Buyer guide

Metal CNC Machine Services: 7 Checks Before You Send a PO

A supplier-by-supplier checklist for engineers and sourcing teams. It covers tolerance reality, inspection records, finishing, order size and the numbers a quote should always contain. Read it and you can score three vendors on the same page.

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±0.005 mmRa 0.2–0.8 μm4,000 mm travelNo MOQ

metal cnc machine services shop floor with machined parts

On this page

1
Key takeaways
2
How to Score Three Suppliers on One Page
3
Tolerance Claims: Read the Method, Not the Number
4
Inspection Records and What They Prove
5
Certifications: Match the Certificate to the Part
6
Order Size, Materials, and Finishing in One Place
7
Lead Time, Quote Content, and the Late-Delivery Question
8
Step by Step: Running a Fair Supplier Comparison

Quick version

Key takeaways

Tolerance needs a method, not a numberAsk which process holds ±0.005 mm, on which features, and how it is measured.

Inspection before shipment is the real filterRaw material check, in-process monitoring and final inspection, with reports on request.

Certification must match your industryISO 9001 for general work; IATF 16949, ISO 13485 and ISO 27001 for the tougher programs.

Small runs should not be penalizedA shop that runs one prototype and a 10,000-part lot on the same floor keeps pricing sane.

Vague quotes cost more laterMaterial grade, finish, tolerance class and inspection scope belong in writing.

Scoring sheet

How to Score Three Suppliers on One Page

Give each vendor a 1–5 score per row. A vendor that fails a critical row cannot be rescued by a strong score elsewhere.

Check	What to ask for	Good answer	Red flag
Tolerance method	Which process and which features?	Named machine, stated feature list	"We hold everything to ±0.005 mm"
Inspection evidence	Reports with the shipment?	Raw material, in-process, final	Final check only, no data
Certification fit	Which cert covers our part?	ISO 9001 plus sector cert	Certificate with no scope detail
Order size	Smallest and largest lot?	One prototype to 10,000+ parts	Setup fee added to one-offs
Lead time basis	Days from what event?	Days from drawing release	Quote with no start point
Quote detail	What is priced in writing?	Material grade, finish, tolerances	One lump-sum number
Finishing	In-house or subcontracted?	Stated clearly, with lead time	Finishing adds a hidden week
Data handling	NDA and file control?	NDA on request, ISO 27001	Files passed to third parties

Check 1

Tolerance Claims: Read the Method, Not the Number

Every supplier page says tight tolerance. The useful question is narrower: which machining process reaches ±0.005 mm, on which features, and how is it verified? A simultaneous 5-axis center holds position on a contoured surface differently from a 3-axis mill running the same wall in two setups.

Position tolerance and size tolerance are not the same thing. A bore can measure on size and still sit off-center. If your assembly stacks three parts, that positional error is what breaks the fit, not the diameter.

Surface finish interacts with tolerance too. A wall held to ±0.005 mm usually needs a finishing pass, and that pass changes the tool load. Roughing-only shops will tell you the number is fine and ship parts that fail the fit check.

Ask for the inspection method: CMM, optical comparator, gauge pins, height gauge. Then ask what happens when a feature lands at the edge of the band. A shop that reworks before shipping is cheaper than one that ships and argues.

1
5-axis, 16 centersContoured and angled features in one setup
2
Mill-turn, 16 centersTurning plus milling on one platform
3
Max part size 4,000 mmLong rails and frames fit one machine
4
Finish rangeRa 0.2–0.8 μm fine, Ra 1.6–3.2 μm as-machined

Check 2

Inspection Records and What They Prove

A certificate of conformance is a signature. An inspection report is data. For a first article, ask for dimensional results on the drawing's critical dimensions, not a single pass or fail stamp.

In-process monitoring matters more on long runs than on prototypes. On a 10,000-part run, tool wear drifts the last thousand parts away from the first. A shop that measures during the run catches drift before the pallet is finished.

Material traceability belongs in the same conversation. If the mill certificate does not name the heat and grade, you cannot prove your 17-4PH part is 17-4PH after anodizing or passivation changes the surface.

Final inspection before shipment is the last gate. GreatLight Metal inspects 100% of parts before shipment and can issue reports on request. That is a floor, not a selling point. Treat it as the minimum any candidate should meet.

1
Raw material checkGrade and heat verified against the PO
2
In-process monitoringDrift caught mid-run, not at final
3
Final inspection100% before shipment, reports on request
4
Qualification rate99.99% across inspected production

Check 3

Certifications: Match the Certificate to the Part

ISO 9001:2015 tells you the quality system is audited. It does not tell you the shop can machine your implant, your EV busbar, or your flight-control bracket. The sector certificate is the one that carries the specific requirements.

IATF 16949:2016 adds automotive process discipline: PPAP thinking, control plans, change control. ISO 13485:2016 brings medical device requirements around traceability and process validation. ISO 27001:2022 covers how your design files are stored and who can open them.

Ask which certificate covers the process that will actually run your part. A shop can hold four certificates and still subcontract your finishing to a supplier outside all four scopes.

If your program needs an NDA before files move, settle that first. GreatLight Metal offers an NDA on request, and uploads are handled as confidential. Get it signed before the RFQ, not after the PO.

Check 4

Order Size, Materials, and Finishing in One Place

Minimum order quantity is where small teams get squeezed. A shop with no MOQ can run one prototype and a 10,000-part production lot on the same floor without a separate price list for each. That matters when your design is still moving.

Material range tells you how many vendors you need. Aluminum 6061-T6, 7075 and ADC12; stainless 303, 316L, 17-4PH; steel 1018, 4140, 4340; titanium TC4; Inconel; magnesium AZ31B. A shop that stocks and machines all of these removes a sourcing step.

Finishing should be quoted with the part, not discovered later. Anodizing in clear, color, hardcoat and conductive; electroless nickel, zinc, silver and gold plating; powder coating and black oxide; bead blasting, tumbling, brushing and polishing.

Ask whether finishing is in-house or subcontracted, and how that affects the schedule. Outsourced anodizing can add several days to a job that was quoted at 3–5 days for machining alone.

1
Prototype to 10,000+ partsNo minimum order quantity
2
Laser markingMinimum character height 1.5 mm
3
Sheet metal and die castingSame vendor as the machined parts
4
3D printing and vacuum castingBridges before hard tooling

Check 5

Lead Time, Quote Content, and the Late-Delivery Question

Lead time without a start event is marketing. Ask for days counted from drawing release, and ask what happens when the drawing changes mid-run. A quote that starts counting from PO signature hides the DFM review window.

A usable quote names material grade, tolerance class, finish, inspection scope, quantity breaks and shipping terms. If one lump sum arrives with no breakdown, you cannot compare it to the next vendor and you cannot tell what changed when the price moves.

Historical performance is fair game. GreatLight Metal reports a historical late-delivery probability below 2%, and quotation with free DFM analysis inside 12 hours. Production can begin within 24 hours once the drawing is released.

Watch the DFM step. A supplier that machines exactly what you drew, including a wall that will chatter, is not helping. Free DFM feedback before cutting is where cost gets removed.

RFQ workflow

Step by Step: Running a Fair Supplier Comparison

Use the same inputs for every vendor. Differences in the answers are the signal.

1
Send one clean packageSTEP file, 2D drawing with GD&T, material grade, finish callout, quantity breaks. No verbal tolerance notes.
2
Request DFM feedback with the quoteAsk each vendor to flag thin walls, deep pockets and features needing 5-axis access. Free DFM analysis is a normal ask.
3
Score tolerance and inspection firstFill the scorecard rows on method and evidence before looking at price. A cheap part that fails fit is not cheap.
4
Match certifications to the programISO 9001:2015 as the base; add IATF 16949:2016, ISO 13485:2016 or ISO 27001:2022 only if your part needs them.
5
Confirm the finishing pathAnodizing, plating or powder coating: in-house or subcontracted, and how many days it adds.
6
Fix the lead-time start eventWrite "days from drawing release" into the PO. Confirm the 24-hour production start and the 3–5 day shipping window.
7
Agree on inspection outputDecide before the run which dimensions get reported and in what format. Reports on request, not on argument.
8
Sign the NDA before files moveIf the program is confidential, get the NDA signed at RFQ stage, not after tooling starts.

FAQs

Questions Buyers Ask Before the First Order

Can one supplier handle both the prototype and the production run?

Yes, and it usually saves money. A shop with no minimum order quantity can machine one piece for fit check and then run 10,000+ parts on the same process, so the prototype data carries into production.

The risk to watch is a process swap between stages. Ask whether the prototype and the production parts use the same machine class and fixture strategy.

Which tolerance should I put on the drawing?

Put the tolerance the assembly needs, feature by feature, not one blanket callout. A ±0.005 mm band on a cosmetic edge adds cost with no benefit.

Reserve the tight band for mating surfaces, bores and datums. Let non-critical surfaces run at Ra 1.6–3.2 μm as-machined.

How do I know the finishing will match the sample?

Ask for the finish specification in writing: anodizing type (clear, color, hardcoat, conductive), plating type, or coating thickness range. Color anodizing varies with alloy, so name the alloy too.

For laser marking, keep character height at 1.5 mm or above. Smaller text fills in and becomes unreadable after coating.

What should be in the first quote?

Material grade, quantity breaks, tolerance class, finish, inspection scope, lead time in days from drawing release, and shipping terms. If any of these are missing, the quote is not comparable.

Free DFM analysis inside 12 hours is a reasonable expectation at RFQ stage. It tells you whether the supplier read the drawing.

Is a 3–5 day shipping window realistic?

For standard machining on released drawings, yes. The clock depends on when production starts, which is why the start event belongs in the PO.

Add days for outsourced finishing, non-standard material stock, and any drawing change after release. Those are the three that move a date.

How is design data protected?

Ask for an NDA before the RFQ, and ask which certificate covers file handling. ISO 27001:2022 is the one written around information security.

Uploads should be treated as confidential, with access limited to the people quoting and machining the part.