CNC Machining Glossary for Buyers & Engineers

A – D

Anodizing — an electrolytic finish for aluminium that grows a hard, corrosion-resistant oxide layer; Type II is decorative, Type III (hardcoat) is thick and wear-resistant. It also takes dye for color.

Billet — a solid block of raw stock (aluminium, steel, etc.) that a CNC machine cuts a part out of. Opposite of casting or forging, which start near-shape.

Burring / deburr — the sharp edge or curl of metal left by a cutting tool; deburring removes it. Tight-tolerance or sealing faces need it gone.

CNC — Computer Numerical Control: a machine tool positioned by programmed coordinates rather than by hand. Covers mills, lathes, routers and more.

CMM — Coordinate Measuring Machine: a probe that maps a part's dimensions in 3D. A CMM report is the proof a part is in tolerance.

Cosmetic tolerance — tolerance on appearance only (a visible face), not on function. Often looser than the fit tolerance.

Datum — a reference point, axis or plane on a drawing that other dimensions and GD&T are measured from. Pick stable, machined faces as datums.

DFM — Design for Manufacturability: shaping a part so it machines accurately, quickly and cheaply without changing what it does. See the design guide and checklist.

Drawing — the 2D PDF (with dimensions and GD&T) that defines the part. Send it with the 3D model; one file alone invites guesswork.

E – H

Ex-works / EXW — a shipping term: the buyer collects the goods at the factory gate. Cheapest unit price, but the buyer owns freight and risk from there. See also FOB, CIF, DDP.

Fixture — a custom jig that holds a part in a repeatable position so every piece is cut the same way. Fixtures cost money up front but pay off in volume.

First Article Inspection (FAI) — a full, documented measurement of the first produced part against the drawing, before production runs. The gate before you approve a batch.

5-axis — a machine that moves the tool or part on five axes at once, cutting complex faces in one setup. Fewer setups = tighter relationships between features. See 5-axis machining.

GD&T — Geometric Dimensioning and Tolerancing: a symbolic language for how a part's geometry must relate (flatness, position, concentricity), not just sizes. The six symbols buyers use are in the GD&T guide.

Hardcoat — see Anodizing, Type III.

Hole tolerance — how far a drilled hole may sit from its nominal position and size. Position tolerance matters more than diameter tolerance on most assemblies.

I – P

ISO 2768 — the general-tolerance standard. "ISO 2768-m" means medium class; if a drawing says only that, the shop works to those defaults. See the tolerance chart.

Lead time — calendar time from purchase order to shipment. Prototype and production lead times differ; state what you need.

Lot traceability — the ability to trace a finished part back to its raw material certificate and batch. Required for medical and aerospace work.

Mill-turn — a machine that both turns (spins the part) and mills (spins a tool) in one chucking, so turned and milled features stay aligned. See CNC turning.

MOQ — Minimum Order Quantity. Many shops (including us) take prototypes from one piece; price just gets better with volume.

NDA — Non-Disclosure Agreement: signed before drawings change hands so your design stays yours. Grab our free NDA template.

Passivation — a chemical treatment that removes free iron from stainless steel so the chromium layer protects it. Common on 300-series medical and food parts.

Prototype — the first few parts made to prove the design before production. Usually 1–100 pieces.

Q – T

Quantity break — the price step where volume starts to matter. Setup and programming don't scale with quantity, so 40 parts cost far more per piece than 4,000. See why small batches cost more.

Ra (Roughness average) — the most common surface-finish number; lower Ra = smoother. Ra 0.8 µm is a typical machined finish; 0.4 µm needs extra passes or polish. See the surface-finish chart.

Setup — mounting and aligning the part (and tools) in the machine. Each extra setup adds time and a small risk of shift. Designing for one setup is the cheapest win.

Spindle — the shaft that spins the cutting tool. Higher rpm helps on small tools and aluminium; rigidity matters more on steel.

STEP / IGES — common 3D model formats shops program from. STEP (ISO) is the modern default; send it with the 2D PDF.

Surface finish — how smooth or textured a machined face is, usually given as Ra. Call it out only where it functions.

Tolerance — the allowed variation on a dimension. ±0.1 mm is routine; ±0.01 mm needs a controlled setup and more checks. Tolerance only what the function demands.

Turning — spinning the part against a stationary tool to make round features (shafts, bushings). See CNC turning. Milling spins the tool instead.

U – Z

Unit price — price per part. On a quote it sits next to setup, tooling and shipping; compare like for like, not headline number alone. See how to read a quote.

Verification / inspection report — the document proving the part matches the drawing. CMM reports and material certs are the usual pair.

Wall thickness — how thin a feature can be and still machine reliably. Sub-millimetre metal walls chatter; keep them ≥ 0.8 mm.

Workholding — how the part is gripped during cutting. Good workholding is most of what keeps a part in tolerance.

XT — Parasolid, another common 3D model format, especially from SolidWorks and NX.