Siemens SINUMERIK 808D Milling Programming And Operating Manual page 14

Introduction
1.3 Coordinate systems
The origin of this coordinate system is the machine zero.
This point is only a reference point which is defined by the machine manufacturer. It does not
have to be approachable.
The traversing range of the machine axes can be in the negative range.
Workpiece coordinate system (WCS)
To describe the geometry of a workpiece in the workpiece program, a right-handed, right-
angled coordinate system is also used.
The workpiece zero can be freely selected by the programmer in the Z axis. In the X axis, it
lies in the turning center.
The figure below shows an example of the workpiece coordinate system.
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Programming and Operating Manual, 12/2012, 6FC5398-4DP10-0BA0
Milling Part 1: Operation