plate.
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  One datum per drawingState it on the print and the machinist will hold it through every operation.
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  Support the floorUnsupported floors vibrate and leave chatter marks at the bottom of a pocket.

Tolerance and surface finish are the two numbers that move price most. A general machining tolerance of ±0.1 mm on a milled aluminium bracket is routine. Tightening one bore to ±0.005 mm adds a finishing pass, a reaming or boring operation, and a CMM check. The cost sits in the extra time, not the material.

Surface finish is measured as Ra, the arithmetic mean roughness. As-machined finish on aluminium lands around Ra 1.6–3.2 μm. A finer pass with a sharper cutter and lighter depth of cut reaches Ra 0.8–1.6 μm. Below Ra 0.8 μm usually needs a polishing or lapping step after machining.

Not every surface needs the tight callout. Engineers who mark an entire print as ±0.01 mm pay for inspection across every feature. Mark only the functional surfaces, usually bearing bores, seal grooves, and mating faces. Leave the rest at general tolerance and the quote drops quickly.

Material choice sets the floor. Aluminium 6061-T6 machines fast and holds a good finish. 316L stainless work-hardens and needs slower speeds. Titanium Ti-6Al-4V and Inconel generate heat at the cutting edge and can need special tooling and more passes for the same geometry.

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  ±0.005 mmAchievable on critical features with correct tooling and inspection.
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  Ra 0.8–1.6 μmStandard high-quality machined finish on aluminium and mild steel.
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  Ra 0.2–0.8 μmFine finish, often followed by polishing on visible surfaces.