CNC machines are often used in metal machining,
like drilling, milling and taping. This kind of machine
usually consists of a servo mechanism controlled by
computer, a high speed spindle and specific tools. This
servo mechanism can be realized in closed loop fashion,
using servo controllers driving DC or synchronous AC
machines, or open-loop fashion, using stepper motors.
The open-loop based machines are only suitable to
small loads. Therefore, industrial sized machines
generally do not, use open-loop machine.
The machine is based on the removal of the material
of a work piece, being the amount of time necessary to
finish the piece proportional to both the difference
between the initial and final volume of the work piece
and to the final piece complexity. Nowadays, there are
machines with six or more axes, which allow the
machining of pieces with such a high level of
complexity that it would not be feasible by other means