Machining centres are individual machines, i.e., all the machining operations are performed on a single (multi-function) machine. These centres must satisfy two conditions : they must carry out several machining operations and they must have automatic tool change, from a magazine or the like in conformity with a machining programme
Machining centers are essentially combination boring, milling and drilling machines which are numerically controlled (CNC) and have some form of automatic tool change, most commonly (probably 99%) an automatic tool changer (ATC).
The Tool & Manufacturing Engineers’ Handbook (TMEH) describes machining centers as multifunction, NC or CNC machines with automatic tool changing capabilities and rotating cutting tools.
In general, the principal operation of a machining center is milling. A CNC milling machine becomes a machining center with the addition of a tool changer as does a CNC combination boring and milling machine.