Correct a registered grinder: its name, burrs, collar scheme, nominal step, layout, range or notes. Only the keys you send in fields change; send null to clear an optional field (an empty string also clears a text field, but not a number or a closed list). Required fields can be changed but not cleared. fields may be omitted when only reason is sent. setting_scheme, nominal_step and components_spec describe the collar every recorded grind label is read through: call with dry_run first to see which recorded settings would read as a different position, and tell the user before saving. Saving records the change in the grinder's maintenance history; when a locked recipe's setting moves it also opens an unmeasured calibration epoch (set_maintenance_offset resolves it). The collar's current setting is marked assumed when it reads differently, or cleared when the new collar cannot read it, and measured offsets are converted when a click changes worth. Changing direction_convention is a correction: it moves no number, but every finer/coarser said about this grinder was the wrong way round.