Continuous CNC Machining Services: 6 Checks Before You Commit

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  Key takeaways
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  Which supplier type fits your program
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  Tolerance: what continuous CNC machining can actually hold
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  Surface finish and secondary operations
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  Lot size, setup cost and the case for no MOQ
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  Certifications and inspection paperwork
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  Lead time, NDA and how quotes are priced
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  Red flags in a low-cost continuous CNC machining quote
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  Six steps to qualify a supplier

A drawing that says ±0.005 mm everywhere is a wish, not a spec. On a 40 mm aluminum housing, that number is routine. On a 1,200 mm frame rail, thermal drift and tool deflection push the real limit closer to ±0.05 mm. Ask the shop to confirm tolerance per feature and per material before you compare prices.

Material changes the answer too. 6061-T6 machines clean and holds size well. 316L stainless work-hardens and needs slower feeds, so thin walls can move after clamping is released. Titanium TC4 (Ti-6Al-4V) cuts hotter and wears tools faster. A shop that quotes the same tolerance for all three has not read your part.

Feature geometry matters more than overall size. A Ø6 mm bore 60 mm deep needs a long-reach tool that deflects. A flat face on the same part is easy. Split the drawing: mark the three or four features that carry function, and hold those tight. Let the rest sit at ±0.1 mm. You will cut cost without losing performance.

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  State tolerance per featureBlanket callouts invite disputes at first article.
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  Flag thin walls under 1.5 mmThey need light passes and possibly stress relief.
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  Confirm the measurement methodCMM, micrometer and optical gauge give different numbers.

Finish is a process choice, not a checkbox. As-machined aluminum sits at Ra 1.6–3.2 μm with visible tool marks. Fine turning or grinding reaches Ra 0.2–0.8 μm, but it adds a second setup and often a second vendor if the shop does not run its own finishing line.

Anodizing, plating and powder coating all change dimensions. Hardcoat anodizing on aluminum can add 25–50 μm per surface. If you call a Ø10.00 mm pin at ±0.01 mm and then hardcoat it, the part is out of tolerance before it ships. Tell the shop the final finish before it sets the pre-plate dimensions.

Laser marking is the same story. Minimum character height is 1.5 mm for a clean mark, and deep engraving can distort a thin wall. Decide what the mark must survive, then pick the depth. A part number that only needs to be readable does not need a 0.3 mm cut.

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  Ask who does the finishingIn-house lines cut days off the schedule and keep one process sheet.
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  Send the final finish spec up frontPre-plate dimensions must account for coating thickness.
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  Keep masking simpleEvery masked zone adds handling and a chance for a miss.

Setup is the fixed cost in machining. On a 3-axis mill, it might be 20 minutes. On a 5-axis center with a custom fixture, it can run two hours. That is why a 10-piece order often costs the same as a 50-piece order. Ask the shop to break out setup from run time so you can see where the money goes.

A no-MOQ policy matters when your design is still moving. If revision C changes a mounting boss, you scrap 10 parts instead of 500. Continuous CNC machining services work best when the first article, the pilot run and the production run come off the same machines and the same inspection plan.

Watch for a hidden minimum in the finishing step. Some shops machine one part but send plating out in batches, which adds a week and a batch charge. Confirm the true minimum across every operation, not just the mill.

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  Separate setup from run timeIt shows how much a design change will cost.
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  Check the finishing minimumPlating and anodizing often batch, not per part.
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  Reuse fixtures across revisionsA stable fixture keeps the second run cheaper than the first.

ISO 9001:2015 covers general quality management. IATF 16949:2016 applies to automotive work and adds traceability and PPAP-style discipline. ISO 13485:2016 is for medical devices. ISO 27001:2022 covers information security, which matters if you send unreleased CAD. A supplier can hold several, but each one applies to specific work. Ask which scope covers your part.

Inspection is the second half of the question. A shop that runs 100% inspection before shipment and keeps raw material, in-process and final records can hand you a report when your customer asks. A shop that checks the first part and the last part will not. Ask for the inspection plan before the first chip is cut.

For regulated programs, confirm that material certificates trace to the heat lot, and that the shop can hold that record for the life of the product. Retrofitting traceability after shipment is not possible.

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  Match certificate to industryAutomotive, medical and general work use different scopes.
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  Request the inspection plan earlyIt should list features, gauges and sampling rate.
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  Confirm traceability depthHeat lot, machine, operator and date are the usual fields.

A fast quote is only useful if it is complete. The first reply should answer four things: tolerance per feature, finish and coating allowance, inspection level and ship date. If the quote ignores one of them, the price will move later. GreatLight returns a quotation and free DFM analysis within 12 hours, and production can start within 24 hours of approval.

Prototype turnarounds of 3–5 days are normal when the shop owns its own machines and finishing. The real test is the second order. A shop that kept your fixture and your process sheet will repeat the first run. A shop that rebuilt everything will not.

Send an NDA before the first STEP file if the design is unreleased. Uploads should be encrypted in transit and stored with access control, which is where ISO 27001:2022 becomes relevant. Do not rely on a verbal promise. Put confidentiality, revision control and data deletion in writing.

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  Require four answers in the quoteTolerance, finish, inspection and ship date.
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  Test the repeat orderIt shows whether the process was ever documented.
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  Sign the NDA firstBefore CAD leaves your network.

A price far below the others is a signal, not a win. The usual causes are vague tolerance, no inspection report, outsourced finishing with no schedule control, or a material grade swap. Any of those turns a cheap part into an expensive rework loop.

The most common swap is aluminum. 6061-T6 and 6082 look alike on a quote but differ in strength and anodizing response. The same happens with 304 versus 316L stainless in wet or marine environments. Ask for the material certificate and check the grade matches the drawing.

The second red flag is a supplier who will not answer a technical question before the order. If they cannot explain how they will hold a Ø6 mm bore 60 mm deep, they will not hold it in production either. A short conversation about tooling and setup tells you more than a price sheet.

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  Demand the material certificateGrade swaps are the cheapest trick and the hardest to catch.
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  Ask one process questionHow would you hold this feature? The answer is the interview.
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  Avoid verbal-only promisesTolerance, finish and date belong in the PO.