Buyer's guide

CNC Machine Service Machining: How to Choose a Supplier

This guide is for engineers and sourcing managers comparing cnc machine service machining quotes. It covers seven checks that decide whether a supplier can actually hold your tolerance, hit your date and keep your drawings confidential. Read it before you release a PO.

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±0.005 mm toleranceNo MOQ12-hour quoteISO 9001 / IATF 16949

On this page

1
Seven essential checks before you send a PO
2
Which supplier profile fits your part
3
Tolerance and machine capability
4
Lead time, MOQ and quote clarity
5
Certifications and inspection scope
6
Confidentiality, materials and finishing in house
7
How to run the supplier check in one week

Key takeaways

Seven essential checks before you send a PO

Tolerance is a capability, not a wishAsk which machine holds ±0.005 mm and on what part size. A number without a machine and a size means nothing.

Quote speed tells you about engineering depthA quote with a DFM note inside 12 hours usually means the shop read your model before pricing it.

MOQ is where small programs dieIf the supplier cannot run one prototype, you will never find out how they handle a 10,000-part run.

Certificates must match your industryISO 9001 covers general work. IATF 16949 and ISO 13485 are the ones that matter for auto and medical.

Inspection scope is a line itemAsk what is measured, on which parts, and whether you get the report. Vague answers cost money later.

Decision table

Which supplier profile fits your part

Match the part to the shop before you compare price.

Part situation	What to require	What to avoid
Prototype, 1-20 pcs	No MOQ, DFM feedback, 3-5 day ship	Shops quoting only at 500 pcs
Tight tolerance ±0.005 mm	Named 5-axis machine and size range	Blanket tolerance claims
Large frame up to 4,000 mm	4,000 × 400 × 150 mm travel	Shops that outsource the big mill
Automotive production	IATF 16949:2016 in scope	General ISO 9001 only
Medical device parts	ISO 13485:2016, traceable material	No material certs on request
Confidential design	NDA before upload, ISO 27001:2022	Public file-transfer links
Mixed material batch	Aluminium, stainless, titanium in house	Single-material job shops

The short version

Pick the supplier whose machine, certificate scope and inspection plan match your part, not the one with the lowest unit price on a lump-sum quote. If a shop cannot answer the tolerance and inspection questions before the PO, it will not answer them after it.

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Check 1-2

Tolerance and machine capability

Every supplier page claims tight tolerance. The useful question is narrower: which machine, at what part size, and repeatably? A shop can hit ±0.005 mm on a 50 mm aluminium bracket and miss it by a factor of three on a 900 mm steel plate, because thermal growth and tool deflection scale with size. Ask for the machine model and the work envelope it runs in.

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. The largest travel is 4,000 × 400 × 150 mm, with medium envelopes at 750 × 1,150 × 550 mm and compact cells at 500 × 500 × 450 mm. That spread matters: the right answer for your part is usually a specific cell, not the whole factory.

Material changes the answer too. Titanium TC4 (Ti-6Al-4V) and Inconel push heat into the tool and the part, so the same nominal tolerance needs slower speeds and more passes. Aluminium 6061-T6 and 7075 cut clean and hold size easily. If a quote does not mention material-specific feeds and speeds, the tolerance claim is probably copied from a brochure.

Surface finish is the second half of the capability question. As-machined parts land around Ra 1.6–3.2 μm. A fine finish at Ra 0.2–0.8 μm needs a separate finishing pass, sometimes a different tool, and always more time. Decide which surfaces actually need it before you ask for it everywhere.

1
Ask for machine and envelopeTolerance without a size range is not a commitment.
2
Name the critical surfacesFinish on every face raises cost for no functional gain.
3
Test with one hard featureA single tight bore or flatness callout reveals real capability.

Check 3-4

Lead time, MOQ and quote clarity

Lead time breaks into three parts: quoting, production start, and shipping. A shop that quotes in 12 hours with a free DFM analysis and starts production within 24 hours is telling you its estimating and programming teams are not queued. Historical late-delivery probability below 2% is a useful number only when the shop can say how it is measured. Ask what counts as late.

MOQ is the quiet filter. No minimum order quantity means one prototype and a 10,000+ part run use the same intake process. That is unusual, and it is the fastest way to find out how a supplier treats small work. A shop that only wants volume will price your prototype high to make it go away.

Read the quote structure, not just the total. Line items for material, machining, finishing, inspection and freight tell you where cost sits. A single lump sum hides the assumption that will bite you: material grade, quantity break, or whether deburring is included. Ask for the unit price at 1, 100 and 1,000 pieces.

Parts ship in 3–5 days on standard programs. That window assumes the drawing is released and the material is in stock. If your alloy is 17-4PH or a titanium grade, confirm stock before you accept the date. Most schedule slips start with material, not with the spindle.

1
Separate the three clocksQuote, production start and ship date are different promises.
2
Get three quantity breaksUnit price at 1, 100 and 1,000 shows the real cost curve.
3
Confirm material stockExotic alloys move the date more than machine time does.

Check 5-6

Certifications and inspection scope

Certificates are only useful when the scope matches your part. ISO 9001:2015 is the baseline for general machining. IATF 16949:2016 applies to automotive and engine hardware. ISO 13485:2016 is the one medical device buyers look for. ISO 27001:2022 covers information security, which matters when you upload proprietary models. GreatLight holds all four.

Ask to see the certificate scope, not just the logo. A certificate can cover one plant, one process, or one product family. If your part is made in a different building, the certificate may not apply. This is a two-minute check that prevents a long conversation during your audit.

Inspection is where tolerance claims get verified. The baseline is 100% inspection before shipment, with raw material check, in-process monitoring and final inspection. Reports come on request. For a first article, ask for dimensional data on the features you flagged, not a general pass note.

For regulated work, request material certificates and heat lot numbers with the shipment. Traceability is not optional in medical and aerospace programs, and it is much cheaper to set up before the run than to reconstruct after it.

1
Match cert to industryAuto needs IATF 16949, medical needs ISO 13485.
2
Check the scope lineA certificate for one plant does not cover another.
3
Define the first-article reportList the features you want measured, with nominal and actual.

Check 7

Confidentiality, materials and finishing in house

Your drawings are the asset. Uploads should be secure and confidential, with an NDA available on request and an information security management system behind it. If a supplier sends you a public file-sharing link for a customer's model, that tells you how yours will travel.

Material range is a practical filter. A shop that stocks aluminium 6061, 6061-T6, 2024, 5052, 5083, 6063, 6082, 7075 and ADC12, plus stainless 303, 304, 316, 316L, 420, 430, 431, 440C and 17-4PH, can quote most industrial work without a broker in the middle. Steel grades 1018, 1045, 4130, 4140, 4340 and A36 cover the common structural parts.

Finishing in house shortens the loop. Anodizing in clear, colour, hardcoat and conductive, electroless nickel, zinc, silver and gold plating, powder coating, black oxide, bead blasting, tumbling, brushing and polishing, plus laser marking with a minimum character height of 1.5 mm. When finishing is subcontracted, the schedule risk moves outside the shop and the shop cannot control it.

One caution: more capability is not always better. A shop with a narrow material and process focus often beats a generalist on a specific part. Use the range to check fit, not to be impressed.

1
Put the NDA firstSign before the model upload, not after the quote.
2
Keep finishing closeIn-house anodizing and plating remove a schedule handoff.
3
Prefer fit over breadthA focused shop usually holds tolerance better on its core work.

Step by step

How to run the supplier check in one week

Seven steps, in order. Each one is designed to eliminate a supplier before you spend engineering time.

1
Send the same RFQ to every candidateOne 3D model, one 2D drawing, one quantity list (1, 100, 1,000 pcs). Note the exact send time so you can measure the reply.
2
Time the first responseA quote and DFM note within 12 hours is the target. Slower replies are not automatically bad, but ask what caused the delay.
3
Check the tolerance line against the partIf the drawing calls ±0.005 mm on a 600 mm feature, ask which machine holds it. Reject blanket tolerance statements.
4
Confirm MOQ and quantity breaksLook for no minimum order quantity and unit prices at three quantities. A flat price at all quantities usually hides a setup charge.
5
Verify certificate scope, not the logoRequest ISO 9001:2015, IATF 16949:2016, ISO 13485:2016 or ISO 27001:2022 as applicable, and read which plant and process the scope covers.
6
Define inspection before the runList critical features, tolerance, gauge and report format. Ask for 100% inspection before shipment and data on your flagged features.
7
Sign the NDA, then release the POConfidentiality first, then production. Confirm the ship window in writing and keep the material certificate with the shipment.

FAQs

Questions buyers ask before the first order

What tolerance can cnc machine service machining actually hold?

On the right machine and part size, ±0.005 mm (±0.0002 in) is achievable, and that is the figure we quote for suitable work. The qualifier matters: small features in aluminium or brass are easier than long steel parts, where thermal growth and tool deflection add error.

Send the drawing with the critical features marked. We will tell you which cell runs it and whether the tolerance is realistic before you commit.

Do you have a minimum order quantity?

No minimum order quantity. The same intake handles one prototype and a 10,000+ part run. Prototypes typically ship in 3–5 days, and production can start within 24 hours of a released drawing.

Small runs are useful for us too. They show how a design behaves before tooling and volume pricing get involved.

Which certifications apply to my program?

ISO 9001:2015 is the general quality baseline. IATF 16949:2016 applies to automotive and engine hardware. ISO 13485:2016 applies to medical devices. ISO 27001:2022 covers information security for uploaded designs.

Ask which scope line covers your part and which plant will run it. A certificate for one building does not automatically cover another.

How is the quote structured, and how fast is it?

Quotation and free DFM analysis come back within 12 hours. The quote breaks out material, machining, finishing, inspection and freight where they apply, with unit prices at low, mid and high quantity.

If a callout is hard to machine or the tolerance is tighter than the function needs, we say so in the DFM note and suggest a change.

Can you keep our design confidential?

Uploads are secure and confidential, and an NDA is available on request. Sign it before you send the model if your program requires it. Information security practices follow ISO 27001:2022.

We do not reuse customer designs or show them to other clients.

What part sizes and materials can you run?

Maximum processing size is 4,000 mm, with a 4,000 × 400 × 150 mm travel on the large cell. Materials include aluminium 6061-T6, 7075 and ADC12, stainless 304, 316L and 17-4PH, steel 4140 and 4340, titanium TC4, Inconel, copper alloys and engineering plastics.

Finishing such as anodizing, plating, powder coating and laser marking is done in house, which keeps the schedule inside one shop.