The term "zero line" is used in two contexts. The first is the zero line of the processing system and the second is the zero line in the straightener.
The term zero line of the processing system is understood to mean that the product to be straightened runs through the processing system without undergoing any additional bending, deflecting or guiding. It is an advantage therefore for wide spools to be axially adjustable in order to always obtain the required zero line. The zero line of the processing system includes the zero line in the straightener.
The term zero line in the straightener describes the situation when all the straightening rolls touch the product to be straightened but do not bend it. This setting is usually made with a caliber, hence the term calibration. The zero line in a straightener depends on the geometry of the product to be straightened and of the straightening roll groove.
See Also
Adapted from “"We do it straight" — Wire Straightening”, p. 205 (ISBN 3-00-005897-4).