Machining has evolved from individual machines which, with the aid of man, performed individual processes to machines capable of performing many processes.

In 1968, a NC machine was marketed which could automatically change tools so that many different processes could be done on one machine. Such a machine became known as a “machining center”—a machine that can perform a variety of processes and change tools automatically while under programmable control.

The study of machining centers begins with the history of numerical control (NC). NC is programmable automation in which certain functions of the machine tools are controlled by coded instructions.

Computer and numerical control is used on a wide variety of machines. These range from single-spindle drilling machines, which often have only two-axis control, to machining centers, which can do drilling, boring, milling, tapping, and so forth with four-axis control. A machining center can automatically select and change as many as 32preset tools. The table can move left/right or in/out and the spindle can move up/down or in/out, with positioning accuracy in the range of 0.0003 in. in 40 in. of travel. The machine has automatic tool change and automatic work transfer so that workpiece can be loaded/unloaded while the machining is in process.

The concept of automatic tool changing has been extended to NCN lathes. The tools are held on a rotating tool magazine and a gantry-type tool changer is used to change the tools. Each magazine holds one type of cutting tool. The versatility is being increased by combining both rotary-work and rotary-tool operations—turning and milling—in a single machine. Tools are changed in six seconds or less. It is also common to provide two or more worktables, permitting work to be set up while machining is done on the workpiece in the machine, with tables being interchanged automatically. Consequently, the productivity of such machines can be every high, the chip-producing time often approaching 50% of the total.

Two new trends are observed in the development of machining centers. One is the growing inteest in smaller more compact machining centers and the other is the emphasis on extended-shift or even unmanned operations. Modern machining centers have contributed significantly to improved productivity in many companies. They have eliminated the time lost in moving workpieces from machine to machine and the time needed fro workpiece loading and unloading for separate operations. In addition, they have eliminated the time lost in moving workpiece loading and unloading for separate operations. In addition, they have minimized the time lost in changing tools, carrying out gauging operations, and aligning workpieces on the machine.