In our work we use gages and other measurement devices on daily basis. And as most things we use in our lives, gages are subject to wear and tear over time.
If this process is not carefully monitored, devices will very possibly fail to measure parts accurately, thereby giving unreliable information about processes and outcomes. Calibration of them requires a disciplined approach: regular intervals between two calibrations, comparison to known reference values. Without this way of performance, the gage may continue to measure within the allowable tolerances. On the other side, if the gage is identified as out of tolerance for various calibration checks, the challenge is in knowing the point at which this gage had begun to have accuracy issues and determining how many parts need to be rechecked again.
When to calibrate?
Find out and weigh what calibration interval is appropriate for your device. Consider the use and environmental conditions, these can have influence for how often those gages are calibrated. Reevaluate requirements that have been passed down from a customer or from regulations described in an industry standard. The financial cuts or a shrinking quality department should not to have an influence on the decision about calibration intervals without regard for the increased risk that this causes for measurement systems accuracy.
Calibration intervals must be set only after thorough analysis. As a baseline interval for start can be used the recommendations from the gage manufacturer or articles in professional journals, but the continued effort is in determining what the best calibration interval is given the device’s anticipated use, environment, and customer requirements.
The best time when to calibrate a gage is the moment just before it begins to lose its accuracy. But we cannot identify this moment so easily. For this reason is useful a stability study to identify when an abrupt change that impacts the gage’s accuracy occurs. The setup for such a study begins after a gage has been calibrated. A user is selected to act as the appraiser for the duration of the study. A part is then chosen, preferably one that is stable and not subjected to drastic environmental changes. A schedule begins for the user to take a measurement reading on the part every few days and record this reading on a log. A simple control chart generated from this data can highlight drastic changes or gradual drifts to indicate wear in the gage.
To get the most of such a study, some considerations must be noted:
- to measure by the same operator,
- to measure near the same time of day, if is done on shop floor,
- to measure the same part,
- the values of measurement should be recorded as they are, not what you may hope that they are.
Three possible results of performed stability study:
- The gage is consistent and stable during the time between its scheduled calibrations. Evidence to support the decision to lengthen the calibration interval is provided.
- The gage begins to wear prior to its scheduled calibration. Shortening of the calibration interval can be justified to avoid risk of using the gage past the point where it is stable.
- A special case is caught and corrected before it negatively impacts the measurement process. An unplanned calibration can be scheduled to address the issue and have the gage back to peak performance with minimal downtime or rework.
Regardless of which result will occur, the quality department or calibration technicians will begin to save time, money, and effort. Unnecessary calibration events can be postponed while focus can be directed to gages that need greater attention. While experience and intuition can be a great asset to any calibrator, evidence and data can significantly reduce the risk of relying on wrong chosen calibration intervals. Continued examination and analysis of gage calibration intervals benefits the whole organization and embodies the spirit of continuous improvement.
Author: Jana Loskotova