10.4 Agreeing
Test Procedures
It is essential that in every case the user
and the manufacturer agree on the necessary and economically
achievable tolerances at an early stage. It is equally important
that consistent measurement procedures (standardised measurement
procedures, wherever possible) are agreed for assessing the
components.
Example 1:
The straightness of large-volume components (slabs, beams
etc.) is usually measured statically in accordance with DIN
40 680 (see Figures 209 and 210). It can be appropriate to
use dynamic measurements to achieve a more meaningful assessment
of the quality of rotationally symmetrical parts such as,
for instance, transport rollers. This is done by turning the
roller once around its own axis of rotation and measuring
the total excursion. In this way it is possible to determine
errors in straightness as well as in ovality. Both are relevant
to the later use of the component.
If the method of measurement is not specified in advance,
customers and manufacturers are presented with wide scope
for interpretation, with corresponding points of friction.
Example 2:
Ceramic sealing discs or rings are checked for flatness using
what is known as the interference method. This is an optical
measurement process by means of which variations in a surface's
flatness of < 1µm can be made visible.
A beam of light of a defined wavelength (monochromatic light)
is directed on to the component through a plane faced glass.
The light is reflected from the component's surface. Extremely
small variations in flatness have the effect of yielding different
refractive deviations on the return journey through the plane
glass, making errors visible and measurable.
The slabs of plane glass are strictly standardised into different
precision classes.
Until about 15 years ago, helium vapour lamps were used to
create the monochromatic light. For a number of reasons, sodium
vapour lamps have since then become increasingly established.
Two types of light source are thus represented on the market.
Because of the different wavelengths of helium and sodium
(although only specialists can distinguish the colours with
the naked eye) the two light sources will provide different
results from the same component.
This example illustrates how, as tolerances become tighter,
it is not only necessary to use the same measurement procedures
but even the same types of measuring instrument!
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