Buyer guide

CNC Processing Service Provider: 7 Checks Before You Order

This guide is for design engineers and sourcing teams comparing machine shops. It covers the checks that separate capable suppliers from expensive ones: tolerance and inspection, machine envelope, material coverage, certifications, MOQ, quote response, and the questions to ask before you release a PO.

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±0.005 mm toleranceNo MOQISO 9001 / IATF 16949Quote in 12 hours

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Key takeaways

Tolerance is a process claim, not a sloganAsk which machine and which inspection method holds a callout, not which number sounds best.

Machine envelope decides feasibilityA 4,000 mm part and a Ø400 mm rotary table job need different suppliers.

Certifications map to your industryISO 9001 for general work, IATF 16949 for automotive, ISO 13485 for medical.

Quote speed signals factory loadA DFM response inside 12 hours usually means engineering is not queued behind sales.

MOQ tells you who they serveSuppliers with no minimum can take one prototype and still run 10,000+ parts.

Judging criteria

What to compare across CNC processing service providers

Use the left column as your question list during the first call.

Check	Weak answer	Strong answer
Tolerance	±0.05 mm, no machine named	±0.005 mm on named 5-axis centers
Inspection	Visual check only	100% inspection, reports on request
Machine envelope	One or two 3-axis mills	Up to 4,000 mm and Ø400 mm rotary
Certifications	ISO 9001 claimed verbally	ISO 9001, IATF 16949, ISO 13485, ISO 27001
MOQ	1,000 pieces minimum	No minimum order quantity
Quote turnaround	5 to 10 working days	Quote and DFM within 12 hours
Material coverage	Aluminum and mild steel	Titanium, Inconel, PEEK, beryllium copper
Confidentiality	No paperwork offered	NDA on request, secure uploads

The short version

Pick the supplier whose machine envelope, inspection method and certifications match your part, then validate with one small order before you scale. Price comes last.

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Check 1-3

Tolerance, inspection and machine envelope

Tolerance is the first filter, but the number alone means little. A shop that claims ±0.005 mm should be able to name the machine, the setup and the inspection method behind it. Tight work usually runs on 5-axis centers with thermal compensation, then gets checked on a CMM rather than a caliper. Ask how they verify a position callout before you send the drawing.

Inspection is where cheap quotes hide their cost. A supplier that checks only the first article will ship a mixed lot sooner or later. Look for raw material verification, in-process monitoring and a final inspection before packing. Reports on request matter for aerospace, medical and automotive buyers who need traceability in their own quality file.

Machine envelope decides whether a part is feasible at all. GreatLight runs 127 high-precision CNC machines, including 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers. Maximum processing size reaches 4,000 mm, with travels of 4,000 × 400 × 150 mm, 750 × 1,150 × 550 mm, 600 × 600 × 600 mm, 500 × 500 × 450 mm and 500 × 310 × 200 mm, plus a Ø400 mm rotary table.

Fit the envelope before you argue about price. A 1,200 mm housing and a 30 mm connector body rarely belong on the same machine, and a supplier who quotes both from one work center is guessing. Ask which spindle takes your part and how many sides need repositioning.

1
Ask for the machine, not the spec sheetA model number tells you whether the tolerance is realistic.
2
Confirm the inspection stageFirst-article only is a risk flag for production runs.
3
Match part size to travelOversized parts force a second setup and add error.

Check 4-5

Materials, finishes and surface requirements

Material coverage is a practical test. Aluminum 6061, 7075, 2024 and 6082 are standard almost everywhere. Titanium TA1, TA2 and TC4 (Ti-6Al-4V), Inconel, magnesium AZ31B and beryllium copper separate the general shops from the ones with tooling and coolant strategy for hard alloys. Stainless grades such as 17-4PH, 316L and 440C also change cutting parameters, so ask whether they have run your grade before.

Plastic parts have their own rules. POM, PEEK, PC and ABS machine well but move with heat, so clamping and feed rates decide whether a thin wall stays straight. Carbon fibre needs dust control and diamond tooling. If your supplier treats plastics like aluminum, expect burrs, melted edges and dimensional drift on the second operation.

Surface finish should be specified as a range, not a feeling. As-machined finishes sit around Ra 1.6–3.2 μm, high-quality functional surfaces at Ra 0.8–1.6 μm, and fine finishes at Ra 0.2–0.8 μm. Each step down costs time and often needs a separate finishing operation, so only tighten the callout on faces that touch something.

Finishing options are worth confirming early. Anodizing in clear, colour, hardcoat or conductive types, electroless nickel and other plating, powder coating, black oxide, bead blasting, tumbling, brushing and polishing all change dimensions slightly. Laser marking needs a minimum character height of 1.5 mm to stay readable.

1
Hard alloys need a named processTitanium and Inconel cut differently from 6061.
2
Plastics reward the right clampThin walls move when the fixture is wrong.
3
Tighten finish only where neededRa 0.2–0.8 μm on every face raises cost fast.

Check 6-7

Certifications, MOQ and quote quality

Certifications tell you which industries a supplier is built for. ISO 9001:2015 covers general quality management. IATF 16949:2016 is the automotive and EV baseline. ISO 13485:2016 applies to medical devices, and ISO 27001:2022 covers information security, which matters if you share CAD files under NDA. A shop with all four has already been audited on documentation, not just on parts.

Minimum order quantity shapes the working relationship. Suppliers with no MOQ can take one prototype and still run 10,000+ parts later, which keeps the same process and the same inspection standard from first article to series production. If a shop pushes you to 1,000 pieces for a design you have not validated, the incentive is their setup time, not your program.

Quote quality is the last check and often the most revealing. A useful quote names the material grade, the machine, the finish, the inspection level and the assumptions behind the price. GreatLight returns a quotation and free DFM analysis within 12 hours, and production can start within 24 hours. Parts typically ship in 3–5 days.

Read the DFM notes carefully. Good feedback flags thin floors, deep pockets, unmarked tolerances and features that need a second setup. If the quote comes back as a single number with no comments, the shop is pricing a drawing it has not fully read.

1
Match certificates to your marketIATF 16949 for automotive, ISO 13485 for medical.
2
No MOQ keeps the same processOne prototype to 10,000+ parts on one route.
3
A quote without notes is a guessDFM comments show the shop read your drawing.

Sourcing workflow

Step by step: qualifying a CNC processing service provider

Run these seven steps in order before you release a purchase order.

1
Send the real drawing, not a simplified oneInclude material grade, tolerances, surface finish and any critical callouts. Mark which dimensions are functional. A stripped drawing produces a cheap quote that collapses at first article.
2
Ask for a DFM response with the priceGood feedback arrives with the number, not weeks later. Look for comments on thin walls, deep pockets, tool reach and setup count. GreatLight returns quote and free DFM analysis within 12 hours.
3
Confirm the machine and the setup countAsk which machine holds the tightest callout and how many setups the part needs. Each extra setup adds error and cost. A part quoted on a 5-axis center with one setup usually beats a four-setup 3-axis plan.
4
Check tolerance against inspection methodIf the drawing calls ±0.005 mm, ask what measures it. A CMM with a reported method is a different answer from a caliper on the bench.
5
Verify certifications and confidentialityRequest the certificate scope, not just the logo. If your files are sensitive, sign an NDA before uploading. GreatLight offers an NDA on request and keeps uploads secure.
6
Test with a small order firstOrder one or two parts before committing to a production run. With no minimum order quantity you can validate dimensions, finish and documentation without a large commitment.
7
Agree on the inspection report before shippingDecide which dimensions appear on the final report and who signs it. 100% inspection before shipment is the baseline. Reports are available on request, which keeps your own quality file complete.

FAQs

Frequently asked questions

What tolerance can a CNC processing service provider actually hold?

On well-setup 5-axis work with thermal compensation, ±0.005 mm (±0.0002 in) is achievable on critical features.

The number applies to specific dimensions, not the whole part. Faces that do not touch anything else can stay at ±0.05 mm and cost far less.

Do I need a supplier with no minimum order quantity?

It helps most during development. You can order a single prototype, check the fit, then scale to 10,000+ parts without changing supplier or process.

Suppliers with high MOQs often change machine and fixture between prototype and production, which resets your validation.

How fast should a quote come back?

For a standard part with a clear drawing, 12 hours is a reasonable benchmark for both price and DFM notes.

If a shop needs a week to respond, ask how long an engineering change will take once you are in production.

Which certifications matter for my industry?

ISO 9001:2015 is the general baseline. Automotive and EV work usually needs IATF 16949:2016. Medical devices need ISO 13485:2016.

ISO 27001:2022 covers information security and is worth checking when you share CAD under NDA.

Can one supplier handle both machining and finishing?

Often yes, and it saves shipping and scheduling time. Anodizing, plating, powder coating, black oxide, bead blasting, brushing and laser marking are commonly offered alongside machining.

Confirm that the finish callout is on the same drawing revision the machinist works from. Finishing changes dimensions by microns, so the tolerance stack needs to account for it.

What should I do if the first article is out of tolerance?

Ask for the inspection data and the setup sheet before discussing rework. The cause is usually fixture movement, tool wear or thermal drift, not the machine itself.

A supplier who can show the data and adjust the process is safer than one who simply remakes the part without explanation.