An extra-deep throat gives you a long reach for measuring sheet metal and other large items. These micrometers have a ratchet-stop thimble with a speeder knob to reduce the time it takes to open and close the micrometer. The thimble also has a constant-force mechanism, so that once the spindle contacts the surface of the measured object, it will stop tightening. 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.