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Whether you calibrate your own instruments and gages or send them to an outside laboratory, you must establish a frequency for doing so.
What are the rules for determining how often equipment should be calibrated? The short answer is: The equipment should be calibrated at intervals that will detect problems before incorrect measurements are taken or bad parts passed. How is that for being specifically vague?
The only way you can realistically achieve this state of enlightenment would be to calibrate your gage or instrument each time it is used. Obviously, management would be all over you for suggesting such a thing. As such, some sort of balancing act is required.
Automatically assigning the same intervals to all equipment is easy to say but may be difficult to implement and expensive as well. You could find yourself over-calibrating some equipment at the same time risky hardware is not reined in.
Where do you start? Intervals used by other companies in your industry can be a helpful starting point. Your supplier of the gage or instrument may be able to pass on some guidance, but remember that the intervals you choose must reflect reality. If you are borrowing from someone else's experience, you must compare their situation with your own. Are their people more or less skilled than yours? Is their working environment the same as yours? If they have a clean operation while yours leaves much to be desired, their calibration interval may take too long to accommodate your situation.
Variations within your company's operations need to be considered as well. For example, a micrometer used on the production floor usually would be calibrated more frequently than one used in an inspection environment. A thread gage used to check components made from an abrasive material needs a shorter interval than if a less abrasive material was involved.
If you have absolutely no guidance on determining intervals, start with short intervals and lengthen them after you have some experience to support the change. In the case of an outside micrometer, I suggest a six-month interval. If the results of calibration after a year or so show no significant change, you would be justified in expanding the interval and documentation to support it.
In addressing a digital micrometer, start with a three-month interval if it is being used on the shop floor, as this tends to be more fragile than its mechanical counterparts, and folks tend to forget about low-battery indicators. If addressing a 12-inch or 300-millimeter-capacity micrometer, follow a closer interval because it can suffer damage such as a bent frame, throwing off measuring-face parallelism when dropped on the floor-more so than with a 1-inch or 25-millimeter model.
These comments assume there is no change-generally speaking-in your company's operations. But if you go from a single shift five days a week to working flat-out on a 24 hours a day, 7 days a week basis, your intervals will have to be adjusted to accommodate this. This is especially true with fixed limit gages where it doesn't take much wear to put them outside of acceptable limits. This is why many people set intervals based on use rather than time.
You can navigate these problems by verifying measuring tool performance with simple intermediate checks that can be performed using specifically made masters for items such as micrometers and calipers. Do not forget to record the results.
Most people use the calendar basis but it is a simple way to write themselves into a corner with their quality manual. They do this by stating what these intervals will be for all of their equipment. As a result, when they find required changes, they then have to paper the country with revisions to quality manuals that are decorating their customers' bookcases. The quality manual should indicate how the intervals will be determined and who will perform the task to avoid this problem. The gage or instrument record can show when and by whom they have been changed.
I recommend studying calibration results carefully to avoid disasters. This is most important with fixed limit gages. When you see one that is within your acceptable limits but wearing close to them, it is time to shorten the calibration interval. It is cheaper to recall the gage for calibration early than to recall thousands of parts for inspection.