CNC Processing Factory: 7 Checks Before You Order
This guide is for engineers and sourcing teams comparing CNC processing factory quotes. It covers the checks that actually predict part quality: tolerance capability, machine mix, certification fit, MOQ, quoting detail and inspection paperwork. Read it before you send a purchase order.
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Key takeaways
What to check on a CNC processing factory quote
Use this table as a scoring sheet when two or three quotes land on your desk.
| Check | What good looks like | Red flag |
|---|---|---|
| Tolerance | Stated per feature, with a measurement method | One blanket ±0.01 mm note on every drawing |
| Machine list | Axis count and travels match your part size | No answer on which machine will run the job |
| Lead time | Quote, first cut and ship dates in writing | Vague '2–4 weeks' with no start date |
| MOQ | One piece accepted, price breaks listed | Minimum quantity raised after RFQ |
| Certification | Certificate scope covers your product type | Certificate number never shared |
| Quoting detail | Material, setup, run time, finishing split out | Single lump-sum price only |
| Inspection | Reports named before the order starts | Inspection sold as an extra after shipping |
Pick the factory that answers process questions
A CNC processing factory worth keeping answers machine, setup, tolerance and inspection questions in specific terms. If the replies stay general, the parts will too.
Tolerance: what a CNC processing factory can hold, and when it cannot
Tolerance is the first filter. A shop that claims ±0.005 mm on every feature is telling you very little. The real question is which features can hold that band on your part geometry, and how the shop will measure them. On a 20 mm aluminum bracket with a single bore, ±0.005 mm is routine. On a 900 mm steel weldment with thin walls, the same number is a different problem.
Machine choice drives the answer. A three-axis mill with a rigid vise can hold tight bores and flat faces when the part has few setups. Add a fourth face and the error stack grows with every re-clamp. A simultaneous five-axis center cuts the part in fewer setups, so datums stay consistent across features. That is why shops quote five-axis time for parts that look simple on the print.
Thermal drift matters more than most drawings admit. Aluminum expands roughly 23 μm per meter per degree Celsius. A shop that machines a 500 mm aluminum part and measures it right off the machine, while the part is warm, can read a size that shrinks after cooling. Ask how long the part rests before final inspection.
Wall thickness and tool access set the practical floor. Features under 0.5 mm wall on aluminum tend to deflect during cutting, so the tolerance you can hold depends on support, not on the control resolution. For very thin ribs, expect the shop to rough, stress-relieve and finish in separate passes.
- 1Routine band±0.01 mm on stable features with good datum access.
- 2Tight band±0.005 mm on bores, slots and faces when the setup is rigid.
- 3Measure itConfirm the shop has CMM capacity and a stated measurement method.
Machine mix and part size: does the factory fit your geometry
A CNC processing factory is a collection of machines with different envelopes. The part envelope decides which machine runs your job, and that decision shows up in the price. Large work, up to 4,000 mm, needs a gantry-style machine with long travel. A 750 × 1,150 × 550 mm envelope covers most enclosures and housings. Compact parts under 500 mm can run on a fast three-axis mill with quick load and unload, which keeps the cycle cost low.
Axis count is the second half of the fit. Parts with features on four or five faces, deep pockets, or undercut geometry benefit from simultaneous five-axis work because the shop cuts more faces in one setup. Parts that are mostly flat with a handful of drilled holes do not need five-axis time at all. Paying five-axis rates for a plate with six holes is a common sourcing mistake.
Turning and mill-turn capability matter for round parts. A shaft with a cross-drilled hole and a milled flat is a mill-turn job, not a lathe job followed by a second setup on a mill. Every extra setup adds a datum shift. Ask the shop to name the machine family and the number of setups in the quote.
Tooling and fixturing are part of the fit too. Deep pockets need long-reach tooling, which deflects more and usually forces lighter cuts and slower feed. If your part has a 6:1 depth-to-diameter pocket, expect the shop to quote more run time and to flag it during DFM review.
- 1Large parts4,000 mm travel for long frames and rails.
- 2Mid-size parts750 × 1,150 × 550 mm covers most housings.
- 3Round partsMill-turn centers cut shafts with cross features in one setup.
Lead time, MOQ and quotingscope — no, keep it English
Lead time is a process number, not a sales number. A shop that quotes and returns a DFM analysis within 12 hours has already reviewed your geometry. Production can start within 24 hours when material is in stock and the setup is defined. Parts typically ship in 3–5 days for standard work. Treat a quote that gives no start date as an unknown.
MOQ is the second commercial filter. Some factories will not open a job for fewer than 100 pieces because the setup cost cannot be recovered. A factory that accepts one prototype and later runs 10,000+ pieces removes a second supplier qualification step from your project. That matters most for products still in design review, where the geometry changes between builds.
Quoting structure decides how comparable two prices really are. Ask for material cost, setup hours, run hours, finishing and inspection as separate lines. A single lump-sum price hides which part of the job is driving cost. When one quote is 30% cheaper, the breakdown usually shows a different material grade, a looser finish, or inspection excluded.
Payment and shipping terms belong in the same conversation. Confirm who owns the freight, what Incoterm applies, and what happens if a part fails inspection. Get the answer in writing before the first chip is cut.
- 1Quote turnaroundQuotation and free DFM analysis within 12 hours.
- 2StartProduction can start within 24 hours.
- 3ShipStandard parts ship in 3–5 days.
Certifications: match the certificate to the industry
Certification is a scope question, not a logo question. ISO 9001:2015 covers a general quality management system and fits industrial machinery, robotics and electronics work. IATF 16949:2016 is the automotive and EV standard, and it brings traceability and change-control requirements that general work does not need. ISO 13485:2016 applies to medical devices and adds process validation and record retention rules.
ISO 27001:2022 is different in kind. It covers information security, which matters when you send CAD files, BOMs and CAD-linked drawings to an outside factory. If your drawings are commercially sensitive, ask how files are stored and who can open them. A factory that offers an NDA on request and keeps uploads confidential is easier to onboard.
Read the certificate scope. A site certificate covers the site, not every process. If finishing is subcontracted, the finishing house holds its own certificate, or it does not. For automotive parts, a plating or anodizing step done outside the certified scope can break your traceability chain.
Records are the practical side. For medical and automotive work, expect material certificates, process records and dimensional reports to be retained and retrievable by lot. Ask how long records are kept before you place the first order.
- 1General industrialISO 9001:2015 is the baseline.
- 2Automotive and EVIATF 16949:2016 with lot traceability.
- 3Medical devicesISO 13485:2016 plus process validation records.
- 4File securityISO 27001:2022 and an NDA on request.
Materials and finishing: where quotes quietly diverge
Material grade is the most common hidden difference between two quotes. Aluminum 6061-T6 and 7075 machine differently and cost differently. A quote that says 'aluminum' without a grade is not comparable to one that names 6061-T6. The same applies to stainless: 303 machines freely, 316L is tougher and galls more, and 17-4PH needs heat treatment to reach its final properties.
Titanium and nickel alloys raise the bar further. Ti-6Al-4V (TC4) and Inconel cut slowly, generate heat at the edge and wear tooling fast. Shops that run these regularly plan for lower feed rates and more tool changes. If your design uses Inconel for a bracket that sees no temperature load, the material may be costing you cycle time for no functional gain.
Finishing changes both price and lead time. Anodizing, electroless nickel, zinc plating, powder coating and black oxide all add a second process step, often off the machine. Bead blasting, tumbling and polishing change surface texture, which can move a critical dimension if the finish is thick. Masking a tight bore before plating is a detail worth confirming.
Surface roughness should be specified per face, not per drawing. As-machined surfaces land around Ra 1.6–3.2 μm. A high-quality finish of Ra 0.8–1.6 μm is common for sealing faces and sliding contact. Fine finishing down to Ra 0.2–0.8 μm is available but costs extra passes. Laser marking needs a minimum character height of 1.5 mm to stay readable.
- 1Name the grade6061-T6, 7075, 316L, 17-4PH — not just 'aluminum' or 'steel'.
- 2Specify finish per faceRa bands belong on the faces that need them.
- 3Mind plating thicknessThick coatings can close a tight bore.
7 steps to qualify a CNC processing factory
Work through these in order. Each step produces a document you can put in the supplier file.
- 1Send drawings and get a DFM replyUpload STEP and PDF files with a tolerance block. A useful factory returns manufacturability notes within 12 hours, flagging thin walls, deep pockets and datums it cannot reach.
- 2Confirm the tolerance per featureWalk through the tight features and ask which machine holds them and how they are measured. Expect a CMM report on the critical dimensions, not a general statement.
- 3Match the machine to the part envelopeGive the maximum part size and ask which machine family will run it. Check the axis count against the number of faces you need cut.
- 4Check the certificate scopeRequest the certificate number and confirm it covers your product type. For automotive or medical work, confirm that outside processes are also certified.
- 5Ask for a line-item quoteRequire material, setup, run time, finishing and inspection as separate lines. Compare quotes line by line, not on the total alone.
- 6Agree on inspection and paperworkDecide before the order what ships with the parts: material certs, dimensional report, first-article report. 100% inspection before shipment is the baseline to ask for.
- 7Run one part, then scaleOrder a single prototype first. Measure it against the drawing, then release the batch. No minimum order quantity makes this step practical.
Questions engineers ask before ordering
How do we know a factory can actually hold ±0.005 mm?
Ask which features on your part need that band, then ask how they are measured. A shop that answers with a specific machine, a specific setup and a CMM method is credible. A shop that repeats the number back without a method is not.
On stable geometry with good datum access, ±0.005 mm (about ±0.0002 in) is a normal capability. On thin walls, deep pockets or long parts, expect a conversation about which features are feasible.
What does a CNC processing factory need to quote quickly?
A 3D model in STEP format, a 2D drawing with the tolerance block, the material grade and the quantity. Add the finish spec and any inspection requirement.
With those files, a quotation and free DFM analysis can come back within 12 hours. Missing material grade or finish spec is the usual reason a quote stalls.
Is there a minimum order quantity for prototypes?
It depends on the factory. Some set a floor of 100 pieces because setup cost cannot be recovered below that. Others accept one piece and scale to 10,000+ part runs on the same process.
Ordering one prototype from the same factory that will run production avoids a second qualification and keeps datums and fixtures consistent between builds.
Which certifications should we require?
Match the certificate to your industry. ISO 9001:2015 is the general baseline. IATF 16949:2016 for automotive and EV. ISO 13485:2016 for medical devices. ISO 27001:2022 if you are concerned about how your design files are stored and accessed.
Always read the scope. A certificate covers the processes at that site. If finishing or heat treatment is subcontracted, confirm the subcontractor's certification separately.
How should surface finish be specified?
Per face, not per drawing. As-machined is typically Ra 1.6–3.2 μm. Sealing faces and sliding contacts often need Ra 0.8–1.6 μm. Fine finishing to Ra 0.2–0.8 μm is available but adds passes and cost.
If the part is plated or anodized, note that coating thickness can close a tight bore. Call out any feature that must be masked.
What inspection records should ship with the parts?
Ask for the report list before production starts. Typical options are a material certificate, a dimensional inspection report on critical features, and a first-article report for the first part of a run.
100% inspection before shipment is the baseline, covering raw material check, in-process monitoring and final inspection. Reports are available on request, so name the ones you need on the purchase order.
Send your drawings and get a process review
We return a quotation and free DFM analysis within 12 hours, with the machine, setup count and inspection plan named in the reply.
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.
