HEIDENHAIN TNC 122 User Manual - Manual (Page 11)

1 Fundamentals of Positioning
TNC 122 11
1
Fundamentals of Positioning
Reference system and coordinate axes
Reference system
In order to define positions on a surface one needs a reference
system. For example, positions on the earth's surface can be
defined “absolutely” by their geographic coordinates of longitude
and latitude. The term “coordinate” comes from the Latin word for
“that which is arranged.” The network of horizontal and vertical
lines on the globe constitute an absolute reference system in con-
trast to the “relative” definition of a position that is referenced to
some other known location.
The illustration at right shows the 0° longitude at the Greenwich
observatory and the 0° latitude at the equator.
0° 90° 90°
0°
30°
30°
60°
60°
Greenwich
+X
+Y
+Z
+X
+Z
+Y
Cartesian coordinate system
On a milling or boring machine, workpieces are normally machined
according to a workpiece-based Cartesian coordinate system (a
rectangular coordinate system named after the French mathemati-
cian and philosopher Renatus Cartesius, who lived from 1596 to
1650). The Cartesian coordinate system is based on three coordi-
nate axes designated X, Y and Z which are parallel to the machine
guideways.
The figure at right illustrates the “right-hand rule” for remembering
the three axis directions: the middle finger is pointing in the posi-
tive direction of the tool axis from the workpiece toward the tool
(the Z axis), the thumb is pointing in the positive X direction, and
the index finger in the positive Y direction. X, Y and Z are the main
axes of the Cartesian coordinate system.
Fig. 1.1: The geographic coordinate system
is an absolute reference system
Fig. 1.2: Designations and directions of the
axes on a milling machine