Siemens SINUMERIK 840D sl Programming Manual page 209

The tool must always be aligned perpendicular to the probe before the measuring cycle is
called, i.e. the tool axis is parallel to the center line of the probe.
Figure 2-33
Length measurement
If the tool diameter is less than the upper diameter of the probe, then the tool is always
positioned at the center of the probe.
if the tool diameter is larger, the tool is positioned offset by the tool radius toward the center
onto the probe. The value of the offset is subtracted.
If no offset axis is specified, if necessary an offset is realized in the 1st axis of the plane (with
G17: X axis).
Figure 2-34
Radius measurement
The tool radius is measured using lateral probing at the probe in the parameterized
measuring axis and measuring direction (see the following diagram).
Measuring cycles
Programming Manual, 02/2011, 6FC5398-4BP40-0BA0
Parallel alignment of tool axis, probe axis and axis of the coordinate system
Length measurement with and without offset
Measuring variants
2.5 Measure tool (milling)
209