These micrometers come with a calibration certificate traceable to NIST that states they've passed a test for accuracy. The anvil and spindle are shaped for measuring small grooves and keyways. Each micrometer has a ratchet-stop thimble with a speeder knob to reduce the time it takes to open and close the micrometer. A constant-force mechanism stops the micrometer’s spindle from tightening once it contacts the surface of the measured object. This ensures repeatable measurements and prevents over-tightening. When the spindle stops tightening, the thimble stops turning, but the speeder knob will continue turning with a clicking sound.