Original Date: 05/01/2000
Revision Date: 01/18/2007
Information : Calibration Intervals
Previously, test equipment at Northrop Grumman Defensive Systems Division (DSD) was calibrated on an arbitrarily chosen interval of six months. If a piece of equipment was within its tolerance limits three consecutive times, then the calibration interval was increased by one month. If it was out of tolerance once, then the calibration interval was decreased by one month. This process resulted in confusion due to having many different intervals for the same equipment models. Additionally, the company experienced frequent manufacturing test station shutdowns for calibrations; numerous requests for priorities and due date extensions; a backlog of more than 1,000 items; and an increase in items being outsourced. In 1994, Northrop Grumman DSD developed a revised method to optimize test equipment calibration intervals and maintain a maximum in-tolerance rate.
Northrop Grumman DSD’s method employs the U.S. Air Force’s 33K-1-100 guidelines document as the basis for setting the initial calibration intervals. In addition, the company uses a five-year analysis for each family of test equipment to adjust the intervals. All equipment calibration needs are reviewed quarterly to determine if the equipment is still in excess of the 95% in-tolerance limit (per family of equipment).
Since implementing its revised method, Northrop Grumman DSD has realized a 60% reduction in backlog; a 50% improvement in turnaround times; and a reduction in calibration outsourcing which has increased quality. In addition, the average calibration interval has increased from nine months in 1994 to 16.9 months in 2000, and manufacturing test station shutdowns for calibrations now occur only twice a year.
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