Precision CNC Machine Service: What to Check Before You Send a PO
A practical buying guide for engineers and sourcing teams choosing a precision CNC machine service. We cover tolerance capability, inspection method, order quantity, lead time and certifications, so you can compare quotes on the same basis instead of on price alone.
In this article
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Key takeaways
What to compare across precision CNC machine service quotes
Use the same questions for every supplier. A quote that cannot answer the middle column is not comparable on price.
| Criterion | What to ask | Why it changes the quote |
|---|---|---|
| Tolerance | Which feature needs ±0.005 mm, and on which machine? | Tight callouts need 5-axis or jig-ground setups and more inspection time. |
| Inspection | First-article, in-process, or final only? | 100% inspection adds labor but removes surprise rejects at assembly. |
| Order quantity | Is there a minimum order quantity? | No MOQ means prototype and production pricing live on the same process. |
| Lead time | Quote turnaround vs. ship date? | A fast quote usually points to a shop that can start within a day. |
| Certifications | ISO 9001, IATF 16949, ISO 13485, ISO 27001? | Regulated industries need the matching system, not a general one. |
| Surface finish | As-machined, Ra 0.8–1.6 μm, or finer? | Fine finishes need slower feeds, extra passes and separate handling. |
| Confidentiality | NDA available? Where do uploads go? | IP-sensitive designs need an agreement before STEP files move. |
Matching part geometry to the right machine
A precision CNC machine service is a set of machines plus the tooling, programming and metrology around them. The machine count matters less than the mix. A shop with 127 high-precision CNC machines may still send your part to the wrong spindle if nobody reads the drawing.
Start with geometry. Prismatic parts with features on five faces belong on a simultaneous 5-axis machining center. Shafts, bushings and threaded bodies with a turned profile belong on a mill-turn center, which cuts both the diameter and the cross-features in one setup. Flat plates with pockets and holes on one face are cheaper on a 3-axis mill, and there is no reason to pay for more axes.
Size sets the floor. GreatLight runs a 4,000 mm maximum processing size with travels of 4,000 × 400 × 150 mm for long parts, and 750 × 1,150 × 550 mm or 600 × 600 × 600 mm for medium frames. Small, high-density work runs on 500 × 500 × 450 mm and 500 × 310 × 200 mm machines. A Ø400 mm rotary table covers parts that need angular features in one fixturing.
The wrong question is 'how many axes do you have'. The right question is 'which machine will you quote this part on, and why'. If the answer is vague, the tolerance will be too.
- 15-axisFive faces, compound angles, undercuts, tight true position on multiple datums.
- 2Mill-turnRound parts with milled flats, slots or cross-holes that must stay concentric.
- 33-axisSingle-face plates, brackets and housings where one setup is enough.
- 4Large travelFrame rails, manifolds and tooling plates up to 4,000 mm long.
Tolerance, surface finish and what drives cost
±0.005 mm (±0.0002 in) is achievable on the right machine with the right fixture, but it is not a blanket tolerance for every dimension on the print. Cost climbs sharply when a tight callout sits on a flexible wall, a deep bore, or a feature far from the datum. A good shop will flag those features during DFM and suggest a datum or a thicker wall instead of quietly adding hours.
Surface finish is the second cost lever. As-machined surfaces sit around Ra 1.6–3.2 μm. A high-finish requirement of Ra 0.8–1.6 μm is common for sealing faces and sliding contacts. Fine finishing at Ra 0.2–0.8 μm is reserved for optical, medical and sealing-critical surfaces, and it usually means separate finishing passes and slower feed rates.
Material behavior matters as much as the spec. Aluminium 6061-T6 and 7075 machine fast and hold tolerance well. Stainless 316L and 17-4PH work-harden, so light radial cuts and sharp tooling keep the dimension stable. Titanium TC4 (Ti-6Al-4V) and Inconel move heat into the tool, which pushes cost up through tool wear rather than machine time.
Ask how the shop plans to hold the tight feature, not just whether it can. The answer tells you if the quote is built on a process or on hope.
Inspection and documentation you should request
Inspection is where a precision CNC machine service earns the word precision. GreatLight inspects 100% of parts before shipment, covering raw material check, in-process monitoring and final inspection. Reports are available on request. That sequence matters: material certification catches a bad heat before machining, in-process checks catch drift before the last operation, and final inspection protects the shipment.
For a first order, ask for a first-article inspection report on the tightest three to five features. On recurring orders, ask for a sampling plan and a copy of the in-process record. If a shop cannot describe its inspection flow in one sentence, the measurement is probably happening after the fact.
The reporting format should match your receiving process. A CMM report against the CAD model is useful when you need true position and profile. A hand-tool record is fine for linear dimensions on simple parts. Sending the wrong report wastes both sides' time.
Certifications support the paperwork but do not replace it. GreatLight holds ISO 9001:2015, IATF 16949:2016, ISO 13485:2016 and ISO 27001:2022. Pick the one that matches your industry: IATF 16949 for automotive, ISO 13485 for medical devices, ISO 27001 when design files and IP need a controlled environment.
Order quantity, lead time and how quotes should be structured
Many engineers assume a prototype shop and a production shop are different suppliers. That split creates a second qualification cycle and a tolerance shift between the sample and the run. GreatLight quotes from one prototype to 10,000+ part runs with no minimum order quantity, so the process that made the sample is the process that makes the batch.
Lead time has two clocks. Quotation and free DFM analysis come back within 12 hours, and production can start within 24 hours of approval. Parts ship in 3–5 days for standard work. The historical late-delivery probability is below 2%. Ask any supplier to separate quote time from ship time; a single blended number hides the bottleneck.
Read the quote structure. A comparable quote lists material, machining, finishing, inspection and any tooling separately. A single lump sum makes it impossible to see what changes when you relax a tolerance or switch a finish.
Confidentiality is a commercial term too. Uploads are handled as secure and confidential, and an NDA is available on request. Sign it before the first STEP file moves if the design is sensitive.
Common mistakes when selecting a supplier
The most expensive mistake is comparing quotes that were built on different assumptions. One shop assumes as-machined finish; another assumes Ra 0.8–1.6 μm and a CMM report. The second quote looks high until you add the missing operations to the first one.
The second mistake is sending a print with no datum strategy. If the drawing calls for a tight position on a hole pattern but the datum is a rough cast surface, every shop will interpret it differently. Fix the datum before you request quotes.
The third mistake is ignoring material availability. A 7075 or 17-4PH grade may need to be ordered in, and that delay lands on your schedule, not the shop's. Ask what is in stock for your alloy.
The fourth is treating finishing as an afterthought. Anodizing, plating, powder coating and laser marking add handling and can shift a dimension if the masking plan is wrong. Laser marking has a minimum character height of 1.5 mm; plan the marking area accordingly.
- 1Mixed assumptionsConfirm finish, inspection and packaging in writing before comparing price.
- 2Weak datumsA tight callout on a rough surface will be quoted differently by every shop.
- 3Alloy availabilityAsk what is stocked before you commit to a delivery date.
- 4Masking plansPlating and anodizing need masked threads and bores specified on the print.
Step by step: qualifying a precision CNC machine service
- 1Send a drawing with datums markedInclude the CAD model, the 2D print and a note on which features are functional. Mark the tight callouts yourself so the DFM reply addresses them.
- 2Request DFM feedback with the quoteA useful reply flags thin walls, deep pockets, non-standard thread sizes and any tolerance that forces a second setup. Expect it within 12 hours.
- 3Ask which machine will run the partGet a specific machine class: 5-axis, mill-turn, 3-axis or large-travel. Match it against your geometry and size.
- 4Confirm the inspection planState whether you need first-article only or 100% inspection, and which features need CMM reports. Agree on the report format.
- 5Place a small first orderOne to five pieces proves the setup, the finish and the paperwork. Check the tight features against the report before releasing a larger run.
- 6Lock the finish and marking specName the finish, the color, the masking zones and the marking height. Laser marking minimum character height is 1.5 mm.
- 7Set the confidentiality termsSign the NDA before releasing sensitive files, and confirm where uploads are stored.
Questions buyers ask us
Can you hold ±0.005 mm on every dimension?
No shop should promise that on every callout. ±0.005 mm is realistic on rigid features with a clean datum and the right machine. It becomes expensive or impossible on thin walls, deep bores and long unsupported sections.
Send the print and we will tell you which features hold that tolerance and which ones need a design change or a wider tolerance.
Is there a minimum order quantity?
No minimum order quantity. We quote from one prototype to 10,000+ part runs on the same floor.
That means the process used for your sample is the process used for production, so the dimensions do not shift between the two.
How fast can I get a quote and parts?
Quotation and free DFM analysis come back within 12 hours. Production can start within 24 hours of approval, and standard parts ship in 3–5 days.
The historical late-delivery probability is below 2%. Complex finishing or special alloys may add time, and we will say so in the quote.
Which materials do you machine most often?
Aluminium 6061, 6061-T6, 7075 and 2024; stainless 303, 304, 316L and 17-4PH; steels 1018, 1045 and 4140; copper and brass grades such as C36000; titanium TC4 (Ti-6Al-4V) and Inconel; and engineering plastics including POM, PEEK and ABS.
Tell us the alloy on the print. If a substitute is acceptable, we can often shorten the schedule.
What certifications do you hold?
ISO 9001:2015, IATF 16949:2016, ISO 13485:2016 and ISO 27001:2022.
Match the certificate to your industry: IATF 16949 for automotive and EV work, ISO 13485 for medical devices, ISO 27001 when design files need a controlled environment.
How do you handle my design files?
Uploads are handled as secure and confidential, and an NDA is available on request.
Sign the agreement before the first file moves if the design is sensitive. We will not share drawings or models with third parties.
Send your drawing and get a comparable quote
Upload the CAD model and 2D print. You get a quote with free DFM analysis within 12 hours, plus a clear note on which machine runs the part and how it will be inspected.
12-hour quoteNo MOQ100% inspectionNDA on request
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