CNC Machining Service Production: How to Choose a Supplier
This guide is for engineers and sourcing staff moving a part from prototype into repeat production. It covers the checks that decide whether a supplier can hold your tolerance, hit your dates, and keep paperwork clean across the whole run. Read it before you send the purchase order.
In this article
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Key takeaways
Supplier check table: what to ask and what a good answer looks like
Use this table in the first supplier call. The right column is the answer we would give for our own shop.
| Check | What to ask | Weak answer | Strong answer |
|---|---|---|---|
| Tolerance | Which features hold ±0.005 mm? | All dimensions, no exceptions | Named features, with the datum called out |
| Machine list | How many 4- and 5-axis centers? | We have many machines | 16 simultaneous 5-axis, 12 four-axis |
| Lead time | When do parts ship? | Depends on the order | Quote in 12 h, ship in 3–5 days |
| Minimum order | Can you run one piece? | We prefer larger lots | One prototype to 10,000+ parts |
| Inspection | What is checked before shipping? | We check quality | 100% inspection, reports on request |
| Certifications | Which standards apply to us? | We are ISO certified | ISO 9001, IATF 16949, ISO 13485, ISO 27001 |
| Finish | Who does anodizing and plating? | We have partners | In-house finishing plus laser marking |
| Data | Will you sign an NDA? | On request, later | NDA available, secure uploads |
The buying rule
Pick the process route first, then the supplier that runs it every week. A shop that matches your feature pattern and your industry certificate will beat a shop with a longer machine list every time.
What cnc machining service production actually covers
Production is not the same job as prototyping. A prototype answers a design question. Production has to repeat the same cut, the same fixture, and the same inspection on every lot for months. The drawing may not change, but the process has to hold still.
That is why the supplier question shifts. In prototyping you ask who can machine this shape first. In cnc machining service production you ask who can machine it again in March and still land inside the tolerance band. Fixture design, tool wear tracking, and incoming material checks start to matter more than the machine's top spindle speed.
GreatLight runs three wholly-owned plants with 127 high-precision CNC machines, including 16 simultaneous 5-axis centers, 12 four-axis mills, 27 three-axis machines, and 16 mill-turn centers. The largest travel is 4,000 × 400 × 150 mm. Those numbers matter only if they map to your part. A 4,000 mm rail and a Ø10 mm connector pin do not need the same machine.
So the first buying decision is not which supplier. It is which process route your part belongs to. Get that wrong and every later check, from tolerance to lead time, is built on the wrong machine.
- 1PrototypeOne to a few pieces, tight feedback, geometry still moving.
- 2Bridge productionSmall lots that fill the gap before tooling or casting is ready.
- 3Repeat productionStable drawing, fixed fixture, inspection plan that repeats per lot.
Tolerance and finish: read the numbers per feature
A tolerance on the title block is not the same as a tolerance a shop can hold on every wall. Thin ribs, deep pockets, and long unsupported sections move during cutting. Ask which features carry ±0.005 mm and which use a general tolerance. A supplier who names those features has already done the DFM work.
Surface finish follows the same logic. As-machined parts sit around Ra 1.6–3.2 μm. A high finish is Ra 0.8–1.6 μm. Fine finish is Ra 0.2–0.8 μm and usually needs a separate operation, a different tool, and extra time. If your callout asks for Ra 0.4 μm on a face that only needs to clear a gasket, you are paying for a step you do not need.
Material choice sets the limit too. Aluminum 6061 and 7075 cut clean and hold fine detail. Stainless 316L and 17-4PH move more and wear tools faster. Titanium TC4 (Ti-6Al-4V) and Inconel cut hot, so heavy stock removal on a thin wall will spring back. A shop that quotes the same cycle time for 6061 and Inconel has not looked at the material column.
The practical move: send the drawing and let the shop return a DFM note with the quote. A free DFM analysis within 12 hours is a normal turnaround for a shop that reads prints daily. If the quote arrives with no comment on tolerance or material, ask what they assumed.
Lead time, MOQ, and what the quote should include
Lead time is a chain, not a single number. Material purchase, programming, first-article inspection, run time, finishing, and shipping all sit inside it. A shop that says "two weeks" without breaking that down is guessing. Our own line runs production start within 24 hours after a confirmed order, and parts ship in 3–5 days for standard runs.
MOQ is the next filter. Some shops price one piece and then quietly add setup to every small lot. Others refuse small lots because they cannot amortize tooling. A no-minimum-order policy from one prototype to 10,000+ part runs means the same fixture and the same inspection plan carry from the first part to the last. That is what you want for a bridge order.
Read the quote line by line. A usable quote lists material grade, machine, finish, inspection level, and what is excluded. Watch for a low unit price with no finishing or inspection line. That gap gets filled later, usually after you have committed the order.
Finishing is the most common hidden line. Anodizing, electroless nickel, zinc plating, powder coating, black oxide, bead blasting, and laser marking all add steps and days. Laser marking has a minimum character height of 1.5 mm, so plan your part numbers and logos around that limit before you approve the artwork.
- 1Ask for the splitMaterial, machining, finishing, inspection, freight.
- 2Ask who finishesIn-house steps keep the schedule inside one shop.
- 3Ask what is excludedTooling, fixtures, first-article report, special packaging.
Certifications and inspection records that survive an audit
Certificates are industry gates. ISO 9001:2015 covers general quality management. IATF 16949:2016 applies to automotive work. ISO 13485:2016 applies to medical devices. ISO 27001:2022 covers information security, which matters when your drawings and CAD files sit on a supplier's server.
Ask which certificate covers the plant that will run your part. A group may hold four certificates across three sites while your work goes to a fourth building. The certificate number and the site address should match the shipping address on your PO.
Inspection is the second half of this check. Raw material verification, in-process monitoring, and final inspection before shipment should all be part of the plan. A 99.99% qualification rate sounds strong, but ask what is measured, on which features, and how often. Reports on request are fine for standard runs; safety-critical parts usually need a defined sampling plan written into the order.
For medical and automotive work, the first article inspection report is not optional. Get it before the run starts, not with the shipment. By then, any fixture error has already repeated across the lot.
When a 5-axis shop is the wrong choice
5-axis machining is not automatically better. It pays off when a part has angled faces, deep pockets reachable from one side, or features that would need three setups on a 3-axis mill. Every extra setup adds a datum shift and a chance to lose the tolerance stack.
For a flat bracket with holes on one face, a 3-axis machine is faster and cheaper. For a part with a single eccentric bore, a 4-axis mill with a rotary table does the job without the cost of simultaneous motion. Choosing 5-axis for simple geometry just moves money from your budget to the machine hour.
The reverse case matters more. A hydraulic manifold with cross-drilled ports at compound angles, or an aerospace housing with a contoured pocket around a boss, will lose accuracy if it is repositioned four times. One setup on a simultaneous 5-axis center holds the relationship between those features.
So sort your parts before you sort your suppliers. Group them by feature pattern, not by industry. The shop's machine list should match that grouping, and the quote should name the machine it plans to use.
- 1Choose 5-axisCompound angles, contoured pockets, tight datum relationships.
- 2Choose 4-axisEccentric bores, slots around a cylinder, index-and-cut work.
- 3Choose 3-axisFlat plates, single-face holes, simple pockets and profiles.
Step by step: qualifying a supplier for production
Run these in order. Each step should produce a document you can keep in the project folder.
- 11. Group your parts by process routeSplit the BOM into flat 3-axis work, index 4-axis work, and simultaneous 5-axis work. Note the largest envelope. Our largest travel is 4,000 × 400 × 150 mm, so anything beyond that needs a different plan.
- 22. Send drawings for DFM and quoteInclude material grade, finish callout, tolerance per feature, and target volume. Expect a DFM note and price within 12 hours. Read the note before the price.
- 33. Check the tolerance mapConfirm which features hold ±0.005 mm and which use general tolerances. Ask for the datum scheme. If the reply is a blanket claim, get a second quote.
- 44. Verify certificates against the plantMatch the certificate site address to the shipping address. ISO 9001:2015 is the floor. Add IATF 16949:2016 or ISO 13485:2016 for automotive and medical work.
- 55. Approve the first articleInspect the first part against the drawing before the run continues. Check finish with a roughness gauge, not by eye. Ra 0.8–1.6 μm and Ra 0.2–0.8 μm look identical on a bench.
- 66. Lock the fixture and inspection planWrite down the fixture, the tool list, and the sampling rate. These three items are what keep lot 10 the same as lot 1.
- 77. Confirm finishing and markingName the finish process and the marking content. Laser marking needs a minimum character height of 1.5 mm, so revise small text before approval.
- 88. Set the shipping and data termsAgree on packaging, freight terms, and NDA coverage. Secure uploads and a signed NDA should be in place before CAD files leave your server.
Questions buyers ask before the first production order
What is the difference between prototype machining and production machining?
Prototyping optimizes for speed and feedback. The drawing is still moving, so the shop may choose a simple setup and accept a looser tolerance on non-critical faces.
Production optimizes for repeatability. The fixture is fixed, the tool list is set, and the inspection plan repeats on every lot. The part does not change; the process around it does.
Can a supplier hold ±0.005 mm on every dimension?
Not on every feature. Thin walls, deep pockets, and long unsupported sections deflect during cutting, so they carry wider tolerances by necessity. The ±0.005 mm figure applies to specific features with a defined datum.
Ask for the tolerance map. It should name the features and the inspection method used to verify them.
Do we need an NDA before sending CAD files?
If the part is not public, yes. Our NDA is available on request, and uploads are kept secure and confidential. ISO 27001:2022 covers information security at the process level.
Sign the NDA before the first file transfer, not after the quote. It is simpler to set the terms once.
Which materials are available for production runs?
Aluminum grades include 6061, 6061-T6, 2024, 5052, 5083, 6063, 6082, 7075, and ADC12. Stainless covers 303, 304, 316, 316L, 420, 430, 431, 440C, and 17-4PH (SUS630).
Steel, copper, brass, titanium, and engineering plastics such as POM, PEEK, PC, and ABS are also stocked. Send the grade from your drawing and we confirm availability with the quote.
How is a small bridge order handled compared with a large run?
There is no minimum order quantity, so one prototype and a 10,000+ part run go through the same fixture and the same inspection plan. The difference is lot size, not process.
That matters for bridge orders because the parts you qualify today should match the parts you receive after tooling or casting comes online.
What should be in the inspection report?
At minimum: raw material verification, the measured features, the gauge or CMM used, and the sampling rate. Reports are available on request.
For automotive and medical parts, add the first article inspection report and the traceability record for the material lot.
Send your drawing and get a production quote
We return a DFM note and price within 12 hours, and run from one part to 10,000+ under ISO 9001, IATF 16949, ISO 13485, and ISO 27001. Uploads stay secure and confidential.
12-hour quoteNo MOQ100% inspectionNDA on request
More from the shop floor
We publish setup notes, tooling trials and inspection data from the factory floor.
