CNC Parts Machining: How Metal Removal Really Works

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  What happens during one CNC parts machining cycle
• 2
  Why tolerance is a system, not a machine spec
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  Choosing 3-axis, 4-axis, or 5-axis for the part
• 4
  When CNC parts machining is the wrong process
• 5
  How material choice changes the machining plan
• 6
  Which machining route fits the part

CNC parts machining is subtractive. A rotating cutter is driven along a programmed path and removes material until the remaining shape matches the CAD model. Nothing is added, so the starting stock must be big enough to contain the finished part plus whatever the fixture needs to hold it.

The cycle starts with CAM output: tool paths, feed rates, spindle speeds, and depth of cut. A roughing pass removes most of the volume with a large cutter. A semi-finish pass evens out the stock left behind. A finishing pass cuts the final surfaces at the programmed tolerance.

Every pass leaves marks. A 12 mm end mill at 0.5 mm radial engagement leaves a different surface than a 6 mm cutter at 0.2 mm. That is why the same part, machined on two different machines, can arrive with two different surface finishes even when the dimensions both pass inspection.

Heat is the quiet variable. Aluminium 6061 conducts heat away quickly and cuts clean at 300–500 m/min. Titanium TC4 (Ti-6Al-4V) conducts poorly, so heat stays at the edge and tool life drops fast. Feed and speed windows are set per material, not per machine.

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  RoughingRemoves bulk stock. Tolerance is loose here, typically ±0.2 mm.
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  Semi-finishLeaves 0.2–0.5 mm of stock for the finishing pass.
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  FinishingSets final dimensions and surface finish.

A machine tool quoted at ±0.005 mm is not a promise that every part will hit ±0.005 mm. That number describes the machine under controlled conditions. The part depends on the whole system: machine, fixture, tool, material, thermal state, and inspection method.

Fixtures matter more than most designers expect. A thin wall clamped on one side will deflect during cutting and spring back after unclamping. A part held in a vise with 2 mm of grip can move. Soft jaws bored to the part diameter usually hold better than hard jaws on finished surfaces.

Tool deflection follows the length-to-diameter ratio. A 6 mm end mill hanging 60 mm out of the holder will deflect roughly ten times more than the same cutter at 20 mm. Deep pockets need either a shorter tool, a smaller stepover, or a different process.

Thermal drift is real. A spindle that has run for two hours is not the same size as a cold one. Shops that hold tight tolerances on long runs let the machine warm up and measure parts at a stable temperature, not straight off the table.

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  Machine geometryStraightness, squareness, and spindle runout.
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  Fixture rigidityHow well the part resists cutting forces.
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  Tool extensionShorter is stiffer. Keep L/D under 4:1 where possible.
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  Inspection methodCMM, micrometer, or optical. Each has its own uncertainty.

Axis count is a fixturing decision first and a geometry decision second. A part with features on five faces can be machined on a 3-axis mill if you are willing to build multiple setups. Each setup adds a datum shift and a chance to lose alignment.

A 4-axis machine adds a rotary table, usually around the X axis. That lets the part rotate while the cutter stays in one orientation. Shafts with cross holes, cam profiles, and parts with radial features are natural fits. GreatLight runs 12 four-axis mills and a Ø400 mm rotary table for this class of work.

A 5-axis machine adds a second rotary axis, so the cutter can approach the part from nearly any direction. Contoured surfaces, deep cavities with undercuts, and parts that would need five separate 3-axis setups are the usual reasons to move up. The trade is programming time and machine cost.

Not every part benefits. A flat plate with holes on one face is faster and cheaper on a 3-axis machine. Moving it to 5-axis adds setup complexity without improving the part. The right question is how many faces need machining and how tight the feature-to-feature tolerance is.

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  3-axisPrismatic parts, one or two faces, simple holes and pockets.
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  4-axisShafts, cams, radial holes, parts that rotate around one axis.
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  5-axisContoured surfaces, undercuts, multi-face parts with tight datums.

CNC machining removes material from a solid block. That is efficient for one part or ten thousand, but it is wasteful for a part that is mostly empty space. A housing with thin walls and large internal cavities may cost less as a casting or a weldment with machined interfaces.

Very hard materials push the process to its limit. Heat-treated tool steel above 50 HRC can be machined, but only with carbide or ceramic tooling, reduced speeds, and a plan for the heat. Inconel and other nickel alloys cut slowly and wear tools quickly. Sometimes grinding or EDM is the better answer.

Parts with internal features that cannot be reached by a rotating cutter are a different problem. A blind internal channel with a sharp internal corner is not a milling feature. It may need EDM, additive manufacturing, or a design change to a radius the cutter can reach.

Surface finish has a floor. As-machined surfaces typically land between Ra 1.6 and 3.2 μm. Getting to Ra 0.2–0.8 μm requires slower finishing passes, smaller stepovers, or a secondary process like polishing or lapping. That is a cost decision, not a machine setting.

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  Mostly hollow partsCasting, die casting, or fabrication may be cheaper.
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  Hardened steelAbove 50 HRC, consider grinding or EDM.
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  Internal sharp cornersRotating cutters leave a radius. Design for it or change process.

Aluminium 6061 is the default for prototypes and many production parts. It machines fast, holds tolerance well, and takes anodizing cleanly. 7075 is stronger but cuts with more chatter and is less forgiving of thin walls. 2024 sits between them and is common in aerospace work.

Stainless 303 is the free-machining grade and is easy to turn. 304 and 316 are tougher, work-harden quickly, and need slower speeds and heavier feeds to get under the hardened layer. 17-4PH can be machined in the annealed state and then aged to strength, which avoids cutting hard material.

Plastics behave differently. POM and PEEK hold dimensions well but move with temperature. ABS and PP are soft and can burr. Carbon fibre reinforced plastics wear tools fast and need dust control. Clamping pressure matters more than on metal because the material deflects.

The setup follows the material. Aluminium can be held lightly and cut aggressively. Titanium needs rigid workholding and a feed that keeps the cutter engaged. Stainless needs a constant feed so the tool does not rub. The same fixture that works for aluminium may not work for titanium.

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  Aluminium6061, 7075, 2024, 6082. Fast, stable, anodizes well.
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  Stainless303, 304, 316L, 17-4PH. Slow speeds, heavy feeds.
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  TitaniumTC4 (Ti-6Al-4V). Rigid setup, heat management.
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  PlasticsPOM, PEEK, ABS, PC. Watch temperature and clamping.