Precision manufacturing increasingly calls for tight tolerances. Through
call-out or implied reference to ISO standards, dimensions are usually
specified at 68°F / 20°C (ISO 1 - 1975: “The standard reference
temperature for industrial length measurements is fixed at 20°C” ).
A few degrees departure from 68°F (20°C), by workpiece, master or gage,
can cause certain critical dimensions to change size by an undesirable
percentage of tolerance. It can be important to identify in advance those
critical dimensions that, when measured on the shop floor, may be subject
to significant thermal variation, so that steps to avoid such errors may
be taken.
