Request a CNC Machining Service: What to Send and What to Check

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  Key takeaways
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  Which supplier tier fits your part
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  What a complete request a CNC machining service package contains
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  Specifying tolerance, finish and material without inflating cost
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  Matching part geometry to machine capability
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  Certifications, security and the paperwork behind the quote
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  How to read a CNC machining quote line by line
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  How to request a CNC machining service in 6 steps

Most slow quotes are not a shop problem. They are a file problem. When you request a CNC machining service, the shop needs enough information to choose a machine, a fixture and a stock size before anyone can price the job. A STEP or Parasolid model gives geometry. A PDF drawing gives intent: which faces are datums, which holes are press fits, which surface is visible to the customer.

Send both. A model alone leaves tolerance and finish open, so the shop either guesses or emails you back. A drawing alone forces the programmer to rebuild geometry and invites interpretation errors on fillets and thread depths. Together they answer the three questions every estimator asks: what shape, how tight, how many.

Add the commercial frame in the same message. Material grade with temper, for example 6061-T6 rather than aluminium. Quantity per revision, including spares. Surface finish by process name, such as clear anodizing or electroless nickel. Any inspection report the end customer requires. If the part is cosmetic, say which faces are visible. That one sentence prevents more disputes than any tolerance callout.

Keep the package under one revision label. If a drawing changes after you send it, resend the whole set with a new revision number and note what moved. Shops that receive a silent revision mid-quote will price the wrong geometry, and the error surfaces at first article, not at quote.

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  GeometrySTEP AP214 plus PDF drawing with datums and notes
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  MaterialGrade and temper, not just the alloy family
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  FinishProcess name and Ra range where it matters
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  QuantityPer revision, plus spare parts

Tolerance is a cost dial. A general machining tolerance of ±0.1 mm covers most bracket and housing features and machines quickly. Tightening a bore to ±0.005 mm is realistic on a CNC machining center, but it requires a finishing pass, a controlled temperature shop floor and a CMM check. Put the tight callout on the two or three features that actually locate or seal, and leave the rest general.

Surface finish follows the same logic. As-machined surfaces sit around Ra 1.6–3.2 μm. A sealing face or bearing seat often needs Ra 0.8–1.6 μm. Optical and vacuum surfaces can reach Ra 0.2–0.8 μm, but that finish comes from a specific tool path and sometimes a secondary polish, so name it only where the function needs it.

Material choice drives both. Aluminium 6061-T6 machines fast and holds thin walls well, which is why it dominates prototype housings. Stainless 304 and 316 resist corrosion but work-harden, so deep pockets and small taps need slower speeds and sharp tooling. Titanium Ti-6Al-4V and Inconel are machinable but eat tool life, and that shows up in price rather than in the drawing.

For plastics, POM and PEEK hold tolerance better than ABS or PP, which move with temperature and moisture. If a plastic part carries a press fit, say so. The shop can then choose a grade that holds the interference instead of advising you after the first article fails.

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  General ±0.1 mmDefault for non-critical features
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  Tight ±0.005 mmReserve for locating and sealing features
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  Ra 1.6–3.2 μmAs-machined, no extra operation
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  Ra 0.2–0.8 μmOnly where optics or sealing demand it

Before you compare prices, check that each shop can physically hold your part. A 4,000 mm frame needs a machine with that travel, and splitting it across two setups adds a joint tolerance that no amount of inspection removes. A part with features on five faces benefits from simultaneous 5-axis work because one setup keeps the datums consistent. A turned part with cross-drilled holes is often cheaper on a mill-turn center than on two separate machines.

Ask for the machine envelope and the setup plan, not the machine list. A shop with 127 machines still cannot quote honestly if the part exceeds its largest travel. The useful answer sounds like: this runs on a 5-axis center in two setups, first op on the A face, second op on the B face, datum transferred through the bore.

Accuracy claims need a measurement path behind them. ±0.005 mm means little without a CMM and a stated inspection stage. Ask when the part is measured, on what equipment, and whether the report ships with the parts. Shops that inspect only at final release catch problems late. The stronger pattern is raw material check, in-process monitoring after critical operations, and final inspection before shipment.

For regulated programs, capability includes documentation. Aerospace, medical and automotive buyers need material traceability, process records and change control. A shop that cannot show how it handles a drawing revision will struggle when your customer audits the supply chain.

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  Envelope firstConfirm travel before comparing price
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  Setup countEach extra setup adds stacked error
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  Inspection stageIn-process beats final-only checking
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  Change controlRevision handling is part of capability

Certificates are a starting point, not a verdict. ISO 9001:2015 covers a quality management system. IATF 16949:2016 adds automotive process discipline. ISO 13485:2016 addresses medical device manufacturing. ISO 27001:2022 covers information security. Each one is worth checking against the scope statement, because a certificate that excludes machining does not help a machining order.

Ask what the certificate changes in practice. A documented process should show up as incoming material inspection, controlled programs, calibrated measuring equipment and a record you can request. If the shop cannot explain the workflow behind the certificate, treat the document as marketing.

Confidentiality matters more than most buyers expect. CAD files carry your product roadmap. Send them through a secure upload rather than a public link, and sign an NDA before transferring a full assembly. A shop that offers an NDA without being asked is usually a shop that handles defense, medical or pre-launch consumer work.

Payment and shipping terms belong in the first exchange, not after the quote. Ask what currency the price is in, what incoterm applies and who handles customs paperwork. Getting this clear early avoids a second round of emails that costs a week before the first chip is cut.

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  Scope checkConfirm the certificate covers machining
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  RecordsAsk for a sample inspection report
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  NDASign before sending a full assembly
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  TermsCurrency, incoterm and customs up front

Two quotes for the same part can differ by 40 percent and both be honest. The difference usually sits in setup amortization, material stock size and inspection depth. A quote built on bar stock and one setup will beat a quote that assumes a custom fixture and three operations, even at the same hourly rate. Ask which assumptions drive the number.

Look for a DFM note. If the shop flags a deep pocket with a 2 mm corner radius as a tool-access problem, or suggests opening a tolerance that does not affect function, it has reviewed the geometry. Silence on a part with thin walls and sharp internal corners is not a good sign.

Check how quantity breaks are structured. Unit price should fall as volume rises, but not because the shop skipped inspection. If the 100-piece price is barely below the 10-piece price, setup dominates and a second supplier may not help. If the 10,000-piece price drops sharply, ask what changes in the process.

Treat lead time as a claim to verify. Ask when production starts after drawing approval, how long the first article takes, and what the shop does when a machine goes down. A realistic schedule with a buffer beats an aggressive date that nobody can hold.

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  Setup assumptionsOne op versus three changes the price
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  DFM feedbackSpecific notes show a real review
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  Quantity breaksFalling unit price should not mean less checking
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  Schedule logicAsk what happens when a machine stops