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Introduction
This interim note covers some of the problems encountered in the
measurement of large sizes, up to 80 inches (200 mm), during
investigations carried out at the College of Aeronautics. A
description of a new design of stick micrometer for internal diameters
and a micrometer frame for external diameters is included, together
with experimental results obtained in using this equipment.
Problems associated with measuring large sizes
These may be classified under three broad headings:-
a) The influence of temperature on the measurement
b) Problems associated with the stiffness of the measuring
equipment.
(c) The elimination of operator feel.
(a) The influence of temperature
In practice, it may be extremely difficult to maintain both the
component and measuring equipment at the internationally agreed
standard of 20°C. Figure 1 shows the effect of variations in the
temperature of steel components with respect to the Fundamental
Tolerance value (B.S. 1916), expressed as a percentage of the
tolerance grade.
The particular case illustrated is for a 3°C. change in
temperature on steel components, (coefficient of expansion assumed
to be 11 parts in a million/°C), manufactured +o tolerance grade
IT.6.
It can be seen from this graph that in the smaller sizes up to
say 10 inches (250 mm), the percentage of the tolerance taken up by
the change in temperature remains relatively small, but with increase
in size the effect of temperature becomes increasingly significant,
until at 100 inches (2500 mm) 82% of the tolerance has been consumed. |
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