Purchasing CNC Machining Service: 6 Essential Checks Before You Send a PO
This guide is for engineers and sourcing managers comparing machine shops. It lists the checks that change a part's cost and delivery: tolerance and finish, machine size, MOQ, certifications, inspection paperwork and how a shop quotes. Read it and you can sort three quotes in under an hour.
In this article
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Six things that decide whether a shop fits your part
What to compare across quotes
Use the same column order for every shop so the numbers line up.
| Check | What good looks like | Warning sign |
|---|---|---|
| Tolerance | ±0.005 mm stated with the machine that holds it | Tolerance promised in the email only |
| Work envelope | 4,000 mm max; Ø400 mm rotary table listed | No machine list, no travel figures |
| Surface finish | Ra 0.8–1.6 μm as a normal machined finish | Ra quoted with no measuring method |
| MOQ | One prototype to 10,000+ parts | Tooling charge before the first part |
| Certifications | ISO 9001, IATF 16949, ISO 13485, ISO 27001 | Certificate numbers refused |
| Inspection | 100% before shipment; reports on request | Sample-only inspection |
| Quote turnaround | Quote and DFM notes within 12 hours | Price after a week with no DFM notes |
Match the shop to the part, not to the price alone
If your part fits the work envelope and the tolerance is reachable in one or two setups, the deciding factors are inspection paperwork, certification scope and how fast the shop answers a technical question. Price follows those. Send the drawing and we will tell you which machine runs it.
Match the tolerance to the machine that can hold it
When you are purchasing CNC machining service, the first number to settle is tolerance. ±0.005 mm (±0.0002 in) is a working target for a well-set-up 3-axis or 4-axis machine. It is not a default. It depends on the material, the wall thickness, the reach of the tool and how the part is held.
A thin aluminum bracket and a 17-4PH stainless housing at the same tolerance are two different jobs. Aluminum moves with cutting heat. Stainless work-hardens and pushes back on the tool. Ask the shop which machine they plan to run and why. A short answer that names the machine is better than a long answer about quality systems.
Geometry matters more than the tolerance callout. Deep pockets, thin floors and holes on five faces usually need a 5-axis setup to avoid re-fixturing. Every extra setup adds stack-up error. If the shop can finish the part in one setup, the tolerance gets easier to hold.
Surface finish travels with tolerance. Ra 0.8–1.6 μm is a normal machined finish. Ra 0.2–0.8 μm needs slower passes and often a finishing tool change. Ra 1.6–3.2 μm is fine for brackets and covers. Tell the shop which surfaces actually matter instead of calling the whole part fine.
- 1One setup beats threeFewer fixtures means less stack-up error.
- 2Name the critical surfacesA finish callout on every face adds cost with no function.
- 3Ask for the machineA shop that names the machine has already planned the job.
Work envelope, order quantity and material availability
Work envelope is the fastest way to disqualify a shop, and the fastest way to save a week. GreatLight runs 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. Medium machines cover 750 × 1,150 × 550 mm and 600 × 600 × 600 mm. Compact machines cover 500 × 500 × 450 mm and 500 × 310 × 200 mm. There is a Ø400 mm rotary table for round parts.
If your part falls outside those boxes, say so in the RFQ. A shop that quietly subcontracts the job adds a middleman you did not plan for. If your part is long and thin, ask how they support it. A 4,000 mm part needs steady support or the middle will chatter.
Order quantity is the second filter. No minimum order quantity means you can buy one prototype and then 10,000+ parts from the same process. That matters when a design is not frozen. If a shop pushes tooling before the first article is measured, you are paying for a guess.
Material availability sets the real lead time. Common grades of 6061, 304 and 1018 sit on shelves. Inconel, TC4 titanium and PEEK often need to be ordered. Ask whether the material is in stock before you accept a ship date. A one-day material slip can push a two-day job into the next week.
- 1Send the bounding boxLength, width, height and the largest diameter is enough for a first screen.
- 2Buy one firstA single prototype proves the process before volume pricing matters.
- 3Ask about stockMaterial on hand is the difference between 3 days and 10.
Certifications and inspection reports that hold up in an audit
Certifications answer a narrow question: can this shop be audited? ISO 9001:2015 is the baseline for general machining. IATF 16949:2016 is what automotive and EV programs ask for. ISO 13485:2016 applies to medical devices. ISO 27001:2022 covers information security, which matters when you send CAD files across borders.
Do not stop at the logo. Ask for the certificate scope and the expiry date. A certificate that covers a different plant or a different process does not cover your part. If the shop cannot send a PDF the same day, that is a data point.
Inspection is the part buyers under-specify. 100% inspection before shipment should be the default, not an upgrade. A normal sequence is raw material check, in-process monitoring and final inspection. Reports are available on request. For a first article, ask for dimensional results on the callouts that matter, not a pass stamp.
Confidentiality belongs in the same conversation. Uploads should be secure and confidential, and an NDA should be available on request. If you are sending drawings for a product that is not public, sign the NDA before the RFQ, not after the order.
- 1Match the standard to the industryISO 9001 for general work, IATF 16949 for automotive, ISO 13485 for medical.
- 2Read the scopeA certificate for the wrong plant or process proves nothing.
- 3Fix inspection in the PO100% before shipment, reports on request.
How a quote should be built, and what a vague one hides
A useful quote has five lines: material, machining, finishing, inspection and freight. When one number appears with no breakdown, you cannot tell whether finishing is included. Anodizing, electroless nickel, powder coating and bead blasting are separate operations with separate lead times.
Finishing also has limits worth knowing. Laser marking has a minimum character height of 1.5 mm. Hardcoat anodizing changes dimensions on tight bores. Black oxide is fine on steel and wrong on aluminum. If the drawing calls for a finish that fights the tolerance, the shop should flag it in the DFM notes, not after the parts are coated.
Free DFM analysis within 12 hours is a reasonable expectation. A real DFM note points at a specific feature and suggests a change. Thin walls that will deflect, a corner radius smaller than the cutter, a tapped hole too close to an edge. If the feedback is only a price, you are buying a commodity, not a process.
Ask how the shop handles changes. A revision after the first article should be requoted in hours, not days, if the geometry change is small. Production can start within 24 hours once the drawing is frozen and material is on hand. Parts ship in 3–5 days on standard jobs.
- 1Demand five linesMaterial, machining, finishing, inspection, freight.
- 2Read the DFM notesSpecific feature feedback is the sign of a real review.
- 3Check the finishing lead timeCoating often takes longer than machining.
Step by step: from RFQ to parts on the dock
- 1Send a complete RFQ pack3D model, 2D drawing with GD&T, material grade, finish callout, quantity and target date. Missing finish callouts are the most common cause of a requote.
- 2Ask for DFM notes with the priceExpect the quote and free DFM analysis within 12 hours. Read the notes before the price. A feature change can move cost more than a shop choice.
- 3Confirm the machine and the setup countAsk which machine holds the tightest tolerance and how many setups the part needs. One 5-axis setup is usually cheaper than three 3-axis setups on complex geometry.
- 4Fix inspection in writingState 100% inspection before shipment and list the callouts you want measured on the first article. Ask for reports on request rather than assuming they ship.
- 5Approve the first articleCheck dimensions on the critical features and the finish on the mating surfaces. If a bore is tight, fix it now. Hardcoat anodizing will not make it looser.
- 6Release productionProduction can start within 24 hours of approval when material is on hand. Standard jobs ship in 3–5 days. Keep one revision of the drawing in the PO.
- 7Confirm finishing and packingAnodizing, plating and laser marking run on separate schedules. Ask for the marking layout before the run if character height is near 1.5 mm.
Questions buyers ask before the first order
What tolerance can a CNC shop actually hold on a normal job?
±0.005 mm (±0.0002 in) is a realistic target on a well-set-up machine. It is not automatic. Material, wall thickness and the number of setups all move the result.
If a feature needs tighter than that, say so on the drawing and ask how the shop will measure it. A tolerance nobody can inspect is not a tolerance.
Is there a minimum order quantity for a prototype?
At GreatLight there is no minimum order quantity. You can order one part for a fit check and then run 10,000+ pieces from the same process.
This matters when a design is not frozen. Buying one part first is cheaper than buying 500 and scrapping them.
Which certifications should an automotive or medical buyer look for?
Automotive and EV programs usually ask for IATF 16949:2016. Medical device work usually asks for ISO 13485:2016. ISO 9001:2015 is the general baseline, and ISO 27001:2022 covers information security.
Ask for the certificate scope and expiry date. A certificate for a different process or plant does not cover your parts.
How long does a quote and the first shipment take?
Quotation and free DFM analysis come back within 12 hours. Once the drawing is frozen and material is on hand, production can start within 24 hours.
Standard jobs ship in 3–5 days. Add time for plating, anodizing or laser marking, which run on their own schedules.
What should be in an inspection report?
At minimum, dimensional results on the callouts you flagged, the measurement method and the date. GreatLight inspects 100% before shipment and provides reports on request.
For a first article, ask for the critical features only. A full CMM report on every surface slows the first shipment without adding information.
How are drawings kept confidential?
Uploads are secure and confidential, and an NDA is available on request. Sign it before the RFQ if the product is not public.
ISO 27001:2022 covers the information security side, which is what most buyers want when files cross borders.
Send your drawing, get a quote and DFM notes in 12 hours
Upload the model and drawing, set the material and finish, and an engineer reviews the part before the price goes out. No minimum order quantity, from one prototype to 10,000+ parts.
12-hour quote100% inspectionNo MOQNDA on request
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