Abstract
The number of tooth profile measuring instruments takes 65% of total number of gear measuring instruments developed since early the 20th century. This means the accuracy of tooth profile most greatly affects gear performance among all of the gear items. This paper describes the change of gear measuring instruments in the world, from mechanical type to CNC type machine. Aiming at the high-speed and high accurate measurement, ultimate measuring method such as "free from the influence of room temperature", "rough centering" and "output of measured deviation based on the gear functional center axis" are proposed. Optical checker to measure whole of the tooth flank at one time is another target of development for the future. In the 21st century, some methods should be established internationally in common to certify the accuracy of profile form measuring instrument. This will be realized by the means of reference master artefact. To certify the accuracy of such reference master, the importance of development of the device and method for this is emphasized.
