Milling is cutting with a rotating multi tooth tool while the workpiece is held still, and the operations are named for which part of the cutter does the work. Knowing the names matters because they map directly onto cycle time and tool cost, and because a designer who asks for a feature that forces the wrong operation pays for it on every part.
Face milling and end milling
Face milling uses the end of a large cutter to sweep a flat surface, taking a wide shallow cut and producing a good finish quickly. End milling uses a smaller cutter whose flutes cut on the periphery and the end, producing slots, pockets, profiles and steps. Face milling is the cheap way to make a surface flat; end milling is the general purpose operation for everything else.
Side milling and slotting
Side milling engages the flank of the cutter to machine a vertical wall, which is how most profiling is done and where surface finish on a wall comes from. Slotting engages the full width of the cutter and is the hardest condition on the tool, because chips have nowhere to go and the cutter is loaded on both sides. A slot machined by trochoidal or plunge strategies is faster and kinder to the tool than one cut full width.
Thread milling and why it beats tapping
A thread mill cuts a thread by helical interpolation with a cutter smaller than the hole, so one tool covers a range of diameters, chips clear easily, and a broken tool can be removed rather than wrecking the part. Tapping is faster on small production threads and less forgiving. Thread milling is the standard answer in hard material, in blind holes and on expensive parts.
Choosing between them at design time
Every internal corner radius sets the smallest end mill that can reach it, and the smallest end mill sets the feed rate. Every pocket deeper than a few cutter diameters forces a longer, more flexible tool and lighter cuts. A designer who opens radii and limits depths is choosing faster operations without changing what the part does.
Questions people ask about what is milling
What is milling?
Machining with a rotating multi tooth cutter against a stationary workpiece, producing flats, pockets, slots, profiles and threads.
What is end milling?
Cutting with a smaller cutter whose flutes work on the periphery and the end, used for pockets, slots, profiles and steps. It is the general purpose milling operation.
What is thread milling used for?
Cutting threads by helical interpolation with a cutter smaller than the hole. It suits hard materials, blind holes and expensive parts because it clears chips well and does not trap a broken tool.
Why are deep pockets expensive?
Because they force a long thin cutter that must run at a light cut and slow feed. Reducing depth or opening the corner radius is often the cheapest change available.