Milling is a machining process in which a rotating tool with several cutting edges, the milling cutter, removes material from a part that generally stays still. It is the tool that rotates: that is the essential difference from turning, where the part rotates and the tool stays fixed.
Milling is done on a milling machine or, increasingly, on a CNC machining centre with three axes (X, Y, Z) or more, up to 5-axis machining. The part is clamped in a vice or on a fixture, then the spindle follows the programmed path. The tool is chosen according to the shape to be produced: a flat-end mill for faces and pockets, a ball-nose mill for freeform surfaces, a face mill with indexable inserts for skimming large flat areas, a thread mill or chamfer mill for finishing. Cutting speed and feed depend on the material and the tool; the cutting speed calculator gives a first order of magnitude.
Milling is the reference process for prismatic parts, that is, parts with flat faces: housings, flanges, tooling plates, enclosures, brackets, slideways, hydraulic manifolds. It creates faces, shoulders, slots, pockets, contours and 3D surfaces, and it follows on with drilling and tapping on the same machine. A single set-up often allows several faces to be machined, which guarantees their relative positions. For milled parts, see the CNC machining page.
In a quote request. State the number of faces to be machined and the position dimensions between them, because every extra set-up adds time. Specify internal corner radii: a round cutter cannot produce a perfectly sharp internal corner, and a larger radius allows a bigger, stiffer cutter, and therefore faster machining. Also mention pocket depth, tight tolerances and the surface finish required.