CNC Mill Basics: How Milling Actually Removes Metal

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  CNC mill basics: what happens where the tool meets metal
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  Why axis count decides what the mill can make
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  How material choice changes the cut
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  Setups, datums and how we check the result
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  Which mill setup fits the part

Every milling operation comes down to one event repeated thousands of times: a flute of a rotating cutter shears a chip off the workpiece. The tool spins at a set surface speed, the table or spindle feeds it through the material, and the chip carries heat away with it. Get the chip load per tooth too small and the edge rubs instead of cutting, which work-hardens stainless and burns the tool. Get it too large and the cutter deflects, the wall tapers, and the finish turns to chatter marks.

Three variables set that event. Cutting speed is surface speed at the tool diameter, expressed in meters per minute: roughly 200–500 m/min in aluminium, 80–150 m/min in mild steel, 30–60 m/min in 316 stainless and titanium. Feed per tooth is how far each flute advances per revolution, usually 0.05–0.15 mm for a 10 mm carbide end mill. Axial and radial depth of cut decide how much of the flute is engaged at once.

Tolerance on a mill is not a single number taken from a spec sheet. It is the sum of machine positioning error, thermal growth, tool deflection, fixturing stiffness and how many setups the part needs. On our 5-axis centers we hold ±0.005 mm on critical features, but that figure assumes a stable setup and a feature you can reach with a rigid tool. A deep pocket cut with a long, thin end mill will move, no matter what the control says.

The practical reading of all this: put your tight tolerances on the features that matter and leave the rest at general machining tolerance. A drawing with ±0.005 mm on every dimension costs more than the part needs and gives the machinist no room to choose a better strategy.

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  Chip load firstIf the tool rubs, increase feed per tooth before you touch spindle speed.
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  Rigidity is toleranceShort, thick tools and a solid fixture buy more accuracy than a finer control setting.
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  Tolerance where it countsReserve tight callouts for mating and functional surfaces.

A 3-axis mill moves the table in X and Y while the spindle moves in Z. The tool always approaches the part from one direction, so every face you need to machine has to be presented to the spindle. That means one setup per face, and each setup introduces a new datum and a new stacking of error.

A 4-axis mill adds a rotary table, usually turning around X or Y. Now the part can be indexed to a new face without unclamping it. Shafts with cross-drilled holes, parts with features on four sides, and any geometry with a repeating pattern around an axis become one setup instead of three or four.

A 5-axis mill adds a second rotary axis, so the tool can tilt relative to the part. This is what lets a cutter reach undercuts, blend a compound curve in a single pass, and keep a short tool engaged in deep cavities. Sixteen of our machines are simultaneous 5-axis centers; the rest of the 127-machine floor splits across 4-axis, 3-axis, mill-turn and large-travel mills.

The cost side matters as much as the geometry side. More axes mean more setup time saved, but also more machine time billed per hour. For a flat bracket with holes, 3-axis is cheaper. For a housing with ports on five faces, 5-axis usually wins because it removes three or four setups and the re-fixturing error that comes with them.

Choose by counting setups, not by prestige. If a part needs four faces machined and the tolerance between them is loose, a 4-axis machine with one index is enough. If the tolerance between those faces is tight, or the geometry is curved, that is when 5-axis earns its rate.

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  3-axisFlat parts, one dominant face, holes normal to the plate.
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  4-axisShafts and parts with features repeated around a centerline.
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  5-axisUndercuts, compound angles, five-face housings, deep cavities.

Aluminium is the easy case. Grades like 6061-T6, 7075 and 6082 cut fast, hold a good finish and let you take deep passes. 7075 is stronger but more prone to stress movement after heavy material removal, so rough it, let it rest, then finish. Plastics such as POM, PEEK and ABS machine cleanly but need sharp tools and air blast rather than flood coolant to avoid melting and burr strings.

Stainless and titanium are the opposite. They work-harden under a rubbing edge, conduct heat poorly, so the heat stays in the tool, and they spring away from the cutter. The fix is a rigid setup, a positive rake tool, moderate speed and a feed that stays above the work-hardening threshold. 303 stainless machines far more freely than 316, and 17-4PH in the H900 condition is harder again.

Surface finish follows from the same variables. As-machined surfaces land around Ra 1.6–3.2 μm, a good fine-milling pass reaches Ra 0.8–1.6 μm, and a dedicated finishing strategy with a small stepover can reach Ra 0.2–0.8 μm. Beyond that you are usually looking at a secondary process: bead blasting, tumbling, brushing or polishing.

Finishes also change dimensions. Anodizing builds a coating that shifts a tight fit, hardcoat more so, and plating adds thickness on all sides. If a bore has to accept a bearing after anodizing, tell the shop before the part is cut, not after.

Laser marking has its own limit: minimum character height 1.5 mm. Below that the mark becomes unreadable. Plan part numbers and traceability marks around that number rather than shrinking them to fit.

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  Free-machining grades303 stainless, 6061 aluminium and C360 brass cut fastest.
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  Stress reliefRough 7075 and 17-4PH, then finish after the part settles.
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  Finish moves sizeAccount for coating thickness on any mating or bearing fit.

Setup is where most tolerance is lost. Each time a part is unclamped and turned, the new zero is measured from a surface that may already carry error. A single-setup 5-axis job holds a tighter relationship between faces than the same part run across three 3-axis operations, and that difference shows up in assembly, not on the individual feature drawing.

Fixturing follows the same logic. Soft jaws machined to the part profile, a vacuum plate for thin panels, or a tombstone for small parts in volume all serve one purpose: keep the part from moving while the cutter pushes on it. A part that rings when you tap it is a part that will chatter.

Deburring is not optional and not free. Milling leaves a burr on every exit edge, and a burr on a sealing face or a sliding fit will fail inspection. We plan deburr into the process rather than treating it as a cleanup afterthought, because hand deburring a burr you did not anticipate can change a critical edge.

Inspection closes the loop. We check incoming raw material, monitor dimensions during the run, and inspect 100% of parts before shipment, with reports available on request. That is also where the qualification rate of 99.99% comes from: the process is designed so a bad part is caught before it leaves, not argued about after it arrives.

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  One setup is betterFewer clampings means fewer stacked datums.
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  Plan deburr earlyDecide edge break at the drawing stage, not after machining.
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  Reports on requestAsk for dimensional reports before the run starts, not after.