Aerospace Precision Machining Company Materials, Documentation, and Tolerance Planning

Aerospace machined parts are where the word "precision" earns its keep: contoured structural brackets, housings with sealing faces, and fittings where the tolerance and the surface finish are functional requirements, not decoration. Buying that work from a precision machining company means buying three things that have to arrive together — the alloy and condition the drawing names, the tolerance and finish pairs that make the part work, and the documentation that proves both were measured rather than assumed. This guide walks a buyer through those three decisions in the order they come up on a machined part.

Why does aerospace machining turn on tolerance and finish working as a pair?

On a machined aerospace part, tolerance and surface finish are usually demanded on the same feature, and both are delivered by the same process, which is why a drawing that treats them separately produces a part that fails. A sealing face that sits exactly where the drawing says but carries the wrong finish leaks. A bearing face at the right finish but out of position by a few micrometres wears unevenly. The functional surface is the pair: the dimension that says where it is, and the finish that says how it behaves.

That pairing is the buyer's most useful lens on an aerospace drawing. Group the tolerance and the finish callout on each functional face and ask what process the pair demands: a face pairing a ±0.003 mm tolerance with a fine machined finish stays on the machine with a controlled finishing pass, while a pair that demands a ground finish moves to precision grinding at ±0.002 mm. When the quote names the process for each functional pair, you can see whether the supplier read the drawing as an engineer or priced it as a shape. And when the documentation reports both the dimension and the finish for the same feature, the part is verifiable rather than hopeful.

Which alloys should an aerospace machining RFQ actually name?

Name the grade and the condition — "aluminum" is not a spec, and on an aerospace drawing the condition carries the mechanical properties the analysis assumed. The grades below cover most precision-machined aerospace work, and they are chosen for how they machine as much as for how they fly:

Alloy	Typical machined aerospace use	Machining behavior	Buyer notes
6061-T6 / -T651	Brackets, housings, non-critical structure	Excellent; clean cuts and predictable finishes	The default aerospace millable aluminum; stress-relieved plate holds flatness best
7075-T6 / -T7351	Highly loaded airframe fittings and structure	Good; thin walls can move as stress releases	Specify stress-relieved stock for thin-wall parts; temper drives stress-corrosion resistance
Ti-6Al-4V	Structural parts where strength-to-weight matters	Poor relative to aluminum; work-hardens and holds heat	Rigid workholding is mandatory; expect slower cycles
17-4PH (S17400)	Fittings, actuators, corrosion-resistant structure	Good in annealed Condition A	Mill soft, then age to the final condition; specify H900–H1150
Inconel 718	Parts near heat sources, engine static hardware	Difficult; severe work-hardening and tool wear	Reserve for genuine high-temperature duty; verify heat-treatment records

Two buying habits keep the material line honest. First, name the exact standard — an AMS or ASTM specification with the condition — and require the material certificate at receiving, tied to the heat number of the stock that is actually cut. Second, resist exotic alloys for the label: if the load case is aluminum, the machining time and the stock price both drop by an order of magnitude, and the finish is easier to hold. Aerospace material selection is about evidence for the load, and evidence is cheapest to produce in a grade that cuts predictably.

How tight should you tolerance a machined aerospace part, and where does that drive cost?

On machined work the tolerance question is really a finish-pair question. A blanket ±0.05 mm over the whole part forces the shop to treat cosmetic surfaces like bearing faces, and a blanket tight band says the drawing has not decided which features are functional. The discipline that works: put a general tolerance where the part is not sensitive, then tighten the features that carry function with GD&T against explicit datums, pairing each tight dimension with the finish the function needs.

What a precision machining floor holds, stated plainly, is ±0.003 mm on milling and turning, ±0.002 mm where the feature is precision-ground, and process capability of Ppk ≥ 1.67 at the initial study on the key characteristics you flag. On a machined part the capability number matters as much as the nominal tolerance, because it says the process holds the dimension and the finish across the run rather than one part happening to measure in. Express contoured features with profile-of-a-surface callouts against the datums — they control form, location, and orientation together — and require the inspection report to show measured deviation rather than a pass/fail tick.

One aerospace-specific caution belongs in every drawing conversation: temperature. Aluminum expands around 23 micrometres per metre per degree Celsius, so a long feature measured on a warm shop floor disagrees with the same feature measured in a metrology lab. Decide where critical features are measured — a CMM at a defined reference temperature — and which instrument wins when the numbers disagree. That is not bureaucracy; it is the difference between a part accepted on evidence and a part accepted on trust.

Which GD&T callouts should an aerospace machining drawing carry for meaningful verification?

The datum scheme on an aerospace drawing is the contract the part is measured against, so it must survive both the machining and the inspection. Datums should sit on features that exist in the machined part and are stable to fixture, because a datum that exists only on the model produces a part nobody can verify. Profile callouts on contoured surfaces, position on hole patterns, and flatness and parallelism on sealing faces are the callouts a machining floor can both hold and measure.

The verification side matters as much as the callouts. A machined aerospace part should ship with a CMM report that maps each callout to a measured value against the drawing datums, and a finish report that maps each Ra or Rz to a measured profile on the same features. When the position of a hole pattern and the finish of a sealing face are reported against the same datum scheme, the drawing, the machining, and the inspection are one system. A report that shows measured values and named instruments is worth more than a certificate of conformance that asserts them.

What documentation should an aerospace machining order ship with?

Aerospace buyers are buying evidence, and for a machined part the evidence has two halves — material and geometry. The list below is the one to put on the order so there is no negotiation after the parts are made:

• Material certificate. The mill's certificate with chemistry, mechanical properties, and the heat or lot number. No mill cert, no traceable material.
• Certificate of conformance. The supplier's signed statement that the parts meet the drawing revision ordered. Useful, but it is a declaration, not evidence on its own.
• Dimensional inspection report. CMM results mapping the GD&T callouts to measured values against the drawing datums, with measured deviations on contoured features.
• Surface-finish data. Ra or Rz measurements on the functional faces, since finish is half of the tolerance-and-finish pair.
• First article inspection. Where the contract requires it, an AS9102-style report verifying every drawing characteristic on a representative part before production commits.
• Capability data on key characteristics. Ppk ≥ 1.67 initial from an initial run, showing the process holds dimension and finish across the batch.

Traceability matters as much as the papers: every part number should trace to the heat or lot that produced its stock, so that a material issue surfacing later is identifiable in minutes. The quality page on this site describes the inspection practice that produces this evidence as routine — it is the standard to hold any precision machining supplier to.

How do you check a precision machining company's aerospace claims before you commit?

Aerospace quality expectations are tiered, and knowing your tier is the first check. Primes and tier-1 suppliers commonly require an AS9100 quality system, AS9102 first-article reporting, and often NADCAP approvals for special processes. A build-to-print machine shop that is ISO 9001:2015, IATF 16949:2016 (product design excluded) and ISO 14001:2015 certified can serve prototype, development, and many production aerospace parts, but if your contract chain demands AS9100 or NADCAP, that must be stated in the RFQ, because it changes the supplier list rather than just the paperwork. Industry standards are the buyer's gate, not a marketing phrase.

Then verify the practice, not the logo. Ask for redacted samples of the reports you will receive — a CMM report with the datum scheme visible, a finish report with the instrument named, a capability study on a tight feature. Ask which aerospace alloys the shop has actually machined, because titanium experience is not aluminum experience. Ask how a ±0.003 mm milled feature and a ±0.002 mm ground feature are verified, and who reads the drawing before the quote. And ask how the shop holds thin-wall parts — the answer tells you whether it understands that a machined aerospace part fails in deflection before it fails in capability. The services page on this site describes the RFQ-to-delivery flow a buyer should expect.

How should prototypes and first articles gate into a production run?

The failure mode in aerospace sourcing is approving a prototype and assuming the production run inherits everything it proved. It does not unless you say so. The clean sequence: machine the prototype or first article, run the full inspection against every drawing characteristic — dimension and finish together — and get written approval on the measured results before the production quantity is committed. The same material condition, the same fixture strategy, the same datum scheme should carry into production; any of those changing means the first article no longer represents the run.

Once parts are moving, control shifts to process stability. Key characteristics should have a sampling plan and a Cpk target of ≥ 1.67, so a dimension or a finish drifting toward the limit is caught before it produces a bad batch. Drawing revisions need the same discipline — a revision that arrives silently halfway through a run invalidates the inspection records for everything before it. Keep the revision letter on the PO, on the drawing, and on the reports, and the traceability stays intact from prototype through production.

What belongs in an aerospace machining RFQ so the quote comes back useful?

An RFQ that produces comparable quotes is a small document with specific content. Send a STEP or IGES model plus a 2D PDF that names the material grade and condition, the general tolerance, and the GD&T on the characteristics that matter — including the surface-finish callouts, because finish is half of the cost on a machined part. State the quantity and the target date, and flag which documents must ship with the parts. If you operate under a quality clause or an approved-supplier list, attach the requirements so the shop can confirm it can meet them before quoting rather than after.

Two things make aerospace machining quotes noticeably better. First, mark which tolerance-and-finish pairs are key: a drawing that flags the three functional faces that need the tight band, the finish, and the Cpk note costs the same to quote as one that marks nothing, but it tells the shop where the effort goes. Second, ask the supplier to confirm its incoming material control — the chain from mill certificate to incoming verification to the part on the shelf is what makes your traceability real. For the questions buyers raise at this stage — materials, tolerances, finishes, and documentation — the FAQ page on this site answers them directly, and the contact page puts you in front of the engineer who reads the drawings.

Frequently Asked Questions

Is AS9100 certification required for every aerospace machined part?

Not for every part — it depends on your tier in the supply chain and the criticality of the part. Primes and tier-1 suppliers commonly require AS9100 and AS9102 first-article reporting, and NADCAP for special processes. A machine shop holding ISO 9001:2015, IATF 16949:2016 (product design excluded) and ISO 14001:2015 can serve many prototype and production aerospace parts, but if your contract demands AS9100 or NADCAP, state it in the RFQ.

Which aerospace alloys are hardest to machine?

Ti-6Al-4V and Inconel 718 are the difficult end of the list. Both work-harden and hold heat at the cutting edge, so they need rigid workholding, conservative speeds, and sharp tooling, which is why the same geometry in titanium carries a longer cycle time and a higher price than in aluminum.

What tolerance and finish should I call out on an aerospace machined part?

Only what the function needs, expressed as tolerance-and-finish pairs on the functional faces. A practical precision band is ±0.003 mm on milling and turning, ±0.002 mm on ground features, with Ppk ≥ 1.67 at the initial study on key characteristics and Cpk ≥ 1.33 on general characteristics. Capability is reported per characteristic: every report names the feature, its tolerance band, the sample size, the sigma method, and the study window., and finish callouts that name the parameter and value the surface needs.

Why does surface finish matter as much as tolerance on a machined part?

Because on a machined part the two are demanded on the same features — a sealing face, a bearing surface, a mating face. A part that sits exactly where the drawing says but carries the wrong finish fails in service, which is why both are measured and both are reported.

What is a first article inspection, and when do I need one?

It is a complete verification of one representative part against every drawing characteristic, commonly reported in the AS9102 format used across aerospace. You need one before committing to a production run when the part is new or a revision changes critical features.

What is the difference between a material certificate and a certificate of conformance?

A material certificate comes from the mill and carries the chemistry, mechanical properties, and heat or lot number — the evidence the material is what the drawing says. A certificate of conformance is the supplier's signed declaration that the parts meet the drawing. For aerospace work, require both.

Can thin-wall aerospace parts hold tight tolerances and finishes?

Yes, within the physical limits of the wall. A thin wall deflects under cutting force, and vibration and chatter destroy finish and dimension at the same time, so rigid workholding and controlled finishing passes do most of the work. Mark which faces are functional and pair each tight tolerance with the finish the function needs.