18.9.1 General

The workpiece features can be inspected by

• gauging, e.g. with a functional gauge (covering the entire feature);

• areal contacting, e.g. with a measuring plate or mandrel;

• tracing, scanning (continuously, consecutive), e.g. with a dial gauge or form measuring instrument;

• probing (assessment of a number of points) (discontinuously), e.g. with a coordinate measuring machine.

With tracing, scanning and probing, the surface is assessed by sampling at selected sections or points. Therefore these methods are approximate. The more sections or points are assessed, the more realistically are the geometrical deviations assessed and the less is the measurement uncertainty.

Figure 18.100 shows form deviations whose amplitude cannot be fully assessed because the spacing of probing (assessed points) is too wide.

Figure 18.100: Incomplete assessment of form deviations because of too wide spacing of assessed points

In order to assess a certain bandwidth of deviations the Nyquist theorem should be respected in all directions of the surface, i.e. the spacing of the assessed points including the distances of sections to be traced or scanned should be at least not more than one half (better not more than one seventh) of the deviation wavelength to be assessed.

Figure 18.101 shows the effect of the spacing c of the assessed points on a surface exhibiting sinusoidal form deviations of the wavelength ?. When the spacing c is greater than half of the wavelength ? the wavelength of the sine wave cannot be assessed (Fig. 18.101(a).