Put simply, the difference between the two processes comes down to which part moves – in general terms, the metal or the machine.
· With milling the metal usually remains still while multi-point cutting tools rotate around it.
· With CNC turning a metal bar is fed into the lathe and rotates at speed while a single-point cutting tool removes portions of the metal to create the desired shape.
CNC Milling
When a part does not have crucial cylindrical features, a milling operation will most likely be used. The work piece is held stationary while a rotating cutter maneuvers around the piece removing material. On a 2 or 3 axis CNC milling machine the work piece will need to be manually rotated to machine the different faces. A 4 or 5 axis machine can rotate the work piece or spindle automatically to machine complex contours and shapes.
CNC Turning
When it comes to creating parts that have symmetrical and cylindrical features, a turning process is often used. the material (typically bar stock) is held in a chuck and rotated at high speeds while a single point cutting tool is positioned against the rotating work piece to remove material.
Turning operations on a CNC lathe can be very effecient for high volume runs. The addition of a bar feeder can help automate the process and minimize manual activity by the machinist.