Siemens SINUMERIK 840D sl Programming Manual

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2.9 Measuring strategy for measuring workpieces with tool offsetThe actual workpiece dimensions must be measured exactly and compared with the setpointvalues to be able to determine and compensate the actual dimensional deviations on theworkpiece. An offset value can then be ascertained for the tool used for machining.FunctionWhen taking measurements on the machine, the actual dimensions are derived from the pathmeasuring systems of the position-controlled feed axes. For each dimensional deviationdetermined from the set and actual workpiece dimensions there are many causes whichessentially 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 internalmeasurement (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 wearor thermal expansion of the leadscrew.● Accidental dimensional deviations, e.g. due to temperature fluctuations, coolant or slightlysoiled measuring points.Assuming the ideal case, only those dimensional deviations that are subject to a trend canbe taken into account for compensation value calculation. Since, however, it is hardly everknown to what extent and in which direction accidental dimensional deviations influencethe measurement result, a strategy (sliding averaging) is needed that derives acompensation value from the actual/set difference measured.Mean value calculationMean value calculation in conjunction with measurement weighting has proven a suitablemethod.When correcting a tool, it can be selected whether a correction is made based on the actualmeasurement, or whether an average value of the measurement differences should begenerated over several measurements which is then used to make the correction.The formula of the mean value generation chosen is:kDMvMvMv ioldoldnew--=Mvnew Mean value new = amount of compensationMvold Mean value prior to last measurementk Weighting factor for mean value calculationDi Actual/set difference measured (minus any empirical value)The mean value calculation takes account of the trend of the dimensional deviations of amachining series. The weighting factor k from which the mean value is derived is selectable.Description2.9 Measuring strategy for measuring workpieces with tool offsetMeasuring cycles42 Programming Manual, 12/2017, 6FC5398-4BP40-6BA1