Optical flats are polished surfaces used to measure the flatness of machined surfaces; they are vital tools to enable precision engineering and manufacturing. This flat was used for more than 40 years by Sydney firm Ross Simpson Engineering Pty Ltd, whose other link with this Museum is that it generously reconditioned parts of locomotives 3830 and 3265 at no charge.

This flat could have been used to determine flatness to approximately 25 nanometres (one millionth of an inch). Both sides of the flat are polished to enable flatness to be determined to the specified resolution. In use, the flat is placed on the surface that is being checked and illuminated from above. Optical interference fringes (bands of light and dark) form at the contact surface and can be seen through the top of the flat. For higher resolution and easier measurement, the flat is illuminated with monochromatic light (light of one wavelength and thus of one colour). Straight parallel fringes indicate that the surface is flat (to within 25 nanometres, or a wider margin of error if white light is used for the test). Curved or irregularly spaced fringes indicate convex, concave, spherical or cylindrical departures from flatness.

Flats are calibrated on a regular basis by registered testing laboratories to determine whether their flatness has changed. This flat was last calibrated in 1995 by the National Measurement Laboratory, part of CSIRO. In 2004 this lab became part of the newly formed National Measurement Institute, which brought together the National Measurement Laboratory, the National Standards Commission and the Australian Government Analytical Laboratories.

Jesse Shore, Curator
13 February 2008

The optical flat was made before 1963 by Lapmaster, a metrology company that was established in 1948.

It is made of glass, perhaps a clear optical fused silica. This material combines durability, resistance to scratching, and low thermal expansion.

This flat was used by Ross Simpson Engineering Pty Ltd, Sydney, NSW, with an optical comparator to measure the flatness of surfaces. Ross Simpson said the flat was used from 1963 to December 2007.

Optical flat, with case, glass / leather / velvet / metal, made by Lapmaster, c. 1960, used by Ross Simpson Engineering, Sydney, New South Wales, Australia, 1963-2007

The optical flat is a thick clear glass disc with top and bottom surfaces polished flat, bevelled upper and lower edges and frosted cylindrical side. The flat is stored in a square grey and green leather covered case with blue velvet lining. The flat fits into a cylindrical recess in the case on top of a blue fabric tab which when pulled allows the disc to be easily removed. The case has two hinges at the back and one clip latch at the front. Marked on top of case.

Made: Lapmaster International; ; 1955 - 1963

Used: 1963 - 2007

Three arrow heads (carets), all pointing the same direction, are lightly etched into (or pencilled onto) the side of the disc.
'M21465 LAPMASTER 1/10 X/2' is etched into the side of the disc.

There is a silver sticker on the upper right of the top outside of the case from the "National Measurement Laboratory" with the CSIRO logo and "CSIRO" under the logo. "Date of Calibration" is printed and "June 1995" is handwritten. "Report No:" is printed and "RS25536" is handwritten.

Above the CSIRO sticker is a black Dymo label imprinted "M21465".

2008/128/2

1955 - 1963