CNC milling machines (also called machining centers) are computer controlled vertical
mills with the ability to move the spindle vertically along the Z-axis. This extra
degree of freedom permits their use in diesinking, engraving applications, and 2.5D
surfaces such as relief sculptures. When combined with the use of conical tools
or a ball nose cutter, it also significantly improves milling precision without
impacting speed, providing a cost-efficient alternative to most flat-surface hand-engraving
work.
In the vertical mill the spindle axis is vertically oriented. Milling cutters are
held in the spindle and rotate on its axis. The spindle can generally be extended
(or the table can be raised/lowered, giving the same effect), allowing plunge cuts
and drilling. There are two subcategories of vertical mills: the bed mill and the
turret mill.