Siemens SINUMERIK 828D Function Manual page 174

B1: Continuous­path Mode, Exact Stop, LookAhead
3.3 Continuous-path mode
• Definition of override points
If the velocity profile of the following block velocity is not sufficient because, for example, very high override
values (e.g. 200 %) are used or a constant cutting rate G96/G961 is active, with the result that the velocity
must be further reduced in the following block, LookAhead provides a way of reducing the programmed
velocity over several NC blocks.
By defining override points, LookAhead calculates a limiting velocity profile for each value. The required
velocity reductions for the current override are derived from these profiles.
The calculated maximum value of the velocity profile is limited by the maximum path velocity.
The upper point should cover the velocity range that will be reached by the maximum value set in the machine
data:
MD12030 $MN_OVR_FACTOR_FEEDRATE [n] (evaluation of the path feed rate override switch)
It can also be reached via the value of the machine data:
MD12100 $MN_OVR_FACTOR_LIMIT_BIN (limit for binary coded override switch)
In this way, a reduction of the velocity continuing into the block in which it is programmed can be avoided.
If velocity reductions across block boundaries are required at a 100 % override, a point should be set in the
lower override range as well.
The number of override points used per channel is specified in the machine data:
MD20430 $MC_LOOKAH_NUM_OVR_POINTS (number of override switch points for LookAhead)
The associated points are stored in the machine data:
MD20440 $MC_LOOKAH_OVR_POINTS (prepared override velocity characteristics with LookAhead)
174
Function Manual, 09/2011, 6FC5397-0BP40-2BA0
Basic Functions