Siemens SINUMERIK 840D sl Function Manual page 204

B1: Continuous-path mode, Exact stop, Look Ahead
4.3 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 feedrate 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)
Example:
Limiting velocity characteristics, whereby:
– Override = 50%, 100% or 150%
– Number of LookAhead blocks = 4
– MD20430 $MC_LOOKAH_NUM_OVR_POINTS = 2
– MD20440 $MC_LOOKAH_OVR_POINTS = 1.5, 0.5
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Function Manual, 01/2015, 6FC5397-0BP40-5BA2
Basic Functions