Rockwell Automation 8520-MUM 9/Series CNC Mill Operation and Programming Manual Documentation Set - Manual (Page 77)

Offset Table and Setup
Chapter 3
3-3
The system installer determines in AMP which axis (or axes) are used by
the control as the tool length axis. Refer to documentation prepared by the
system installer for details on what axes have been selected for the tool
length axis. This manual assumes that the Z axis is used as the tool length
axis.
Figure 3.2
Tool Length Offset
amount of
tool offset
cutting tool
gauge
line
The term “gauge line” is used to define the precise point on the spindle or
tool holder from which all programmed tool paths originate. Offsets refer
to the distance from this gauge line to the end of the tool that contacts the
part being cut.
For tool length offset data, measure the value for the Z axis from the gauge
line to the end of the cutting tool. The values may be entered as either
positive or negative values depending on which G-code is used to activate
the offset (G43 or G44).
Important: For a typical end milling machine (with tool/chuck
configuration similar to Figure 3.2), keep the following in mind. When
programming tool offsets with a G43, the geometry offset value should be
positive and the wear offset data should be negative when entered in the
offset tables. When programming tool length offsets with a G44, the
geometry offset value should be negative and the wear offset data should
be positive when entered into the offset tables.