CNC Machining for Aerospace Parts: Tolerances and Documentation
Short answer: aerospace CNC parts are machined to ±0.005 mm or tighter on critical features, verified with a CMM and often specified with GD&T per ASME Y14.5, and documented with material certificates, a first-article inspection report, and traceability from raw stock to finished part. The tolerance is rarely the hard part; the paper trail and the process control are what qualify a supplier. BQUQ machines precision parts in Dongguan under ISO9001 and returns a drawing-based quotation within 12 working hours.
Aerospace buyers do not shop on price alone. A part that is dimensionally correct but undocumented is worthless in an audit, and a part with a perfect report but a process that cannot repeat is equally risky. The work splits cleanly into three questions: can the shop hold the tolerance, can it prove it, and can it repeat it on the next batch. This guide answers all three.
Which Tolerances and Standards Govern Aerospace Machining?
Most aerospace drawings use GD&T rather than simple plus/minus dimensions, because the function of the part depends on form, orientation, and location, not just size. A bore can be within size tolerance and still be out of position; a face can be flat enough by caliper but tilt enough to fail a mating surface. GD&T captures that intent with datums and feature control frames.
On the shop floor, that means the drawing is read against ASME Y14.5 (or the ISO equivalent), and the inspection plan is built around the same datum structure. Critical features typically land in these bands:
| Feature type | Typical tolerance | Verification method |
|---|---|---|
| Bearing bores and pilot diameters | ±0.005 mm | CMM, bore gauge |
| Flatness / perpendicularity | 0.01–0.02 mm | CMM, surface plate |
| Hole position (true position) | Ø0.05–Ø0.10 mm | CMM |
| General machined profile | ±0.1 mm | Calipers, micrometers |
| Surface finish on sealing faces | Ra 0.4–0.8 µm | Profilometer |
The honest rule: hold the tight tolerance only where the function needs it. Blanketing a drawing with ±0.005 mm drives cost, adds inspection time, and increases scrap without improving the part. A supplier worth using will ask which features are critical rather than quietly quoting the whole part as if everything were.
What Documentation Does an Aerospace Customer Need?
Documentation is where generic machining and aerospace machining part ways. A typical package includes:
- Material certification, tying the finished part lot back to the mill certificate for the raw stock.
- First-article inspection (FAI) report on the first part of the order, showing measured values against every dimension on the drawing.
- A dimensional inspection report per batch, with the measured values and the gauge used.
- Traceability records so any finished part can be traced to its material lot and its inspection data.
- A certificate of conformance (CoC) stating the part meets the drawing and purchase-order requirements.
| Document | When it is produced | What it proves |
|---|---|---|
| Material certificate | At raw material receipt | Grade and heat/lot identity |
| FAI report | On first part of an order | All dimensions measured vs drawing |
| Batch inspection report | Every shipment | Batch meets spec |
| CoC | Every shipment | Conformance statement |
| Traceability log | Maintained continuously | Raw lot to finished part |
Several of these documents are standard output at a serious shop; others depend on what your quality system requires. The correct time to agree the package is at the RFQ stage. Asking for an FAI report after the parts are made is too late, and asking a shop to reconstruct traceability after the fact is a sign the process was never controlled.
Material and Process Choices for Aerospace Parts
Aerospace work leans on a small set of materials: aluminum 6061 and 7075 for structural housings and brackets, 17-4 PH and 15-5 PH stainless for high-strength corrosion-resistant parts, titanium for weight-critical components, and engineering plastics for non-structural parts. Each choice reshapes machining time and cost.
| Material | Typical aerospace use | Machinability | Notes |
|---|---|---|---|
| 6061 aluminum | Brackets, housings | Very good | Low cost, easy to finish |
| 7075 aluminum | Structural, high-strength | Moderate | Higher strength, higher cost |
| 17-4 PH stainless | High-strength fittings | Moderate | Heat treatment affects final size |
| Titanium Grade 5 | Weight-critical parts | Poor | Slow, tool wear, heat control |
| PEEK | Non-structural | Good | MRI-compatible, chemical resistant |
Process choice matters as much as material. A prototype bracket may be worth milling from billet; the same bracket at high volume may be cheaper as a stamping or a casting. At BQUQ we run CNC, metal stamping, springs, and heat sinks in one factory, so we can tell you when a machined part should really be stamped, and when a tight tolerance genuinely requires machining. That comparison, covered in our CNC prototyping and low-volume guide, often saves more than a small discount on the machining alone.
How Does BQUQ Handle Aerospace Work?
We machine precision parts on stable CNC turning and milling equipment, verify critical features with a CMM, and ship each batch with a dimensional inspection report as standard. Our production capability is ±0.005 mm on critical features, and we hold tighter only where the process can defend it — we would rather tell you a callout is beyond a machining process than deliver parts that fail audit.
For aerospace prototypes, small batches are normal. There is no mold to amortize, so single-piece and low-volume orders are routine, and revisions between iterations are expected. Send the model, the drawing with GD&T, the material, the quantity, and the documentation requirement. If a feature needs grinding or a special process we do not run, we say so and, where it helps, coordinate the right step instead of pretending a mill can do everything.
What Belongs in an Aerospace RFQ?
A complete request removes the safety margin a factory would otherwise add. Include the 3D model and a dimensioned 2D drawing with datum callouts, the material grade, quantity and any expected annual volume, the finish and any surface treatment, the documentation package you require, and the delivery target. If the part is part of an assembly, send the mating parts too; tolerance stack-up across several parts is usually the real problem, not any single one. Our tolerance stack-up guide explains how that analysis changes the tolerances you should specify.
What Goes Wrong With Aerospace Machined Parts?
Most aerospace machining failures are not dramatic. They are quiet, and they show up late. The recurring causes are worth knowing before you place an order:
- Tolerance over-specification. A drawing where every dimension is ±0.005 mm forces inspection on features that do not need it, raises scrap, and hides which surfaces are actually critical. The result is a higher price without a better part.
- Undefined datum structure. If the drawing does not clearly state which surface is the primary datum, the shop and the inspector can each choose a different reference. The part passes locally and fails at assembly.
- Heat treatment after final machining. For precipitation-hardening stainless such as 17-4 PH, hardening can move dimensions. If the sequence is wrong, the part arrives correct off the machine and out of tolerance after treatment.
- Weak traceability. A finished part that cannot be linked to a material lot is a problem in any audit, and rebuilding that link after production is rarely possible.
- Deburring and edge condition left unspecified. A sharp edge on a handling surface becomes a crack initiation point; a loose burr becomes foreign object debris.
None of these are exotic. Each is prevented by deciding the critical features, the datum structure, the process sequence, and the documentation package before the first cut. That is why an aerospace quote worth accepting comes from a supplier that read the drawing, not one that priced a photo.
Frequently Asked Questions
Q: Can a Chinese machine shop meet aerospace tolerances?
A: Yes, for machined parts. ±0.005 mm on critical features is achievable on modern machines and is verified with a CMM. What you should test is not the tolerance claim but the documentation and repeatability: ask for an FAI report and a sample batch before committing to volume.
Q: What is an FAI report and do I need one?
A: A first-article inspection report documents the measured value of every drawing dimension on the first part of an order. It is standard practice for aerospace and other regulated work, and it proves the process produced the part as drawn. It should be agreed at the RFQ stage.
Q: How do you handle GD&T on a drawing?
A: We read the drawing against ASME Y14.5 (or the ISO equivalent), build the inspection plan on the same datum structure, and verify form, orientation, and location with a CMM. If a feature control frame is ambiguous, we ask before machining rather than guessing.
Q: What materials do you machine for aerospace parts?
A: Aluminum 6061 and 7075, 17-4 PH and 15-5 PH stainless, titanium, and engineering plastics such as PEEK. If your drawing calls for a process we do not run, we tell you and help route the work instead of forcing it onto the wrong machine.
Q: How fast can I get a quote for machined aerospace parts?
A: Send the drawing and model to sc@bquq.com or WhatsApp +86 13713157787 with material, quantity, finish, and documentation requirements. We return a quotation within 12 working hours.
Related Resources
- CNC Aerospace Prototypes — how aerospace prototype parts move from model to verified first article.
- CNC machining services — precision turning and milling from a Dongguan source factory.
- About BQUQ — an ISO9001:2015 factory running CNC, stamping, springs, and heat sink lines under one roof.
- Contact us — send your drawing and get a quote within 12 working hours.
Authored by the BQUQ Engineering Team. BQUQ is an ISO9001-certified source factory in Dongguan, China, running CNC machining, metal stamping, custom springs, heat sink and collet lines under one roof. Send drawings to sc@bquq.com or WhatsApp +86 13713157787 for a quote within 12 working hours. www.bquq.com
