CNC machining aircraft components: how it works and where it stops

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  What actually happens when a cutter meets an aerospace alloy
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  Tolerance and surface finish: knowing the real floor
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  Why five-axis workholding decides the result
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  When CNC machining is the wrong choice for an aircraft part
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  From drawing to inspected aircraft part
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  Which aerospace material suits which part

CNC machining aircraft parts is a subtractive process: a rotating cutting tool removes material from a solid block, forging or extrusion until the geometry matches the CAD model. Nothing about that changes for aerospace. What changes is how tight the machine has to hold position and how carefully the heat has to be managed.

Aluminum 7075 and Ti-6Al-4V sit at opposite ends of the problem. Aluminum cuts fast and throws heat into the chip, so a 12 mm carbide end mill can run at 8,000 rpm with a healthy feed per tooth. Titanium does the opposite. It keeps heat at the cutting edge, work-hardens if the tool rubs instead of cuts, and will burn a cutter in seconds if the feed is too light. On Ti-6Al-4V we typically run surface speeds around 40–60 m/min and never let the tool dwell.

That difference drives the whole setup. A thin aluminum rib can be machined at high speed with light finishing passes because the material is stiff enough relative to the load. The same rib in titanium deflects under cutting force, so we support it, reduce radial engagement, and accept a longer cycle.

The machine matters less than the process window. A five-axis center helps because it reaches the part from more angles without re-fixturing, which is where most aerospace errors come from. Repositioning a part four times adds four chances to lose 0.02 mm.

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  Climb millingStandard on aerospace alloys. The cutter exits on the thin chip, which reduces work hardening.
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  Rigidity firstShort tool overhang and a supported workpiece beat a bigger machine every time.
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  Heat pathAluminum sends heat into the chip. Titanium sends it into the tool. Plan coolant and speed accordingly.

On aircraft work GreatLight holds ±0.005 mm (±0.0002 in) on critical features. That is achievable on a well maintained machine with temperature-stable stock, but it is not free. Reaching it means roughing, letting the part relax, semi-finishing, then a light finishing pass with a sharp tool.

The mistake we see most often is a drawing that calls ±0.005 mm on a 500 mm long aluminum spar. Over that distance, thermal expansion of aluminum is roughly 0.012 mm per 1 °C. A shop that is 5 °C warmer in the afternoon than in the morning loses the tolerance before the cutter touches the part. On long parts, call out the tight tolerance only on the features that need it.

Surface finish follows the same logic. Ra 0.8–1.6 μm is our normal machined finish on aluminum and steel. Where a seal, bearing bore or fatigue-critical fillet needs better, we can reach Ra 0.2–0.8 μm with a finishing pass at low feed and a fresh insert. Below that, you are usually buying a polished or lapped operation, not a machining operation.

As-machined at Ra 1.6–3.2 μm is fine for brackets, covers, and non-contact housings. Specifying a finer finish than the function needs adds cycle time and cost without adding service life.

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  Fine finishRa 0.2–0.8 μm for seal bores, bearing seats and fatigue-critical fillets.
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  Standard finishRa 0.8–1.6 μm covers most structural brackets and housings.
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  As machinedRa 1.6–3.2 μm for non-contact covers and internal frames.

Most out-of-tolerance aircraft parts are not cut wrong. They are held wrong. A bracket clamped on its thin web will spring when the vise opens, and the hole pattern that measured perfectly on the machine no longer matches the mating part.

Five-axis machining reduces this problem because the tool can reach five faces in one setup. Our 16 simultaneous five-axis centers take parts up to 4,000 × 400 × 150 mm on the large travel, down to 500 × 310 × 200 mm on the compact machines. The rotary table is Ø400 mm. For a part that needs three or four faces machined, one five-axis setup usually beats three three-axis setups on both accuracy and lead time.

Soft jaws machined to the part profile, vacuum plates for thin panels, and sacrificial tabs on long slender parts all do the same job: they hold the workpiece without imposing force that will release later. On titanium, we also plan the tool path so the finishing pass takes a continuous load. Stop-start cutting on titanium is where chatter starts.

If your design has a feature that can only be reached from one direction, say so on the drawing. It lets us choose the setup before quoting instead of discovering it at the machine.

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  One setup beats threeEach re-fixturing adds a datum error you cannot inspect away.
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  Machined soft jawsMatched to the part profile so clamping force goes where the part is stiff.
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  Sacrificial tabsKeep long thin ribs from vibrating during the finishing pass.

CNC is a poor fit when the part is mostly empty space. A large thin-walled duct or a complex bracket with deep internal ribs costs far more to mill from solid than to cast or fabricate. If more than about 70% of the stock ends up as chips, ask whether casting, die casting, or sheet metal fabrication gets you there faster.

It is also the wrong call for parts that must have forged grain flow. A landing gear component or a high-load fitting gets its fatigue life from directional grain, and a machined-from-plate version does not have that. Machining still finishes those parts, but it does not replace the forging.

Composites are a separate case. Carbon fibre laminates are trimmed and drilled, not milled to shape. We do run the drill and trim operations, but the layup and cure belong to another process.

Where CNC wins is low to medium volume, tight tolerance, and geometry that changes between revisions. No tooling cost, no minimum order quantity, and a design change is a new program rather than a new mold. From one prototype to 10,000+ part runs, the same setup scales.

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  High material removalIf most of the block becomes chips, casting or fabrication is usually cheaper.
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  Grain-critical fittingsForged grain flow cannot be machined in. Start from a forging.
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  Polymer compositesTrimmed and drilled, not milled from solid.