How to judge a leading CNC processing service

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  Key takeaways
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  What to check against what it tells you
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  Machine capability behind a leading CNC processing service
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  Tolerance and surface finish: where the claims break down
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  Lead time, MOQ and quotation terms
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  Certifications and finishes that match the end use
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  Confidentiality, uploads and NDAs
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  Capacity, plants and the red flags to walk away from
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  Step by step: qualifying a supplier in one week

A leading CNC processing service is only as good as the spindles it runs. Ask for the machine list in writing: how many 3-axis, 4-axis and simultaneous 5-axis centers, and what travel each one has. A shop with 127 high-precision machines spread across 16 simultaneous 5-axis centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers can route work without waiting on a single overloaded spindle.

Travel matters as much as count. GreatLight runs work up to 4,000 mm, with large-platform travel of 4,000 × 400 × 150 mm, medium platforms at 750 × 1,150 × 550 mm and 600 × 600 × 600 mm, and compact cells at 500 × 500 × 450 mm and 500 × 310 × 200 mm. If your part is 900 mm long and the shop's biggest mill is 600 mm, the quote will come back as a weldment or a subcontracted job.

The rotary table is the quiet spec most buyers miss. A Ø400 mm table with simultaneous 5-axis motion lets one setup cut five faces on a housing, which removes the re-fixturing error that shows up as a bore-to-bore misalignment. On a 3-axis machine the same part needs three or four setups, and each one adds stack-up.

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  Ask for travel, not just tonnagePart size decides which cell the job lands in.
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  Count simultaneous 5-axis centersIndexed 3+2 is not the same as full 5-axis contouring.
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  Check mill-turn capacityIt removes a second op on shaft-type parts.

Every shop advertises tight tolerance. The useful question is which tolerance holds across which feature and which material. GreatLight works to ±0.005 mm (±0.0002 in) on qualified features, and finish bands of Ra 0.2–0.8 μm for fine work, Ra 0.8–1.6 μm for general high-finish, and Ra 1.6–3.2 μm as-machined. Those bands are not interchangeable. A sealing face and a bracket mounting pad do not need the same callout.

Material changes the achievable result. Aluminium 6061 and 7075 hold tight tolerances with less tool wear. Stainless 316L and titanium TC4 (Ti-6Al-4V) move more under cutting heat, so a ±0.005 mm callout on a thin titanium wall needs a different strategy than the same callout on a solid aluminium block. A supplier that quotes both the same way has not read your drawing.

The 99.99% qualification rate quoted by shops is a process statistic, not a part guarantee. Ask what it counts. In our case it reflects raw material check, in-process monitoring and final inspection on every order, with 100% inspection before shipment.

One more trap: finish callouts that cannot be measured on the drawing. If you specify Ra 0.4 μm on an internal corner, the tool radius may make it unreachable. Say so on the RFQ and let the shop propose a workable band.

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  Tie tolerance to features±0.005 mm on datums, looser on clearance holes.
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  Match finish to functionRa 1.6–3.2 μm is fine for most non-sealing surfaces.
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  Flag thin walls earlyUnder 1 mm in titanium, expect to talk about support and stepdown.

Lead time is where most first orders go wrong. A workable benchmark is a quotation and free DFM analysis within 12 hours, production starting within 24 hours of approval, and parts shipping in 3–5 days. That is achievable on standard materials and mid-complexity parts. It is not achievable on a part that needs a custom fixture and a heat-treat cycle, and any shop promising it without asking about the fixture is guessing.

Historical late-delivery probability below 2% is the number to ask for, not an on-time percentage. On-time rates are easy to game by padding the promised date. A low late probability on a short quoted window is a harder claim to fake.

MOQ is the fastest way to tell who a shop is built for. No minimum order quantity, from one prototype to 10,000+ part runs, means the shop has a prototyping cell and a production cell that share the same inspection standard. A shop with a 500-piece minimum is telling you its setup cost structure does not tolerate small work.

Watch the quotation structure. A quote that lists machining only, then adds material, finishing and inspection as line items, will land higher than the headline number. Ask for a landed price per part at your quantity, including the finish you actually need.

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  Ask for the late-delivery probabilityIt is harder to manipulate than an on-time rate.
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  Confirm the DFM stepFree DFM analysis before production catches most rework.
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  Get a landed priceMaterial, machining, finish and inspection in one figure.

Certifications are a filter, not a badge. ISO 9001:2015 covers general quality management. Automotive work usually needs IATF 16949:2016. Medical device parts need ISO 13485:2016. Data-handling clauses such as ISO 27001:2022 matter if you are sending proprietary CAD to an overseas supplier. A shop holding all four has built the paperwork layer that regulated buyers audit.

Finishing is the second half of the part. A machined surface is rarely the final surface. Anodizing in clear, colour, hardcoat or conductive grades changes wear resistance and electrical contact. Electroless nickel, zinc, silver and gold plating handle corrosion and conductivity. Powder coating and black oxide cover appearance and mild protection. Bead blasting, tumbling, brushing and polishing set the cosmetic grade.

Laser marking is a small detail with a real limit: minimum character height 1.5 mm. If your traceability code is smaller than that, the mark will not read reliably and the shop should push back before production rather than after.

When comparing suppliers, list the finishes you need and ask which are done in-house. Subcontracted anodizing adds a week and a second quality record to chase.

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  Match the certificate to the industryIATF 16949 for automotive, ISO 13485 for medical.
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  Confirm finish is in-houseOutside processing adds days and a second supplier to audit.
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  Respect the 1.5 mm marking limitSmaller characters will not scan.

Your CAD file is the product. Before uploading, confirm how the shop stores it, who can open it and how long it is kept. Secure and confidential upload handling is the baseline. ISO 27001:2022 is the auditable version of that promise, and it is the certificate to ask for if you work in defence, medical or any sector with export-sensitive geometry.

An NDA on request is standard practice, not a favour. If a supplier hesitates to sign one before seeing the drawing, treat that as a signal about how they handle customer data in general.

This matters most on multi-supplier sourcing. If you are running the same part through two shops, the second shop's confidentiality standard is only as good as its weakest employee access control. Ask how project folders are permissioned.

For buyers in the UK and EU, data location is part of the conversation. Knowing which plant holds the file, and whether it crosses a border, is a legitimate question to put in the RFQ.

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  Ask for the NDA firstBefore the drawing leaves your server.
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  Check ISO 27001:2022It is the auditable data-security layer.
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  Ask where files are storedCross-border transfer should be a stated fact.

Capacity spread across three wholly-owned plants, 7,600 m² and 150 technicians is a different risk profile from a single rented unit. If one site goes down, the work moves rather than stops. Ask how many sites actually run your process, not how many the group owns.

GreatLight has been running since 2011, which is 15 years of production data on tool wear, fixture design and material behaviour. That history shows up in quoting speed and in how quickly a DFM concern is raised. It is also why the shop can take a one-off prototype and a 10,000-piece run through the same inspection gate.

Red flags worth acting on: a quote with no DFM feedback, a tolerance claim with no part-size qualifier, a lead time shorter than the finishing cycle, and a refusal to name the machine your part will run on. Any one of these usually means the enquiry went to a broker rather than a factory.

The final check is the sample. Before committing a production order, run a sample part through the supplier's process and measure it against the drawing. A sample center exists for exactly this step, and it is cheaper than a scrapped batch.

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  Count the sites that run your processGroup ownership is not the same as process redundancy.
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  Treat missing DFM as a red flagIt usually means the quote came from a reseller.
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  Run a sample before volumeMeasure it against the drawing, not the render.