How to Choose a CNC Machined Service Supplier

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  Key takeaways
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  What to compare before you send an RFQ
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  Tolerance and machine capability
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  One-stop scope: machining plus finishing
• 5
  MOQ, prototype runs and volume
• 6
  Quote speed, lead time and hidden cost
• 7
  Inspection, documentation and data security
• 8
  Capacity and industry fit
• 9
  Six steps to qualify a CNC machined service supplier

A tolerance number on a website means little until you know which machine holds it. GreatLight quotes ±0.005 mm (±0.0002 in) and backs it with 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers. That mix matters because a 40 mm bracket and a 3,000 mm frame are not the same job.

Ask two questions on the first call. What is the largest part you routinely hold ±0.005 mm on, and how do you check it? A supplier running 4,000 × 400 × 150 mm travels will answer differently from one whose largest machine is 500 × 500 × 450 mm. Both can be honest. Only one fits your drawing.

Surface finish behaves the same way. As-machined aluminum lands around Ra 1.6–3.2 μm. A sealing face or a bearing bore often needs Ra 0.8–1.6 μm, and optical or gasket seats can call for Ra 0.2–0.8 μm. Tell the supplier which faces carry the finish callout, not the whole part.

One more thing. Tight tolerance on a deep pocket with a small corner radius is a tooling problem before it is a tolerance problem. If the corner is 1.5 mm and the pocket is 30 mm deep, the tool will deflect. A supplier who flags that in DFM is reading your drawing. One who quotes it silently is not.

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  Ask for the machine, not the promiseMachine model, travel and spindle speed for your specific part.
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  Separate critical facesFinish and tolerance callouts on 5 % of faces cut most of the cost.
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  DFM before POFree DFM analysis in the quote tells you if the design is machinable as drawn.

A machined service supplier that only mills and turns leaves you to manage the rest. Anodizing, electroless nickel, zinc plating, powder coating, black oxide, bead blasting, polishing and laser marking all sit downstream of the machine. If they happen in a different plant, your parts travel twice.

Travel is not the real cost. The real cost is what happens when a batch comes back with a coating thickness that closes a bore, or an anodize run that changes the color between two lots. You then own a three-way conversation between the machine shop, the finisher and your assembly line.

GreatLight keeps finishing in the same plant. Laser marking handles character heights down to 1.5 mm, which covers most serial numbers and traceability marks. Hardcoat, conductive anodize and electroless nickel are quoted with the machining, so one PO covers the finished part.

This is also where certification scope matters. ISO 9001:2015 covers general quality. IATF 16949:2016 applies to automotive and EV programs. ISO 13485:2016 covers medical devices. ISO 27001:2022 covers information security, which is what your customer's drawings actually need. Match the certificate to your industry, not to a logo wall.

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  Count the handoffsEvery extra plant is another chance for a lost lot and a schedule slip.
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  Ask for finish samplesColor and gloss vary between anodize lots. See a coupon before the run.
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  Confirm marking height1.5 mm minimum character height keeps laser marks readable.

Minimum order quantity is the fastest way to learn who a supplier is built for. A shop that wants 500 pieces before it turns on a machine is a production shop. A shop that runs one prototype on Monday and 10,000 parts on the same fixture is a different animal.

GreatLight has no minimum order quantity. A single bracket and a 10,000-piece run go through the same quoting path. That sounds like a sales line until you look at the setup. One-off work needs fast programming and a DFM pass. Volume work needs fixture design, tool life planning and a process control sheet. Both live in the same building.

Volume also changes the material conversation. Prototypes usually run in 6061 or 6061-T6 aluminum because it machines fast and welds well. Production parts may move to 7075 for strength, 17-4PH for corrosion, or Inconel and TC4 (Ti-6Al-4V) where temperature and load demand it. Each switch changes tool wear and cycle time.

Plastics follow the same logic. ABS and POM cut easily. PEEK, PA and carbon fibre need sharper tools, slower feeds and more attention to clamping. Tell the supplier the end use, not just the material name. A housing that sees 80 °C and one that sees 200 °C are not the same quote.

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  No MOQ means no setup penaltyOne piece is priced as a job, not as a favor.
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  Material choice moves the price7075 and Inconel cost more in tool wear than in stock.
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  State the environmentTemperature, load and chemical exposure decide the alloy.

A quotation and free DFM analysis within 12 hours tells you two things. The supplier has capacity now, and someone read the drawing. A quote that arrives in four days with no comments usually means nobody opened the file.

Production can start within 24 hours after approval, and parts ship in 3–5 days. Historical late-delivery probability sits below 2%. Those numbers come from process, not luck. Material is booked against the PO, fixtures are made before the first cut, and in-process monitoring catches a drift before the lot is finished.

The hidden cost trap is narrower than people think. It is not usually the unit price. It is a second setup because a feature was quoted as 3-axis and needs 5-axis, a rework loop because a bore was anodized after final sizing, or a partial shipment because one operation went to a subcontractor. Each one adds a purchase order, a freight leg and a week.

Read the quote line by line. Does it include material certification? Does it include finishing? Does it name the inspection method? A quote that lists only "CNC machining" is a starting point, not a total cost.

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  Free DFM in the quoteComments on wall thickness and tool access save the second setup.
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  In-process monitoringFirst article plus periodic checks catch drift before the lot ends.
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  Ask what is excludedMaterial certs, finishing and marking are common exclusions.

GreatLight inspects 100% of parts before shipment. That covers a raw material check, in-process monitoring and a final inspection, with reports on request. The qualification rate is 99.99%. For aerospace and medical work, ask for the dimensional report and the material certificate with the shipment, not after.

Documentation should match the industry. A medical device customer may need traceability from melt lot to finished bore. An automotive customer may need PPAP-style records. An electronics customer may care most that the fixture did not scratch a cosmetic face. Say which one you are on the first call.

Data handling is the part buyers forget. Drawings carry your customer's geometry. Uploads through the quote portal stay secure and confidential, and an NDA is available on request. If your customer requires ISO 27001:2022, confirm the supplier holds it before you send the STEP file.

One practical habit: send a marked-up drawing. Highlight the critical dimensions, the finish callouts and the faces that must not be touched by a clamp. A supplier who quotes from a clean print will guess. A supplier who quotes from a marked print will ask.

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  Reports on requestDimensional and material reports ship with the lot when specified.
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  Mark the critical dimsA marked print reduces quoting errors more than a longer email.
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  NDA before the fileSign first if the geometry is customer-owned.

Capacity is not only machine count. GreatLight runs 127 high-precision CNC machines across three wholly-owned plants, 7,600 m² of floor space and 150 technicians, with a location in Dongguan, China and a factory in Singapore. The Singapore site matters for buyers who need a second shipping origin.

Industry fit decides the rest. Aerospace and robotics work leans on 5-axis for one-setup geometry on joints and housings. Automotive and EV programs lean on IATF 16949:2016 and repeatable volume. Medical devices lean on ISO 13485:2016 and clean handling. New energy and industrial machinery sit between those poles.

Ask what share of the shop's work is in your sector. A supplier with 30 % automotive work already knows the PPAP rhythm. A supplier with none will learn it on your PO. Neither answer is disqualifying, but you should know which one you are buying.

Finally, check the secondary services. Sheet metal fabrication, die casting, vacuum casting, 3D printing and rapid prototyping in the same group mean a program can move from prototype to production without a new supplier qualification. That saves weeks on a program schedule.

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  Two shipping originsDongguan and Singapore give buyers a choice of export route.
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  Match the certificate to the sectorIATF for auto, ISO 13485 for medical, ISO 27001 for data.
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  Ask about program handoffPrototype to production in one group skips a supplier audit.