Editor's note: Columnist Hill Cox continues his multipart series that focuses on developing custom calibration procedures particular to an individual manufacturer's circumstances. Accurately written calibration procedures are critical in today's manufacturing environment, especially as new standards require detailed documentation.
Most calibration of measuring instruments is functional or "indirect" calibration. This means that the manufacturer only needs information regarding the limits the tool works within rather than all of its performance elements. This makes calibration easier and cheaper.
There is a great deal of freedom when it comes to what is included in a calibration procedure, but common sense dictates that there is a minimum amount of information to be reported. A calibration report should include:
• What is to be calibrated and what is to be reported. When calibrating a micrometer, how many points through its range are to be checked? When the calibration is completed, is the tool to be reported to be within plus or minus a given value overall or is a correction chart to be provided that shows deviations at the calibration points?
• The principal calibration tools to be used. This helps avoid situations where someone gets clever and decides to use a handheld micrometer to calibrate a plug gage, instead of correctly using a comparator. It also establishes a basic element for an uncertainty budget for the calibration.
• Accessory items such as gage blocks and thread wires. If several sets of gage blocks are in use, and they are all in the same accuracy range, they can be referred to collectively. Conversely, if there is one set that is more accurate than the others, and it is the only set suitable for a particular calibration, specify that fact.
• Instructions for using the tools, accessories and for making the report. Adequate detail will ensure that the calibration job is done correctly each time it is undertaken.
• Decision points in the process, if any. If the technician has been instructed to check for repeatability, what must it be within? What does he do if it isn't there? There may be a number of points in the procedure where the calibration procedure should be halted, the supervisor called in or the marines called out. Specify those instances.
• Environmental conditions. If your laboratory is subject to significant temperature fluctuations, some of your calibration work will have to wait until conditions are appropriate, or the calibration may need to be outsourced. The calibration procedure should indicate when such decisions are to be made, especially if there isn't an adequate laboratory in which to do the work. This topic should also be covered in procedures dealing with work done on equipment outside a laboratory setting, such as with coordinate measuring machines, optical comparators and surface plates.
Other considerations in setting a calibration procedure include setting an uncertainty budget for each procedure. The budget must relate to the specified tools, masters and accessories used, and the environment in which they are used.
Sooner or later documentation will be needed to demonstrate that the calibration procedure has been verified, through round-robin studies, along with evidence that the procedure is being followed. In addition, supporting documentation may be needed regarding acceptance criteria.
It becomes quickly evident that a simple calibration procedure has the potential to attract a lot of paper. Because a manufacturer's focus is the calibration procedure and not getting lost among paper, it's handy to have a file folder for each procedure. Using such a method, all the necessary documents are readily accessible when a review or revision is under way. Keep the master printed copy of the calibration procedure in the file, and use only "approved" copies in the laboratory and elsewhere.
Most calibration procedure documentation can be done on a computer. But, photocopies of related documents may be easier to acquire and keep than electronic scans. The use of an old-fashioned file folder means everything an auditor may want to know about the calibration procedure can be kept in one place, without concern that the computer's hard drive will crash.