Custom CNC Processing Services: How to Pick the Right Shop

• 1
  What matters most when you compare shops
• 2
  Which shop fits which job
• 3
  Start with the part, not the brochure
• 4
  What ±0.005 mm really buys you
• 5
  Read the schedule backwards from your build
• 6
  MOQ, certifications and quote clarity
• 7
  Six things that stall a custom CNC order
• 8
  Step by step: from RFQ to accepted parts

Every custom CNC processing services quote should begin with the geometry. Send a STEP file and a 2D drawing with the critical dimensions marked. A shop that quotes from a photo alone will guess at tolerance and material, and the guess shows up as a rejected part.

Look at feature count and reach. A plate with holes on one face runs well on a 3-axis mill. A part with angled ports, deep pockets or undercuts usually needs 4-axis or simultaneous 5-axis so the tool reaches the feature without re-fixturing. Each extra setup adds positional error and queue time.

Part size sets the floor. At GreatLight, large work goes on machines with 4,000 × 400 × 150 mm travel, medium parts on 750 × 1,150 × 550 mm or 600 × 600 × 600 mm tables, and small tight-tolerance parts on 500 × 500 × 450 mm centers. If your part does not fit the quoted machine, the shop will sub it out and the schedule slips.

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  Send a STEP plus a 2D drawingMark datums and the two or three dimensions that really matter.
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  Count the setupsFewer setups means tighter position between features.
• 3
  State the fitA bearing bore needs a number, not 'tight'.

Tolerance is the first number buyers compare, and the first one misread. General machining tolerance and a controlled critical dimension are different things. A shop can hold ±0.005 mm on a bored hole while holding ±0.05 mm on the rest of the part, and that is normal.

Finish follows the same logic. As-machined surfaces sit around Ra 1.6–3.2 μm. A high-quality machined finish is Ra 0.8–1.6 μm, and a fine finish of Ra 0.2–0.8 μm usually needs a separate operation such as fine turning, grinding or polishing. Specify finish per surface, not for the whole part.

Ask how the shop measures. Calipers cannot verify ±0.005 mm. You want to hear about micrometers, bore gauges, height gauges and a CMM, plus a report on request. If inspection is only described as 'we check it', the tolerance claim is not backed by a process.

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  Split the toleranceCritical features tight, everything else standard.
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  Name the finish per faceRa 0.8–1.6 μm on sealing faces is often enough.
• 3
  Request inspection reportsRaw material, in-process and final checks.

Buyers compare delivery days and forget everything that happens before the spindle turns. A workable timeline is: quotation and free DFM analysis within 12 hours, production start within 24 hours, parts shipping in 3–5 days. That assumes a released drawing and material in stock.

Material is the usual hidden delay. 6061-T6 aluminium and 303 stainless are common. 17-4PH, Inconel, Ti-6Al-4V and PEEK may need to be ordered, and that adds days before machining starts. Say whether an equivalent grade is acceptable, or accept the wait.

Finishing runs on its own schedule. Anodizing, plating, powder coating and laser marking are separate steps, and laser marking needs a minimum character height of 1.5 mm to stay legible. A shop with finishing in-house controls that queue. If it is outsourced, add buffer.

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  Confirm material stock earlyCommon grades move fast, exotics do not.
• 2
  Ask where finishing happensIn-house queues are easier to track.
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  Do not plan to the last dayLeave room for one rework loop.

A shop built for production will push you toward volume. If your project starts with one prototype and later scales to 10,000+ parts, check that both ends are supported. No minimum order quantity is a real filter here: it tells you the shop keeps small jobs in the same queue as large ones.

Certifications should be matched to your industry, not collected. ISO 9001:2015 covers general quality systems. IATF 16949:2016 is what automotive programs expect. ISO 13485:2016 applies to medical devices. ISO 27001:2022 covers information security, which matters when you upload proprietary CAD. Ask which certificate applies to the plant that will run your part.

Then read the quote line by line. It should state material grade, tolerance, finish, quantity, inspection scope and whether programming or fixturing is included. A single lump-sum number with no breakdown is where surprise charges live. Uploads should be treated as secure and confidential, with an NDA available on request.

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  Check the plant, not the groupCertificates attach to a site and a scope.
• 2
  Demand an itemized quoteMaterial, machining, finish, inspection.
• 3
  Confirm NDA before uploadEspecially for unreleased products.

Most delays are documentation problems, not machining problems. A missing datum, an undefined finish or a thread callout without a class will send the job back for clarification, and the queue restarts.

The second cluster is material and process mismatch. Hardened tool steel at 60 HRC will not be cut with the same setup as 6061 aluminium. Thin-walled parts deflect, so wall thickness and support strategy need to be agreed before cutting. A shop that flags this in DFM is doing its job.

The third is scope creep after the quote. Adding a finish, tightening a tolerance or splitting a quantity changes the price and the schedule. Lock the drawing revision when you approve the quote.

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  Undefined datumPosition cannot be inspected against nothing.
• 2
  Thin wallsBelow about 1 mm, plan for support or accept deflection.
• 3
  Revision driftFreeze the drawing before the PO.