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It
i s well known that the size of most physical objects changes with temperature. For this
reason it was decided that physical length standards must always be measured or compared
at a temperature of exactly 20°C (68OF). For example, a "1-inch" gage block is defined to
be I-inch long if (and only if) it is at 20°C. At any other temperature it will probably not be
1-inch long even though it is called a "I-inch" gage block. I n addition, if three different
"I-inch" gage blocks made of different materials are compared and found to be exactly
the same length at 20°C they will not be the same length at any other temperature. For this
reason the "true" size of a material object i s commonly accepted as the size of the object at
20°C (68OF). I n order to accurately machine or measure a part
it
is recommended that the
part be at 20°C during the machining or measuring operation. If the path is not at 20°C its
"true" size cannot be defined unless three things are known:
1.
The temperature of the part.
2.
The size of the part at that temperature.
3.
The thermal coefficient of expansion of the part (refer to Appendix D).
The "true" (20°C) size of the part can then be determined using the following relation:
Let
Lo
=
the
size
of
the
part
at
20°C
(68OF)
LT
= the size of the part at Temperature T
a
= Coefficient of Expansion
then
This means that an additional compensation must be made for thermal expansion or con-
traction of the part whenever a laser-controlled machine is operated at a temperature other
than 20°C (68OF). Table 2-7 shows how to calculate the exact compensation factor, and
Table 2-2 shows how to calculate total compensation.
Table
2-7.
Calculation
of
Exact
Compensation
Factor
A
set
of
compensation
factor
charts
is
provided
with
the
Laser
Transducer
System.
However,
the
following
formulas
can
be
used
to
calculate
the
exact
compensation
factor
to
an
accuracy
of
0.1
ppm
if
desired:
T
=
Air
Temperature
P
=
Air
Pressure
R
=
Relative
Humidity
in
%
C
=
Compensation
thumbwheel
setting,
ppm
(XXX.X)
where
N is
given
in
English
and
Metric
systems
by:
English
(T
in
degrees
Fahrenheit,
P
in
inches
of
mercury,
R
in
%)
Metric
(T
in
degrees
Celsius,
P
in
millimetres
of
mercury,
R
in
%)
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