Buyer guide

Main CNC Machining Services: How to Pick the Right Supplier

This page is for engineers and sourcing leads who need to compare main CNC machining services before sending a PO. It covers the checks that actually predict part quality: tolerance capability, machine mix, certification scope, quote detail, and MOQ. Read it and you can shortlist two or three shops with reasons you can defend to your team.

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±0.005 mm tolerance16 five-axis centersNo minimum order quantityISO 9001 / IATF 16949

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Key takeaways

Tolerance is a floor, not a headlineAsk which machine and fixture hold ±0.005 mm, and on what part size. The number alone tells you nothing.

Machine mix decides part sizeA shop with 4,000 mm travel and Ø400 mm rotary tables handles parts a small 3-axis cell cannot.

Certification scope mattersISO 9001, IATF 16949, ISO 13485 and ISO 27001 cover different risks. Check which one applies to your program.

Quote speed reveals process depthA quote that arrives with a DFM note is worth more than a price list with no comments.

MOQ should be one partIf a supplier cannot run a single prototype, they will struggle with a 10,000-part run.

Decision table

Process Selection by Part Geometry

Match the geometry to the machine before you compare shop rates.

Part type	Process	Why it fits	Watch out for
Shafts, pins, bushings	CNC turning	Rotational symmetry, tight diameter control	Live tooling needed for cross holes
Prismatic housings	3-axis milling	Flat faces and pockets, low setup cost	Deep cavities need long reach tools
Parts with side features	4-axis milling	One setup, fewer fixture moves	Rotary table size limits part envelope
Complex contoured surfaces	5-axis machining	Tool reaches nearly any angle in one setup	Programming time is higher
Turned part with milled flats	Mill-turn	No second op, datums stay aligned	Not for very large diameters
Thin-wall or flexible parts	5-axis with light passes	Lower cutting force, less deflection	Chatter if fixturing is weak
Prototypes under 10 parts	3-axis or mill-turn	Fast setup, cheap tooling	Hand finishing may still be needed

Capability check

What Main CNC Machining Services Should Cover

A shop that advertises main CNC machining services is claiming it can take a drawing and return a finished metal or plastic part. That claim is broad. The useful question is which processes sit behind it. Milling covers prismatic work, turning covers round work, and mill-turn covers parts that need both without a second setup. If one of those is missing, your part gets subcontracted and you lose visibility on quality and timing.

Machine count is a rough proxy for capacity, but the mix matters more. A shop with 127 high-precision CNC machines but no five-axis centers will still quote your impeller, then farm it out. Ask directly: how many simultaneous 5-axis machining centers do you run? At GreatLight the answer is 16, plus 12 four-axis mills, 27 three-axis machines, and 16 mill-turn centers.

Size limits are the other hard filter. Maximum processing size here reaches 4,000 mm, with travels of 4,000 × 400 × 150 mm, 750 × 1,150 × 550 mm, and 600 × 600 × 600 mm across the larger machines. Smaller cells run 500 × 500 × 450 mm and 500 × 310 × 200 mm. If your part exceeds the travel, no amount of skill on the floor will fix it.

Materials decide the cutting strategy, not just the price. Aluminum 6061 and 7075 cut fast and hold tight tolerances. Stainless 316L and 17-4PH work-harden, so feeds and speeds need care. Titanium TC4 and Inconel push tool wear hard, and PEEK or carbon fibre bring their own problems. A supplier who lists all of them should be able to say which ones they run weekly.

1
Milling, turning, and mill-turn under one roofFewer handoffs, one set of datums.
2
Five-axis for contoured or angled featuresOne setup instead of three.
3
Travel limits checked against your drawingConfirm before quoting, not after.
4
Material experience you can verifyAsk what they run weekly, not what they list.

Tolerance and finish

Tolerance, Surface Finish, and What They Cost

±0.005 mm is achievable on the right machine with the right fixture. It is not achievable on every feature of every part. Datum features, bore diameters, and mating surfaces usually carry the tight tolerances. Cosmetic surfaces, clearance holes, and non-critical edges rarely need them. When a drawing puts ±0.005 mm on everything, the quote goes up and the shop starts making judgment calls you never approved.

Surface finish follows the same logic. As-machined surfaces land at Ra 1.6–3.2 μm. A high-finish pass gets you Ra 0.8–1.6 μm, and fine finishing reaches Ra 0.2–0.8 μm. Each step adds cycle time. If a sealing face needs Ra 0.4 μm, say so on that face only. Blanket finish callouts are the fastest way to overpay.

Inspection is where tolerance claims become verifiable. Ask what gets measured, on what equipment, and whether reports come with the shipment. A supplier running 100% inspection before shipment, with raw material checks, in-process monitoring, and final inspection, can back up the numbers. One that inspects a sample lot cannot.

Tight tolerance and fine finish also affect lead time. Adding a finishing pass or a stress-relief step moves the part to the back of the queue. Plan for it. The alternative is a part that measures well on the bench and drifts after assembly.

1
Put tight tolerance only where it functionsMating surfaces, bores, datums.
2
Specify finish per faceRa 0.8–1.6 μm on sealing faces only.
3
Ask for inspection reportsDimension, material, and finish data.
4
Budget time for finishing stepsThey are not free and not instant.

Certifications

Certifications and Which Ones Your Program Needs

ISO 9001:2015 is the baseline. It says the shop has a documented quality system and follows it. For general industrial parts and prototypes, that is usually enough. It does not say anything about automotive traceability or medical device controls.

IATF 16949:2016 adds automotive requirements: PPAP discipline, traceability, change control, and stricter process monitoring. If you make automotive or EV components, a supplier without it will need extra oversight from you, and your own auditor may object. ISO 13485:2016 is the medical device equivalent, covering validation, clean handling, and documentation that survives an FDA or notified-body review.

ISO 27001:2022 is different. It covers information security, not part quality. It matters when you send CAD files, drawings, and BOMs to an overseas supplier and need evidence that those files are protected. For defense-adjacent or IP-heavy programs, this is often the certification that gets the shop approved.

Ask for the certificate scope, not just the logo. A certificate that covers one plant does not cover a sister plant. Check the expiry date and the issuing body. Also ask who signs the NDA and how uploads are handled. GreatLight keeps uploads secure and confidential, with an NDA available on request.

1
ISO 9001:2015 for general workThe minimum bar.
2
IATF 16949:2016 for automotiveRequired for most Tier 1 and OEM programs.
3
ISO 13485:2016 for medical devicesValidation and documentation controls.
4
ISO 27001:2022 for data securityProtects your design files.

Lead time and MOQ

Lead Time, MOQ, and Quote Quality

Lead time promises are easy to make and hard to verify. Ask for the sequence, not the number. Quotation and DFM analysis within 12 hours is a front-end commitment. Production start within 24 hours applies once the PO and files are clean. Parts ship in 3–5 days for standard work. Anything outside that range should come with a reason.

MOQ is the quiet filter. Some shops will not quote below 100 pieces because setup cost eats the margin. If you are at prototype stage, that shop is not your supplier. GreatLight quotes from one prototype to 10,000+ part runs with no minimum order quantity. That matters most in the first three iterations of a design.

The quote itself tells you how the shop thinks. A number with no assumptions is a number you cannot compare. Look for material spec, tolerance callouts, finish spec, machine assignment, inspection plan, and shipping terms. A quote that flags a design issue and suggests a change is worth more than one that simply accepts the drawing.

Delivery reliability is a separate data point. Historical late-delivery probability below 2% is a track record, not a guarantee. Ask how the shop handles schedule slips when they happen. A supplier that tells you early and offers options is easier to work with than one that goes quiet.

1
Ask for the sequence, not the promiseQuote, DFM, production start, ship.
2
No MOQ helps early iterationsOne prototype through 10,000+ runs.
3
A quote should list assumptionsMaterial, tolerance, finish, inspection.
4
Ask about slip handlingEarly warning beats silence.

Sourcing process

Step by Step: Vetting a CNC Supplier

1
Send the full drawing setInclude 3D model, 2D drawing, material spec, finish callouts, and quantity range. Missing finish or material specs are the top cause of quote revisions.
2
Read the DFM notes firstBefore comparing price, read the comments. A shop that flags a 0.5 mm wall or an unreachable pocket is doing engineering work for you.
3
Confirm machine assignmentAsk which machine will run the part and whether the travel covers it. For 5-axis work, confirm simultaneous capability, not just 3+2 positioning.
4
Check tolerance per featureWalk through the tight tolerances and confirm they apply where you intended. Move ±0.005 mm callouts off cosmetic faces.
5
Ask for the inspection planRequest which dimensions get measured, on what equipment, and whether reports ship with the parts. 100% inspection should be the default for functional parts.
6
Verify certification scopeRequest the certificate number, scope, and expiry date. Match it to your industry: IATF 16949 for automotive, ISO 13485 for medical.
7
Place a small first orderRun one or a few parts before committing to a production volume. No minimum order quantity makes this practical.
8
Review the first articleMeasure the first part against the drawing before the run continues. Fixing a process after 500 parts is expensive.

FAQs

Frequently Asked Questions

What tolerance can main CNC machining services hold?

GreatLight works to ±0.005 mm (±0.0002 in) on critical features. That applies to dimensions where the machine, fixture, and material allow it.

Not every feature on a part needs that. Clearance holes and cosmetic surfaces usually run looser, and keeping tight callouts off them reduces cost and lead time.

Do I need to order a minimum quantity?

No. GreatLight has no minimum order quantity and quotes from one prototype to 10,000+ part runs.

This suits design teams that need three or four iterations before locking a revision. Each iteration can be quoted and run as a small batch.

Which certifications should I ask for?

ISO 9001:2015 is the baseline for general industrial work. Add IATF 16949:2016 for automotive and EV programs, ISO 13485:2016 for medical devices, and ISO 27001:2022 when design-file security matters.

Ask for the certificate scope and expiry date. A certificate held by one plant does not automatically cover another.

How fast can parts ship?

Quotation and free DFM analysis come back within 12 hours. Production can start within 24 hours once files and the PO are clear, and standard parts ship in 3–5 days.

Complex parts, exotic materials, and multi-step finishing extend that. Ask for a specific schedule with your quote.

What surface finishes are available?

Machined surfaces run Ra 1.6–3.2 μm, high-finish passes reach Ra 0.8–1.6 μm, and fine finishing reaches Ra 0.2–0.8 μm.

On top of that you can add anodizing, electroless nickel, zinc, silver or gold plating, powder coating, black oxide, bead blasting, tumbling, brushing, and polishing. Laser marking goes down to 1.5 mm character height.

How are my design files protected?

Uploads are handled as secure and confidential, and an NDA is available on request before files move. ISO 27001:2022 certification covers the information security side.

If your program has export-control or IP sensitivity, raise it at the quoting stage so the right controls are applied from the start.