Aviation CNC Parts: Tolerances, Materials and Process Choices

Structural brackets, ribs, stringers, actuator housings, fuel and hydraulic manifolds, avionics chassis, and ground-support tooling all start as billet or plate. The common thread is low volume and tight geometry rather than high volume. A run of 40 wing ribs is normal. A run of 40,000 is not, which is why machining stays competitive against casting and forging for this class of part.

Five-axis work earns its cost when a part has features on more than three faces, when a contoured surface must hold position to a datum, or when the setup itself would introduce stack-up error. A one-piece bracket replacing three riveted plates is a typical case. Machining it in one setup removes the assembly tolerance entirely.

Not every part needs five axes. Flat plates, simple bushings and turned fittings are cheaper and faster on a 3-axis mill or a lathe. Putting them on a 5-axis center adds spindle time without improving the part. We route work to the machine that fits the geometry, not the other way around.

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  Good fit for 5-axisContoured pockets, angled bosses, features on four or more faces
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  Good fit for 3-axisFlat plates, covers, shims, simple drilled patterns
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  Good fit for mill-turnShafts and fittings with milled flats or cross holes
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  Consider a different processVery thin-walled ducting or large hollow shells

Aluminum 7075-T6 is the default for loaded structure because of its strength-to-weight ratio, but it is less forgiving than 6061. It machines cleanly at the right feeds and chips badly at the wrong ones. We run it on rigid setups with high-pressure coolant. If a part is mostly cosmetic or lightly loaded, 6061-T6 costs less and cuts faster.

Titanium Ti-6Al-4V (TC4) is where process control matters most. It conducts heat poorly, so the cutting edge takes the temperature. Tool life drops fast if speeds and feeds drift. Roughing with high-feed toolpaths and finishing with light radial engagement keeps heat down and holds the ±0.005 mm tolerance on long features. Expect longer cycle times than aluminum by a wide margin.

Stainless 17-4PH (SUS630) and 316L cover corrosion-exposed fittings and brackets. 17-4PH machines better in the solution-treated condition and gains strength after aging. Inconel and magnesium AZ31B or AZ91D appear in exhaust-adjacent and weight-critical housings. Magnesium needs chip control and fire-safe handling, so it is quoted separately.

A blanket ±0.005 mm callout on every dimension raises cost and inspection time without improving function. Apply tight tolerance to the features that locate the part: bore diameters, mounting hole positions, mating faces. Leave non-critical thickness and edge dimensions at ±0.1 mm or looser. The drawing should show which is which.

Surface finish follows the same logic. A sealing face may need Ra 0.2–0.8 μm, while an internal pocket at Ra 1.6–3.2 μm as machined is fine. Hardcoat anodizing adds 0.02–0.05 mm per surface, so a tight bore should be masked or the pre-plate dimension adjusted. We flag this during DFM review rather than after the parts come off the machine.

Feature size limits matter. Deep pockets with a small corner radius need a long, thin tool that deflects. If a pocket is 60 mm deep with a 2 mm corner, the tool will chatter. Opening the corner radius to 3 mm or splitting the pocket into two operations often solves it. These calls are easier to make at the drawing stage than in the vise.

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  Tolerance±0.005 mm (±0.0002 in) on critical features
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  Fine finishRa 0.2–0.8 μm for sealing and bearing faces
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  Standard finishRa 0.8–1.6 μm for most mating surfaces
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  As-machinedRa 1.6–3.2 μm for non-contact pockets

Every batch goes through raw material verification, in-process checks and a final inspection before it leaves the floor. We report on request. That report matters more on aviation work than on most industrial parts because the paper trail is part of the deliverable, not an extra.

First-article inspection covers the critical dimensions from the drawing, and the setup is not released to production until those numbers are signed off. In-process checks catch tool wear on long runs. Final inspection confirms the part matches the released drawing and that surface finish and any plating thickness are within spec.

The shop holds ISO 9001:2015, IATF 16949:2016, ISO 13485:2016 and ISO 27001:2022. The last one covers how we handle customer drawings and models. Uploads are treated as confidential and an NDA is available on request before any file changes hands.

Quotation and a free DFM analysis come back within 12 hours of a complete RFQ. Production can start within 24 hours once the drawing and material are confirmed. Standard parts ship in 3–5 days. The historical late-delivery probability across our work is below 2%, which is the number we track rather than a promise on any single order.

There is no minimum order quantity. A single prototype and a 10,000-part run go through the same quoting path. For aviation programs, most work sits between 1 and 500 pieces, where machining beats tooling cost. When a design stabilizes and volume climbs past that range, we will say so and point to casting or forging instead.

The shop runs 127 high-precision CNC machines, including 16 simultaneous 5-axis centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers. Maximum processing size is 4,000 mm, with a Ø400 mm rotary table for parts that need positioning on a trunnion. Three wholly-owned plants and 150 technicians cover the floor.