Rockwell Automation 8520-MUM 9/Series CNC Mill Operation and Programming Manual Documentation Set - Manual (Page 569)

Cutter Diameter Compensation
(G40, G41, G42)
Chapter 21
21-51
At times (especially possible during cutter compensation) the control may
not have enough look-ahead blocks to correctly execute the current block.
When this happens, the control automatically starts disabling the block
retrace feature. The block retrace feature uses one set-up buffer for every
retraceable block. The number of re-traceable blocks is set in AMP by the
system installer (a maximum of 15 is possible). As the control starts
disabling the block-retrace feature, it decreases the number of available
retraceable blocks until either there are sufficient set-up buffers available
to successfully execute the current program, or until there are no more
block-retrace blocks left. The control displays a message on line 2 of the
CRT if it needs to disable some of the block-retrace feature.
Note that using too many buffers for block-retrace is not a recommended
method of operating the control. The larger the number of look ahead
blocks that the control is scanning, the more efficiently the control
executes programs. It is recommended that the number of retrace blocks
that is available to the block-retrace feature be limited to the minimum
number required on the system (set the number of block-retrace blocks as
low as possible for a specific application). This greatly improves program
execution and prevent the necessity of the control deleting block retrace
blocks.
Error detection for cutter compensation blocks can be separated into three
categories:
Backwards motion detection
Circular departure too small
Interference
Backwards Motion Detection
The compensated tool path is parallel to, but in the opposite direction of,
the programmed tool path.
21.7
Error Detection for Cutter
Compensation