Helicopter bearing precision manufacturing

42-inch bearing held to ±0.005 inches (one part in 10,000) by submerging the bar mic in the same oil bath as the workpiece. Used to illustrate that thermal expansion governs precision at parts-per-million tolerances.

I've worked on some helicopter bearings on some of the world's largest helicopters. 42-inch bearing, half-inch raceway, quarter-inch balls. It was 42 inches plus or minus 5 thousandths — that's one part in 10,000. So they put the mic — which is just a big bar mic to measure this 42 inches — right on the bed that was filled with oil while they were machining, so it'd be at the exact same temperature. Because the measuring tool has to be at the same temperature as the part you're measuring, or you're going to get an inaccurate measurement. So they bathed the measuring tool in the same cooling and lubricating bath they did the part. So again I'll ask you: what is the world's most precise manufacturing, in parts per thousand or parts per million of accuracy tolerance?