Numerical control (NC) is the automation of machine tools that are operated by precisely programmed commands encoded on a storage medium, as opposed to controlled
manually via hand wheels or levers, or mechanically automated via cams alone. Most NC today is computer numerical control (CNC), in which computers play an integral
part of the control.
In modern CNC systems, end-to-end component design is highly automated using computer-aided design (CAD) and computer-aided manufacturing (CAM) programs.
The programs produce a computer file that is interpreted to extract the commands needed to operate a particular machine via a post processor, and then loaded into
the CNC machines for production. Since any particular component might require the use of a number of different tools – drills, saws, etc., modern machines often
combine multiple tools into a single "cell". In other installations, a number of different machines are used with an external controller and human or robotic
operators that move the component from machine to machine. In either case, the series of steps needed to produce any part is highly automated and produces a part
that closely matches the original CAD design.
Motion is controlled along multiple axes, normally at least two (X and Y), and a tool spindle that moves in the Z (depth). The position of the tool is driven by
motors through a series of step-down gears in order to provide highly accurate movements, or in modern designs, direct-drive stepper motor or servo motors.
Open-loop control works as long as the forces are kept small enough and speeds are not too great. On commercial metalworking machines closed loop controls are
standard and required in order to provide the accuracy, speed, and repeatability demanded. As the controller hardware evolved, the mills themselves also evolved.
One change has been to enclose the entire mechanism in a large box as a safety measure, often with additional safety interlocks to ensure the operator is far enough
from the working piece for safe operation. Most new CNC systems built today are completely electronically controlled. CNC-like systems are now used for any process
that can be described as a series of movements and operations. These include laser cutting, welding, friction stir welding, ultrasonic welding, flame and plasma
cutting, bending, spinning, hole-punching, pinning, gluing, fabric cutting, sewing, tape and fiber placement, routing, picking and placing (PnP), and sawing.