re: glass scale expansion vs steel

For curiousity, I calculated the temperature change needed to cause a length
measurement error of 0.0001"/ft between glass and cast iron.
The glass is BK7, a typical borosilicate glass used for many purposes in
optics, including scales. CTE=3.8 10^-6 /°F.
Cast iron CTE is taken to be 6 10^-6 /°F, per my materials book.
Then a temperture change of about 4 °F causes a 0.0001"/ft differential
expansion between the glass and iron.

This type of temperature error can easily be corrected by measuring the
scale and/or machine temperature, and applying a fudge factor in software.
A good feature for computer based DRO! Some care must be taken in the
software, particularly if the temperature changes during the measurement,
rather than just requiring a scale change.

Elliot Burke