CNC Sub Contract Machining Service: What to Check Before You Award the Job

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  Key takeaways
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  What to Verify on a CNC Sub Contract Machining Service
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  Why the Machine Mix Decides Whether the Job Is Accepted
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  Material Range and What It Costs You in Lead Time
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  Inspection, Certifications and the Paper Trail
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  MOQ, Quoting and Confidentiality Terms
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  How to Qualify a Subcontract Partner in 6 Steps

A CNC sub contract machining service is judged on the work it can take without splitting the part across three suppliers. A shop with 5-axis, mill-turn and 3-axis capacity in the same building can move a job between machines when a fixture is awkward or a deadline moves. That flexibility is worth more than a single high-end spindle.

GreatLight runs 127 high-precision CNC machines across three wholly-owned plants: 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers. Maximum processing size is 4,000 mm, with travels of 4,000 × 400 × 150 mm on the large frames and 750 × 1,150 × 550 mm on the medium ones. A Ø400 mm rotary table covers most turned-and-milled work.

The limit is not always the envelope. Thin walls, deep pockets and features on five faces push a part toward 5-axis or mill-turn, because fewer setups means fewer datum shifts. If a supplier quotes a five-face part as three separate 3-axis operations, ask how the datums are transferred. That answer usually predicts the tolerance you will see on the third batch.

Parts that fit a 3-axis envelope and have one or two critical features do not need the expensive route. Sending simple brackets to a 5-axis cell adds cost without adding capability. Match the process to the geometry, not to the machine list.

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  5-axis and mill-turnFewer setups, tighter position between features
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  3-axisRight call for flat parts with one critical face
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  Large framesUp to 4,000 mm for long bases and rails
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  Rotary tableØ400 mm for turned features with milled flats

Aluminium and stainless are the easy part. Grades like 6061, 7075, 304, 316L and 17-4PH sit in most subcontract shops, and the machining parameters are well understood. Where a supplier separates itself is in the less common alloys: TC4 (Ti-6Al-4V), Inconel, magnesium AZ31B and AZ91D, plus engineering plastics such as PEEK and carbon fibre.

Titanium and Inconel cut slowly. Tool wear is high, coolant strategy matters, and a shop without prior data will burn cutters learning your part. Ask how many jobs of that alloy they ran in the last year. A vague answer is a schedule risk, not a quality risk.

Plastics behave differently again. POM and PA move after machining, so a shop that measures straight off the machine will ship parts that fail inspection a day later. For PEEK and carbon fibre, ask about stress relief and whether they finish to final dimension after a stabilization pause.

Mixed-material assemblies are the common trap. If a housing is 6061 and the insert is 316L, thermal expansion during a tight-bore fit can close the clearance. Tell the supplier the assembly temperature, not just the drawing tolerance.

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  Common grades6061, 7075, 304, 316L, 17-4PH, 4140
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  Hard alloysTC4, Inconel, magnesium AZ31B and AZ91D
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  PlasticsPOM, PA, PEEK, carbon fibre need stabilization
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  Mixed assembliesState the service temperature, not only the tolerance

Anyone can print a tolerance on a quotation. The question is what happens on part 400. Look for a process that checks raw material on arrival, monitors dimensions during the run, and inspects before shipment. A 100% inspection before shipment with reports on request is a reasonable baseline for subcontract work, and it is what we run on every order.

Certifications answer narrow questions. ISO 9001:2015 covers a general quality system. IATF 16949:2016 is what automotive and EV programs expect. ISO 13485:2016 applies to medical devices. ISO 27001:2022 covers how your drawings and data are handled, which matters if the part is defense-adjacent or still under patent review.

Ask for the certificate scope, not just the certificate number. A valid ISO 13485 certificate that excludes machining does not help a machined implant housing. The same applies to IATF: check that the site doing your work is inside the audited scope.

A qualification rate is only useful with a definition. Ours is 99.99% against the drawing, measured on the final inspection step. If a supplier quotes a number with no definition, treat it as marketing and ask for the last three months of first-pass yield instead.

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  Raw material checkGrade and condition verified before the first cut
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  In-process monitoringDimensions checked as the batch runs
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  Final inspection100% before shipment, reports on request
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  Scope checkCertificate must cover the process and the site

No minimum order quantity is the norm for subcontract machining, but the setup cost does not disappear when the lot is one piece. A transparent quote separates programming, fixturing, material and run time, so you can see what a second piece actually costs. If a quote gives one lump number, ask for the breakdown before you compare it with another shop.

Quoting speed tells you something about the shop's process. A quotation and free DFM analysis within 12 hours means drawings are reviewed by someone who machines parts, not a queue. Production can start within 24 hours after that, and parts ship in 3–5 days for standard work.

Confidentiality is a commercial term, not a promise. Uploads should be handled as secure and confidential, and an NDA should be available on request when your drawings carry unannounced features. Get the NDA signed before the DFM review, not after the quote.

Watch for quotes that hide material markup in the run time, or that price a fixture as a one-off charge but reuse it for the next order. Both are normal, but only one is fair to a repeat buyer.

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  Ask for the breakdownProgramming, fixturing, material, run time
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  12-hour quoteIncludes free DFM analysis on your drawings
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  NDA before DFMSign first if the design is not public
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  Fixture ownershipConfirm whether it is billed once or reused