Overview of CNC Machining: What to Check Before You Order

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  Key takeaways
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  Which CNC process fits your part
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  What the overview of CNC machining actually covers
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  Tolerance, finish and inspection: the numbers to pin down
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  Materials and how they change the cutting plan
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  Certifications, confidentiality and the paperwork side
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  Six steps to source CNC machining without rework

A useful overview of CNC machining starts with the machine, not the brochure. Cutting metal is a subtractive process: a rotating tool removes material along a path the CAM software defines. The machine only decides how many setups that path needs. Fewer setups mean fewer re-fixturing errors, which is usually where size drift comes from on a multi-face part.

Every shop you shortlist should be able to name its spindle hours, its maximum part envelope and its tolerance floor. At GreatLight the floor is ±0.005 mm on qualified features, with 127 high-precision machines across three plants and a 4,000 mm maximum processing size. If a supplier cannot state a number, treat the quote as provisional.

The process also sets the surface. As-machined finishes land around Ra 1.6–3.2 μm. A finer Ra 0.8–1.6 μm usually needs a second pass with a smaller stepover or a different tool, and that shows up in the price. Ra 0.2–0.8 μm belongs to polishing or grinding, not to a standard milling cycle.

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  Geometry drives the machine choiceDeep pockets, thin walls and 5-axis contours are different problems.
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  Setup count drives the toleranceEach re-clamp adds stack-up error.
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  Finish is a separate operationSpecify Ra on the drawing or you get the default.

Tolerance is the most misread line on a CNC quote. A blanket ±0.1 mm title block is cheap. The same part with three ±0.005 mm bores is a different job, because those features need a finishing pass, temperature control and a CMM check. When you compare quotes, compare the callouts, not the headline number.

Inspection depth matters more than the tolerance claim. Ask whether the shop checks incoming bar stock, monitors in-process, and inspects the finished part. GreatLight inspects 100% of parts before shipment and sends reports on request, with a qualified-rate figure of 99.99% on released production.

Surface finish follows the same logic. Bead blasting, anodizing or electroless nickel will hide tool marks but not fix a dimension. Put the functional tolerances on a pre-plate drawing, because plating adds thickness and can close a thread or a bore.

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  Mark tight features individuallyA global tolerance note pushes cost onto every dimension.
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  Ask for the inspection report formatFirst-article, key-characteristic or full layout are different documents.
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  Separate pre-plate and post-plate sizesNickel and anodizing shift dimensions by microns to tens of microns.

Aluminium 6061 and 7075 machine fast and hold good finishes, which is why most prototypes start there. Stainless 304 and 316 work-harden, so the tool has to stay in cut and the feed must not dwell. Titanium Ti-6Al-4V and Inconel need low surface speed, rigid tooling and flood coolant; a shop without that setup will burn tools and your budget.

Plastics behave differently again. POM and PEEK cut cleanly at moderate RPM, while ABS and PMMA are more likely to chip at the edge and need sharp tooling and light finishing passes. Carbon fibre composites are abrasive and require carbide or diamond tooling plus dust control.

Material availability drives lead time as much as machining does. A common 6061 plate may already be on the rack. A specific 17-4PH condition, a medical-grade titanium or a magnesium AZ31B bar may need to be ordered, and that wait sits in front of the machine time. Ask which grades the shop stocks before you assume the quote is comparable.

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  Hardened steelsCarbide tooling and high-pressure coolant, not a standard cycle.
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  MagnesiumChips are flammable; the shop needs dedicated handling rules.
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  CompositesAbrasive dust shortens tool life and needs extraction.

Certifications tell you which markets a shop can serve without extra auditing. ISO 9001:2015 covers general quality systems. IATF 16949:2016 is the automotive requirement, and it brings PPAP-style documentation with it. ISO 13485:2016 applies to medical devices, where traceability and process validation are part of the contract. ISO 27001:2022 covers information security, which matters if you send CAD data across borders.

Confidentiality is a practical question, not a legal one at the start. Ask how uploads are stored, who can open the files and whether subcontractors see them. An NDA available on request is a reasonable baseline, and it should be signed before the first drawing is shared on a competitive design.

Paperwork also includes the material certificate and the inspection record. If your customer audits the end product, you need those documents to trace back to a heat number. Request them at the quote stage so the shop prices the documentation time into the job rather than adding it later.

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  AutomotiveIATF 16949 plus control plans and PPAP if required.
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  MedicalISO 13485 with clean handling and validated processes.
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  ITAR or export-controlled workConfirm eligibility before sending drawings.