Thread Axis : The axis of pitch cone of the thread, and the longitudinal central line through the threads. In basic thread design, all length measurements are related to the thread axis. (RP 5A5, RP 5B1)
The Mudblogger (Oilfield, Energy and Related Photographs), Oil Field Glossary, TX, USA 2014. http://mudblogger.com/oilfield-glossary/, (Oil - Amer) {777}