High Quality Parts CNC Machining: How to Vet an Overseas Shop

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  Key takeaways
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  What high quality parts CNC machining requires per part type
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  Tolerance: what the drawing asks for vs what the machine holds
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  Size envelope: the limit that eliminates most shops
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  Lead time: separate the promise from the schedule
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  MOQ, tooling and how the quote is built
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  Certifications and inspection: what the paperwork actually covers
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  How to run the first order, step by step

Every drawing has a tolerance block. The question is whether the shop's machines can hold it on your specific geometry, not on a test coupon. A 5-axis center with thermal compensation holds ±0.005 mm on a 100 mm aluminum part. The same machine on a 300 mm steel part with interrupted cuts will drift more, because heat builds up in the tool and the part.

Ask for the number of 5-axis and 4-axis centers the shop runs, and whether they use in-process probing. A shop with 16 simultaneous 5-axis centers can keep tight parts running while a second job is set up. A shop with two centers will queue your job behind whatever came in first.

Tolerance also depends on material. Aluminum 6061 cuts clean and stays stable. Stainless 316 work-hardens and pushes back on the tool. Titanium Ti-6Al-4V needs low cutting speeds and rigid setups, or the wall thickness wanders. If the supplier quotes the same tolerance for all three without asking about geometry, the quote is a guess.

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  Ask for the machine listCount 5-axis, 4-axis and mill-turn centers, not total machines.
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  Ask about probingIn-process probing catches drift before the part is finished.

Most CNC shops in the 20 to 50 machine range top out around 600 to 800 mm of travel. That covers brackets, housings and small assemblies. It does not cover a 2,000 mm rail, a large vacuum chamber flange or a full engine block mold base.

GreatLight runs travel up to 4,000 × 400 × 150 mm on large machines, plus 750 × 1,150 × 550 mm and 600 × 600 × 600 mm on medium centers, and compact cells at 500 × 500 × 450 mm and 500 × 310 × 200 mm. That range matters because a part that fits on one machine in one setup will hold position better than the same part spread across two setups.

When you send an RFQ, state the finished envelope and the stock size. A shop that quotes before checking both is quoting a part it cannot make. Long parts also need matched fixtures, so ask how the part is supported along its length. Without support, a 1,500 mm aluminum beam will sag under its own weight during finishing passes.

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  Give stock size, not just finished sizeStock drives the fixture and the number of setups.
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  Ask how many setupsEach extra setup adds position error and cost.

A quote that says 15 days with no breakdown is not a schedule. Ask for four dates: quote and DFM return, production start, first article, and ship date. GreatLight returns quotation and free DFM analysis within 12 hours, can start production within 24 hours, and ships parts in 3 to 5 days for standard work.

The dates that slip are rarely the machining. They are material lead time, finishing queue and inspection. If your part needs hardcoat anodizing or electroless nickel, that step sits in a separate queue. Ask whether finishing is in-house or subcontracted. Subcontracted finishing adds a truck trip each way and a second queue you cannot see.

For repeat orders, ask for the historical late-delivery rate. GreatLight's figure is below 2%. Treat any number above 5% as a planning risk and build buffer into your assembly schedule. A cheap part that arrives late costs more than a slightly higher price that arrives on the date promised.

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  Get four dates in writingQuote, production start, first article, ship.
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  Ask where finishing happensIn-house finishing removes one queue and one risk.

Minimum order quantity is the fastest way to find out whether a shop wants your job. Some exporters set an MOQ of 100 or 500 pieces because their setup cost only makes sense at volume. If you need 3 prototypes, that shop is not a fit, no matter how good the price looks at 5,000 pieces.

GreatLight has no minimum order quantity, running from one prototype to 10,000+ part runs. That works because setup is quoted separately from cycle time. When you compare two quotes, split them the same way: material, setup, cycle time, finishing, inspection and freight. A single lump sum hides where the money goes and makes negotiation impossible.

Tooling is the other hidden line. Soft jaws and simple fixtures are usually included in setup. Custom work-holding for a long part, or a dedicated fixture for a repeat order, should be listed as a separate item with a quantity break. If it is not listed, assume it is inside the cycle time and ask for the split.

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  Ask for a cost breakdownMaterial, setup, cycle, finishing, inspection, freight.
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  Check the quantity breaksA real quote shows price at 1, 10, 100 and 1,000 pieces.

Certifications tell you which management system the shop runs, not which part you will receive. ISO 9001:2015 covers general quality process. IATF 16949:2016 applies to automotive production. ISO 13485:2016 applies to medical devices. ISO 27001:2022 covers information security, which matters when you send proprietary CAD files.

Ask which certificate scope your order falls under, and ask for a sample inspection report. GreatLight inspects 100% of parts before shipment, with raw material check, in-process monitoring and final inspection. Reports are available on request. A shop that cannot show a report format before you order will not produce one after.

For tight parts, ask how the measurement is done. A caliper reads to 0.02 mm and is fine for a bracket. A part held to ±0.005 mm needs a CMM or a vision system with a stated uncertainty below that. If the shop's only metrology is calipers, the tolerance on the drawing is not being verified, only assumed.

Confidentiality belongs in the same conversation. Uploads should be secure and confidential, and an NDA should be available on request before you send drawings. Ask for it early, not after the PO.

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  Match certificate to industryISO 9001 for general, IATF for auto, ISO 13485 for medical.
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  Ask for metrology, not just a reportCMM uncertainty must be below the part tolerance.