Introduction: Don't Trust That Factory Test Report Too Much

You finish commissioning. The positioning accuracy is dead-on, repeatability looks gorgeous. You figure the job's done.

Six months later, yield starts tanking. You dig in and find the position has wandered by a handful of microns. The maintenance guys tell you the guide sounds rough, and the ballscrew feels a little loose. You stare at the shiny factory test report and wonder what the hell happened.

I've seen this movie way too many times. In automation, initial accuracy is just for the spec sheet. Long-term accuracy is what actually keeps the line running. That dozen or so test cycles the stage did back at the factory tell you nothing about how it'll behave after millions of starts and stops, under real load, with temperature swings. The skill that really matters is making precision stay, not just making it show up. Here's what I've picked up along the way—some of it the painful way.