Measuring Strategy And Correction Value Determination; Types Of Dimensional Deviation - Siemens SINUMERIK 840 User Manual

1 Introduction

1.6 Measuring strategy and correction value determination

1.6 Measuring strategy and correction value determination
Precise determination of actual workpiece dimensions is required to determine the actual
dimensional deviations of the workpiece and to be able to correct these during subsequent
machining operations and thus to obtain a permanent high standard of machining accuracy.
When measuring on the machine, the actual dimensions are derived from the path measuring
systems of the position-controlled CNC feed axes. For each dimensional deviation determined
from the set and actual workpiece dimensions there is a multitude of causes which essentially
can be classified in 3 categories.
1.6.1

Types of dimensional deviation

• Dimensional deviations the causes of which do n o t folllow a trend, such as
position dispersion range of the feed axes or differences in measured values between
internal measurements (probe) and external measuring device (micrometer, measuring
machine etc.).
Here, the possibility is provided of using so-called empirical values (R11) which are stored
in separate memories. The set/actual difference determined is corrected automatically by
the empirical value. See the descriptions of the various cycles and Section 2 for more
details.
• Dimensional deviations the causes of which follow a trend, such as tool wear or
thermal expansion of the ball screw.
These deviations are corrected by specifying fixed threshold values (e.g. R34) (see
Section 1.7).
• Random dimensional deviations, e.g. due to temperature variations, coolant or slightly
contaminated measuring points.
Presuming the ideal case, only those dimensional deviations can be taken into account for
correction value determination the courses of which follow a trend. Considering the fact,
however, that it is never known to which extent and in which direction random dimensional
deviations influence the measurement result, a strategy (floating average value generation)
is needed that derives the compensation value from the measured set/actual difference.
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SINUMERIK 840/850/880 (BN)
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