8 Master measurement
Measurements that use only one probe are usu-
ally comparative measurements, in other words,
deviations from a nominal value are measured
and displayed. These nominal values are ob-
tained by using measurements on gage blocks,
setting plug gages, or special workpieces with
known dimensions – known as master work-
pieces (“masters” for short). These master mea-
surements are carried out prior to the start of the
actual test measurement.
A distinction should be made between one-
point and two-point master measurements.
In the case of a one-point master measure-
ment, contact is made with only one master
(gage block) and the indicated value compared
with the nominal value noted on the master. This
establishes whether the measured actual value
deviates from the nominal value and, if so, to
what extent. It is assumed here that the devia-
tion established in this fashion is constant over
the entire measuring range and the raw values
of the subsequent workpiece measurements
can then be corrected by the value thus estab-
lished before measurement results are displayed.
For example, if the indicated actual value is + 10
µm larger than the nominal value of the mas-
ter, the raw values of all subsequent workpiece
measurements are reduced by 10 µm before the
measurement result is displayed. In contrast, if
the master measurement returns an actual value
of only 495 µm when the nominal value is 500
µm, 5 µm are automatically added to the raw
value of each workpiece measurement before
the measurement result is displayed.
The probe’s characteristic curve therefore un-
dergoes a parallel shift in the display range (cf.
Fig. 3).
In the case of a two-point master measure-
ment, contact is made with two masters (gage
blocks) of different sizes, one after the other, and
the indicated actual values are compared with
the nominal values noted on the masters.
Initially, as in the one-point master measurement,
measurement on the first (smaller) gage block is
used to establish the deviation from the nominal
value which is then corrected. The probe’s char-
acteristic curve again undergoes a parallel shift
in the display range (see Fig. 4A). All raw values
from subsequent workpiece measurements are
corrected by the value thus established (i.e. re-
duced if the measuring chain is too sensitive, or
increased if the measuring chain is too insensi-
tive). In the first stage therefore, the probe’s char-
acteristic curve again undergoes a parallel shift in
the display range.
In addition, however, a correction factor is cal-
culated from the deviation between the two
nominal values in the first measurement (on the
smaller gage block) and the deviation between
actual and nominal values MSTR.MAX — MSTR.
MIN). This factor is then used to correct the
slope of the probe’s characteristic curve.
It is therefore important to decide whether a
one-point or two-point master measurement is
required before initiating a measuring series. The
corresponding methodology then has to be set
in the catalog of functions and parameters (cf.
Chapter 8.1).
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