CNC Precision Machining Service Factory: 7 Checks Before You Commit

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  Key takeaways
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  What to check, and what a good answer looks like
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  Tolerance: match the claim to the machine
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  Machine mix and capacity: count the spindles, not the brochures
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  Certifications: read the scope, not the logo
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  MOQ, lead time and how the quote is built
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  Materials, finishing and confidentiality
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  Step by step: qualifying a CNC precision machining service factory

Every supplier will tell you they hold tight tolerance. The useful question is which machine holds it, on which feature, and at what size. GreatLight states ±0.005 mm (±0.0002 in) as its working tolerance, and that number comes from the 16 simultaneous 5-axis machining centers and 16 mill-turn centers, not from the whole floor. If your part has a Ø20 mm bore and a flat face, a 3-axis mill can hold it. If it has an angled port, a compound surface and a true position callout across two datums, you need 5-axis or you need two setups with a fixture.

Size changes everything. A 4,000 mm long part behaves differently from a 50 mm fitting. Thermal drift over a long bed eats tolerance before the tool wears. Ask your supplier what their maximum processing size is and how they control temperature on long parts. GreatLight's floor covers travel of 4,000 × 400 × 150 mm for large work, 750 × 1,150 × 550 mm and 600 × 600 × 600 mm for medium work, and 500 × 500 × 450 mm plus 500 × 310 × 200 mm for compact parts. A Ø400 mm rotary table handles round features that would otherwise need a second operation.

Surface finish is the other half of tolerance. A bore can be dimensionally correct and still leak because the wall is Ra 3.2 μm. Where a seal or an O-ring sits, specify Ra 0.8–1.6 μm. For optical or sliding surfaces, Ra 0.2–0.8 μm is achievable but adds a finishing pass and inspection time. If you only need clearance, Ra 1.6–3.2 μm as-machined is enough and cheaper. Put the finish callout on the drawing; do not leave it to the shop's default.

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  Tight geometryAngled features and multi-datum callouts go to 5-axis.
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  Long partsVerify machine travel before you send the model.
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  Sealing facesSpecify Ra 0.8–1.6 μm where an O-ring sits.

A factory's machine list tells you what work it can keep in-house. Outsourced operations add a week and a quality handoff. GreatLight runs 127 high-precision CNC machines across three wholly-owned plants: 16 five-axis centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers, plus lathes, grinders, EDM, vacuum forming and SLM, SLA and SLS printers. The 7,600 m² floor and 150 technicians support that mix. When a shop has turning and milling under one roof, a shaft with a cross-hole does not travel between vendors.

Ask which machine will run your part, then ask what else is booked on it. Capacity is a scheduling question, not a spec-sheet question. A shop with 127 machines can absorb a 10,000-piece run and a one-off prototype in the same week. A shop with four machines cannot, no matter what tolerance it claims.

Prototype and production should not be separate suppliers if you can avoid it. The fixture and program written for the first article are the same assets that make the production run repeatable. Moving from a prototype shop to a mass-production shop means re-qualifying the process, and that is where dimensions drift.

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  In-house operationsFewer handoffs means fewer tolerance stacks.
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  SchedulingAsk what else is on the machine in your week.
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  One supplierPrototype and production share fixtures and programs.

Certificates are only useful if they cover the process you are buying. GreatLight holds ISO 9001:2015 for quality management, IATF 16949:2016 for automotive, ISO 13485:2016 for medical devices and ISO 27001:2022 for information security. The last one is the one buyers forget. If your drawings are the product, an information-security management system matters as much as a quality system.

Ask for the certificate scope and the expiry date. A certificate for a sales office does not cover a machine shop. A certificate that expired last year is a warning sign, not paperwork.

Inspection belongs in the same conversation. GreatLight inspects 100% of parts before shipment, with raw material checks, in-process monitoring and final inspection, and provides reports on request. Ask what report you get: a simple pass note, or dimensional data with the measured values. For a first article, request the measured values. That is the document that tells you whether the ±0.005 mm claim is real on your geometry.

A qualification rate of 99.99% is a useful number only if you know what counts as a defect. Ask how they define it. If a scratch on a cosmetic face counts, the number is meaningful. If only dimensional failures count, it says less about the parts you will receive.

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  ScopeConfirm the certificate covers the machine shop.
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  Data, not stampsRequest measured values on the first article.

Minimum order quantity is where a prototype project dies. GreatLight has no minimum order quantity and runs from one prototype to 10,000+ part runs. That matters because most hardware programs start with a single machined housing that has to fit a board and a connector before anyone commits to tooling. A factory that will not open a job for one piece is telling you its setup cost is fixed and high.

Lead time has three parts: quote, production start and shipping. GreatLight returns a quotation and a free DFM analysis within 12 hours, can start production within 24 hours, and ships parts in 3–5 days. Those are working numbers, not guarantees. The historical late-delivery probability is below 2%. Treat that as a track record to ask about, not a promise for your specific part.

The quote itself is a quality signal. A price with no DFM note means nobody opened your model. A useful quote flags thin walls, deep pockets, sharp internal corners, threads that need a specific tap, and surfaces that will need hand work. That feedback is free engineering time, and it usually saves more than the price difference between two shops.

Payment terms, shipping method and who handles customs are the last three lines to confirm. They rarely change the part, but they change the date it reaches your dock.

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  MOQOne piece is a valid order size for prototypes.
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  DFM noteA quote without feedback is incomplete.
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  LogisticsConfirm Incoterms before the run starts.

Material range tells you how many process windows a shop manages. GreatLight machines aluminium grades 6061, 6061-T6, 2024, 5052, 5083, 6063, 6082, 7075 and ADC12; stainless 303, 304, 316, 316L, 420, 430, 431, 440C and 17-4PH (SUS630); steels 1018, 1045, 4130, 4140, 4340, A36 and tool steel; copper and brass including C101, C103, C110, beryllium copper, C27400, C28000 and C36000; titanium TA1, TA2 and TC4 (Ti-6Al-4V); Inconel; magnesium AZ31B and AZ91D; and plastics ABS, PC, PMMA, POM, PA, PEEK, PP, HDPE and carbon fibre.

Not every grade machines the same way. 304 stainless work-hardens if the feed is too light, so a shop that only cuts aluminium will burn tools on it. Titanium needs coolant discipline and lower surface speed. PEEK needs sharp tools and controlled heat or it smears. Ask your supplier what they cut most often. A shop that runs stainless daily will give you a better price and a better part than one that treats it as an exception.

Finishing is usually the last thing specified and the first thing to cause a rework. GreatLight offers anodizing in clear, colour, hardcoat and conductive; electroless nickel, zinc, silver and gold plating; powder coating and black oxide; bead blasting, tumbling, brushing and polishing; and laser marking or engraving with a minimum character height of 1.5 mm. If your laser mark is a 0.8 mm date code, it will not hold. Change the mark or change the process.

Confidentiality is part of the service, not an add-on. Uploads are secure and confidential, and an NDA is available on request. If your program has an unannounced enclosure, sign the NDA before you send the STEP file, not after.

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  Grade familiarityMatch the alloy to the shop's daily work.
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  Laser markingMinimum character height is 1.5 mm.
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  NDA timingSign before the model leaves your network.