Measuring Strategy For Measuring Workpieces With Tool Offset - Siemens SINUMERIK 840D sl Programming Manual

Description

2.9 Measuring strategy for measuring workpieces with tool offset

2.9 Measuring strategy for measuring workpieces with tool offset
The actual workpiece dimensions must be measured exactly and compared with the setpoint
values to be able to determine and compensate the actual dimensional deviations on the
workpiece. An offset value can then be ascertained for the tool used for machining.
Function
When taking measurements on the machine, the actual dimensions are derived from the path
measuring systems of the position-controlled feed axes. For each dimensional deviation
determined from the set and actual workpiece dimensions there are many causes which
essentially fall into three categories:
● Dimensional deviations with causes that are n o t subject to a particular trend, e.g.
positioning scatter of the feed axes or differences in measurement between the internal
measurement (probe) and the external measuring device (micrometer, measuring machine,
etc.).
In this case, it is possible to apply empirical values, which are stored in separate memories.
The set/actual difference determined is automatically compensated by the empirical value.
● Dimensional deviations with causes that a r e subject to a particular trend, e.g. tool wear
or thermal expansion of the leadscrew.
● Accidental dimensional deviations, e.g. due to temperature fluctuations, coolant or slightly
soiled measuring points.
Assuming the ideal case, only those dimensional deviations that are subject to a trend can
be taken into account for compensation value calculation. Since, however, it is hardly ever
known to what extent and in which direction accidental dimensional deviations influence
the measurement result, a strategy (sliding averaging) is needed that derives a
compensation value from the actual/set difference measured.
Mean value calculation
Mean value calculation in conjunction with measurement weighting has proven a suitable
method.
When correcting a tool, it can be selected whether a correction is made based on the actual
measurement, or whether an average value of the measurement differences should be
generated over several measurements which is then used to make the correction.
The formula of the mean value generation chosen is:
Mv
Mv
new
Mv
old
k
D
i
The mean value calculation takes account of the trend of the dimensional deviations of a
machining series. The weighting factor k from which the mean value is derived is selectable.
42
-
Mv D
= -
Mv old i
new old
k
Mean value new = amount of compensation
Mean value prior to last measurement
Weighting factor for mean value calculation
Actual/set difference measured (minus any empirical value)
Programming Manual, 12/2017, 6FC5398-4BP40-6BA1
Measuring cycles