Siemens SINUMERIK 840D sl Function Manual page 364

Compensations (K3)
Interpolatory compensation
Leadscrew and measuring system errors
The measuring principle of "indirect measurement" on NC-controlled machines is based on
the assumption that the lead of the ball screw is constant at any point within the traversing
range, so that the actual position of the axis can be derived from the position of the drive
spindle (ideal case).
However, manufacturing tolerances result in dimensional deviations of varying degrees of
severity on spindles (so-called leadscrew errors).
Added to this are the dimensional deviations (differences in reference division) caused by
the measuring system as well as its mounting on the machine (so-called measuring system
errors), plus any machine-dependent error sources.
Sag errors
Weight can result in position-dependent displacement and inclination of moved parts since it
can cause machine parts and their guides to sag.
Large workpieces, too, e.g. cylinders, sag under their own weight.
Angularity errors
If moving axes are not positioned in exactly the required angle (e.g. perpendicular) with
respect to one another, increasingly serious positioning errors will occur as the deviation
from zero point becomes greater.
Compensation table
Because the above mentioned dimensional deviations directly affect the accuracy of
workpiece machining, they must be compensated for by the relevant position-dependent
compensation values. The compensation values are derived from measured error curves
and entered in the control in the form of compensation tables during commissioning. A
separate table must be created for each compensation relation.
The compensation values and additional table parameters are entered in the compensation
tables using special system variables.
Note
Compensation tables can be loaded only if the following machine data are set:
MD32700 $MA_ENC_COMP_ENABLE (interpolatory compensation)=0
or
MD32710 $MA_CEC_ENABLE (enable sag compensation)=0
364
Function Manual, 03/2009, 6FC5397-1BP10-4BA0
Extended Functions