Buyer guide

CNC Machine Service Supplier: 9 Checks Before You Order

This guide is for engineers and purchasing teams about to place a first order with a CNC machine service supplier. It covers the checks that actually change part quality and schedule: machine mix, tolerance capability, material range, certifications and quote terms. Read it before you send the RFQ, and you can tell in one call whether a shop fits your part.

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±0.005 mm toleranceNo minimum order quantityISO 9001, IATF 16949, ISO 13485Quote + DFM in 12 hours

cnc machine service supplier machining a precision part

On this page

1
Key takeaways
2
How a CNC machine service supplier compares by order type
3
Machine mix tells you what a CNC machine service supplier can actually hold
4
Tolerance and surface finish: what to demand from a CNC machine service supplier
5
Material range separates real capability from a catalog listing
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Certifications and order terms that protect your program
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How to qualify a CNC machine service supplier step by step

Quick answers

Key takeaways

Match machine mix to geometryA shop with 5-axis, mill-turn and EDM capacity handles undercuts and hardened steel without extra setups.

Ask for the tolerance they hold dailyA quoted best-case number means little. Daily capability of ±0.005 mm is what shows up on the inspection report.

Check the material list, not the sloganInconel, Ti-6Al-4V and PEEK need different speeds, feeds and tooling than 6061 aluminum.

Certificates must match your industryISO 9001 covers general work. IATF 16949 and ISO 13485 add the traceability that automotive and medical buyers audit.

Read the quote line by lineSetup, material, finishing and inspection should each appear. A single lump sum hides where cost can move.

Selection criteria

How a CNC machine service supplier compares by order type

Use this table to shortlist shops before you send drawings. Each row is a common buying situation.

Order type	What matters most	What to verify	Where it goes wrong
One-off prototype	Speed to first part	Quote and DFM within 12 hours	Shop only runs 10,000+ part batches
Bridge production	Repeatability across lots	In-process monitoring records	No inspection data between lots
Tight tolerance fits	Daily capability, not best case	Report showing ±0.005 mm	Tolerance quoted from a brochure
Exotic material parts	Tooling and feed knowledge	Prior Inconel or titanium jobs	Shop learns on your material
Automotive parts	Traceability and PPAP flow	IATF 16949:2016 certificate	General ISO shop with no lot trace
Medical components	Clean handling and records	ISO 13485:2016 certificate	No material certs per lot
Large frame parts	Travel and workholding	4,000 mm maximum processing size	Part split into welded sections
Cosmetic parts	Finishing control	Ra 0.8–1.6 μm in writing	Finish left to a third party

Capability

Machine mix tells you what a CNC machine service supplier can actually hold

A shop's machine list is the fastest way to judge fit. GreatLight runs 127 high-precision CNC machines across three wholly-owned plants, including 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers. That spread matters because a part with compound angles and an undercut cannot be finished on a three-axis machine without a second or third setup, and every extra setup adds positional error.

Travel limits decide whether the part is machined whole. The largest capacity reaches a 4,000 mm maximum processing size with 4,000 × 400 × 150 mm travel. Mid-size work runs in 750 × 1,150 × 550 mm or 600 × 600 × 600 mm envelopes. Compact parts sit in 500 × 500 × 450 mm or 500 × 310 × 200 mm work zones, with a Ø400 mm rotary table for parts that need indexing around a bore.

Ask which machine will run your part, not which machines the shop owns. A supplier with 16 five-axis centers may still schedule a simple bracket on a three-axis mill, which is fine and cheaper. The question is whether the shop can move the part to the right machine when the tolerance tightens. If the answer is always the same model, capacity is thinner than the brochure suggests.

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5-axisOne setup for angled faces, fewer datum shifts.
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Mill-turnShafts and housings finished without re-chucking.
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EDM and grindingHardened steel, sharp internal corners, micron-level fits.
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Rotary tableØ400 mm indexing for radial holes and slots.

Tolerance and finish

Tolerance and surface finish: what to demand from a CNC machine service supplier

Every shop can quote a tight number. Fewer can hold it across a run. Ask for the tolerance that appears on routine inspection reports, not the best result ever achieved. At GreatLight that figure is ±0.005 mm (±0.0002 in), and it is checked with raw material verification, in-process monitoring and final inspection before shipment. Reports are available on request.

Surface finish follows the same logic. As-machined faces typically land at Ra 1.6–3.2 μm. A high-quality finish for sealing or bearing surfaces sits at Ra 0.8–1.6 μm. Fine finishes reach Ra 0.2–0.8 μm and usually need a separate operation, slower feed rates or a finishing pass with a smaller tool. Tell the supplier which faces are functional and which are cosmetic, or you will pay for fine finish on surfaces that only needed to be flat.

Watch for tolerances that fight each other. A ±0.005 mm bore with a Ra 0.2 μm wall is harder than either callout alone, because the finishing pass changes the dimension slightly. Shops that handle this well will tell you the sequence up front: rough, semi-finish, measure, then finish. If the quote has no room for that sequence, the first article will drift.

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±0.005 mmDaily capability, verified on inspection reports.
2
Ra 0.8–1.6 μmTypical functional sealing and bearing surfaces.
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Ra 0.2–0.8 μmFine finish, separate operation and slower feeds.
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100% inspectionBefore shipment, with reports on request.

Materials

Material range separates real capability from a catalog listing

Any supplier will say they machine aluminum and steel. The useful question is which grades and how often. GreatLight works aluminum 6061, 6061-T6, 2024, 5052, 5083, 6063, 6082, 7075 and ADC12; stainless 303, 304, 316, 316L, 420, 430, 431, 440C and 17-4PH; and steel grades 1018, 1045, 4130, 4140, 4340, A36 and tool steel. Copper and brass cover C101, C103, C110, beryllium copper, C27400, C28000 and C36000.

The harder test is titanium and high-temperature alloys. TA1, TA2, TC4 (Ti-6Al-4V), Inconel and magnesium AZ31B or AZ91D behave nothing like aluminum. Titanium work-hardens at the cut and pulls heat into the tool, so speeds drop and coolant strategy changes. Inconel is worse on tool life. A shop that machines these weekly will quote realistic cycle times. A shop that machines them once a year will quote low and then ask for a tolerance waiver.

Plastics are their own trap. ABS, PC, PMMA, POM, PA, PEEK, PP, HDPE and carbon fibre all move differently after machining. PEEK and carbon fibre are abrasive, so tool wear shows in the last parts of a run. If your part is plastic and the tolerance is tight, ask how the shop controls temperature and clamping pressure. The answer tells you whether they have machined it before.

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Aluminum6061, 7075, 2024, 5052, 6082 and more.
2
Stainless303, 304, 316L, 17-4PH (SUS630).
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Titanium and InconelTC4 (Ti-6Al-4V), TA1, TA2, Inconel.
4
PlasticsPEEK, POM, PC, PA, carbon fibre.

Compliance

Certifications and order terms that protect your program

Certifications are not decoration. They define what records the supplier keeps and how it handles a nonconformance. GreatLight holds ISO 9001:2015 for general quality management, IATF 16949:2016 for automotive work, ISO 13485:2016 for medical devices and ISO 27001:2022 for information security. The last one matters more than buyers expect: your drawings and CAD files are intellectual property, and uploads should be handled as confidential with an NDA available on request.

Order terms decide whether the relationship scales. There is no minimum order quantity here, so a single prototype and a 10,000+ part run go through the same shop. Quotation and free DFM analysis come back within 12 hours, production can start within 24 hours, and parts typically ship in 3–5 days. Historical late-delivery probability is below 2%. Those numbers are worth asking about directly, because they tell you how the shop schedules.

Finishing is the last place programs slip. Anodizing, electroless nickel, zinc, silver and gold plating, powder coating, black oxide, bead blasting, tumbling, brushing and polishing are handled in one flow. Laser marking and engraving are available down to a minimum character height of 1.5 mm. If your supplier outsources finishing, add a week to the plan and expect the tolerance conversation to restart.

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ISO 9001:2015General quality management baseline.
2
IATF 16949:2016Automotive traceability and PPAP flow.
3
ISO 13485:2016Medical device process control.
4
ISO 27001:2022Information security for your drawings.

Workflow

How to qualify a CNC machine service supplier step by step

Nine steps, in the order that catches problems early.

1
Send the full drawing setInclude 2D prints with GD&T, a 3D STEP file, material grade and finish callouts. Missing GD&T is the most common cause of a wrong quote.
2
State the function of critical featuresMark which bores seal, which faces mate and which dimensions are reference only. This lets the shop choose the process instead of guessing.
3
Ask for a DFM responseA real supplier returns comments within 12 hours: thin walls, deep pockets, tool reach and any tolerance that will drive cost.
4
Confirm the machine and setup countAsk which machine will run the part and how many setups. More than two setups on a tight part is a warning sign.
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Request the tolerance capabilityAsk for the daily figure, not the best case. For precision work expect ±0.005 mm backed by inspection records.
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Check certifications against your industryISO 9001 for general work, IATF 16949 for automotive, ISO 13485 for medical. Verify the certificate scope, not just the logo.
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Test with one part before a batchOrder a single unit, inspect it, then release the run. This costs little and exposes setup and fixturing problems early.
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Agree on inspection and reportingDecide which dimensions get measured, at what frequency, and in what format. 100% inspection before shipment is the baseline.
9
Lock the finishing and marking specName the finish, the color code and the marking height. Laser marking needs at least 1.5 mm character height to stay legible.

FAQs

Frequently asked questions

What tolerance can a CNC machine service supplier hold on a production run?

For precision work, ±0.005 mm (±0.0002 in) is a realistic daily figure when the shop measures in process and inspects before shipment.

Tighter than that usually needs grinding, EDM or a temperature-controlled environment, and it should be discussed before quoting rather than after the first article.

Is there a minimum order quantity?

Not at GreatLight. A single prototype and a 10,000+ part run both go through the same process.

That matters for bridge production, where you need the same supplier to carry a design from first article into a small batch without a re-qualification.

How fast can a quote and first parts arrive?

Quotation and free DFM analysis come back within 12 hours. Production can start within 24 hours, and parts typically ship in 3–5 days.

Historical late-delivery probability is below 2%. Ask any supplier for their own figure and how they measure it.

Which certifications should I check for automotive or medical parts?

Automotive programs generally expect IATF 16949:2016, which covers traceability and nonconformance handling across lots.

Medical work looks for ISO 13485:2016. General industrial parts are usually fine with ISO 9001:2015. Check the scope statement on the certificate, not just the name.

Can a supplier handle my drawings confidentially?

Uploads should be treated as secure and confidential, with an NDA available on request.

ISO 27001:2022 is the certification that covers information security management, which is the one to ask about if your files are sensitive.

What surface finishes are available without a second vendor?

Anodizing in clear, color, hardcoat and conductive versions, plus electroless nickel, zinc, silver and gold plating.

Powder coating, black oxide, bead blasting, tumbling, brushing and polishing round out the list. Laser marking and engraving go down to 1.5 mm character height.