CNC Machining Glossary — Terms & Definitions

Overview

This glossary defines the terms a buyer or engineer meets when ordering custom machined parts and fasteners. It is written for the person who owns the order rather than the person who runs the machine: the words below are the ones that appear on drawings, quotes and material documents, explained in plain English. Where a term carries a tolerance or a callout, we note the value we work to, so a drawing you write means the same thing to us as it does to you. For the full flow from file to finished part, start with our how-to-order guide.

Tolerance — the permitted variation in a dimension, stated as a plus-or-minus range. We hold ±0.01 mm on precision features.

ISO 2768-m — a general tolerance standard applied to any dimension that carries no individual tolerance on the drawing; the m denotes the medium class.

Surface finish (Ra) — the roughness of a machined surface, expressed in micrometres; a lower value means a smoother, usually more expensive surface.

Datum — a reference surface, edge or axis from which other dimensions are located; every good drawing names its datums.

DFM (design for manufacturability) — feedback on how a design can be made more cheaply and reliably before the order is committed; we send it with every quote, and the DFM guide expands on it.

MTR (material certificate) — the document that records the grade and heat of the material used for your parts; it ships with every order.

Workholding — the way a part is gripped in the machine; the grip must be rigid enough to hold tolerance without marking the part.

Hardness — a material’s resistance to indentation, usually given in Rockwell or Brinell; it is set by material choice and, where needed, heat treatment.

How it works step by step

These are the terms you meet as a part moves from raw stock to a clean, protected surface.

Burr — a thin raised edge left where a cutter exits the metal.

Deburr — the removal of burrs so edges are clean, safe to handle and ready to assemble.

Chamfer — a flat, angled cut on a corner or edge, often there to break a sharp edge or guide a mating part into place.

Fillet — a rounded inside corner, usually the radius of the cutter; generous fillets make a part cheaper to machine.

Tap — the tool, and the act, of cutting an internal thread inside a hole.

Die — the tool, and the act, of cutting an external thread on a rod or stud.

Thread pitch — the distance between two adjacent thread crests; metric threads state it in millimetres, inch threads as threads per inch.

Passivation — a surface treatment for stainless steel that removes free iron from the surface and improves corrosion resistance.

Anodising — an electrochemical coating grown on aluminium for hardness and colour; we arrange it through our vetted partner network, alongside the finishes on our surface finishes page.

Knurl — a cross-hatch pattern rolled or cut into a surface to give fingers or a mating part grip.

Requirements and file formats

These terms appear in the drawing or model file you send — STEP, IGES, DWG, DXF or PDF — and getting them exactly right decides whether the part assembles. The drawing requirements page lists the formats and annotations we accept.

UNC — Unified National Coarse, the common coarse inch thread series used in general fastening.

UNF — Unified National Fine, a finer inch thread used where more threads per inch give better grip or finer adjustment.

Counterbore — a flat-bottomed enlargement at the mouth of a hole, cut so a socket-head screw sits flush with the surface.

Countersink — a cone-shaped recess cut so a flat-head screw sits flush with the surface.

Spotface — a shallow flat seat machined on a rough or angled surface so a bolt head or nut bears square.

Blind hole — a hole that does not pass all the way through the part; its depth must be stated on the drawing.

Through hole — a hole that passes entirely through the part.

Shoulder — a step where a shaft changes diameter, creating a flat face that a mating part seats against.

Common mistakes to avoid

These are the terms most often misread on a drawing, and the misread is usually expensive.

Clearance fit — a fit where the shaft is always smaller than the hole, so the two parts can slide or rotate freely.

Interference fit — a fit where the shaft is larger than the hole and must be pressed in; the interference holds the parts together.

Concentricity — how well the centre of one feature shares an axis with another; it is a relationship between features, not a single dimension.

Runout — how far a surface deviates from a true circle as the part rotates around an axis; a part can have low runout and still not be concentric, so the two are not interchangeable.