The Importance of Scheduling Calibration
November 24, 2008
Many people are familiar with calibration from the stickers on measuring devices and systems that indicate when calibration was last performed. Some people learn about calibration when they go to use a measuring system and find it is not available because it is being calibrated.
There is so much information out there about calibration and calibration frequency that it can be confusing. However, it is important to understand the basics of establishing calibration frequency.
What is Calibration?Calibration is the process of determining the numerical relationship between the observed output of a measurement system and the value of the characteristic being measured based on reference standards. Calibration also describes any steps required to ensure the accuracy and reliability of the procedure.
In reality, the device is considered calibrated if its measurements are within a predetermined range of the known value. If measurements are outside the accepted range, adjustments are usually made to the measuring system to make its readings more closely match the known value. Low-cost measuring devices, in which no adjustment is possible, are simply discarded when they become outside of the accepted range. In some cases a deviation chart is generated to “correct” the measured value.
Do I Even Need to Calibrate?I bought this expensive, highly accurate measuring machine because I know it can measure everything I make. Why should I ever have to calibrate it?
The price of a measuring device has little to do with whether or when to calibrate it. Even though manufacturers design systems for stability and long-term accuracy, many factors can affect calibration. Simple things such as large temperature swings, rough handling in shipment, workplace accidents and operator error can either be a cause for or justify recalibration. It can be pretty expensive to reject parts because they measured out of specification when the real problem was a measuring device giving wrong readings. An appropriate calibration schedule will prevent these situations.
How Often Should My System be Calibrated?Any dimensional measuring device, from a simple dial caliper to a large-capacity coordinate measuring machine, has a measurement range and resolution. It also has manufacturer specifications for measurement accuracy, which the device is usually calibrated and verified to before shipment. It needs to be verified that all devices or systems meet purchased specification after receipt and before use. After verification, such devices can be accepted and used, with calibrations performed as recommended or required.
Recommended or Required?So you thought all calibration requirements were satisfied when you purchased your newest piece of measurement equipment. The equipment was purchased based on the manufacturer’s published specifications and a pre-purchase gage repeatability and reproducibility study or similar evaluation of your requirements. Your measurement equipment should be looked at as a means to guarantee meeting your specific needs and satisfying all internal and external requirements. What you care about is running smoothly, on schedule and with high confidence. Correct scheduling of calibration can directly affect your business.
The manufacturer’s recommendations for calibration usually are supported by international or industry standards, the manufacturer’s years of testing and the inherent quality of the system. However, how you use a system and your historical data help to determine specific requirements for and frequency of calibration. Also, there are many other factors that enter into the frequency of calibration, for example, the environment, part tolerances, regulatory agencies and inherent machine stability.
System maintenance is important in determining calibration frequency. Well-maintained systems will require calibration less often than systems that are poorly maintained or abused. Seemingly minor maintenance issues can be factors.
For instance, failure to properly lubricate stage mechanisms can cause them to perform differently over time, gradually affecting measurements. Failure to keep the measuring device clean can allow debris to contaminate lubricants or obscure optical systems, causing degradation in performance over time. A proper calibration frequency assumes consistent performance of the device, which in turn depends on proper maintenance.
Developing a Calibration Frequency ProcessThe frequency of calibrations is important. In the past it was not unusual for the organization that performed the calibration to put a sticker on the machine with a due date for the next calibration. That practice is not allowed according to ISO/IEC 17025: 2005-General Requirements for the Competence of Testing and Calibration Laboratories. The equipment operator must determine the frequency of calibration.
Ultimately, calibration frequency depends on how important the measuring device is to the manufacturing process. Rarely used measuring machines that are well maintained, kept in a controlled environment and used properly may need calibration once a year. Measuring machines used to support production or used for product and process development might need quarterly, monthly or more frequent calibrations.
Collecting DataIf your system is new, then you must start gathering calibration and verification data immediately to help establish its calibration values and frequency.
How do I find what data I need to help me decide the calibration frequency for my system? Do I use the manufacturer’s specification or recommended calibration frequency? Do I use regulatory suggestions, internal company standards or policies? I don’t have any historical data. How do my part tolerances enter into this collection of data?
Whether you realized it or not, the collection of data started when you first reviewed the system specifications and discussed them with the manufacturer. You also collected data when you asked the manufacturer to measure some of your parts and ran a geometric dimensioning and tolerancing study. You did that, right? Maybe you inherited this system without prior involvement and were asked to set it up and get it running. Or maybe you just acquired a used machine.
It will be assumed that you at least have the manufacturer’s system manual and have had some communication with the manufacturer or sales representative about the installation, service and calibration of the system. The data needed can be collected from all three-installation, service and calibration.
Installation and calibration are the starting points of historical data collection. They form the basis for all future data comparisons.
So, now you have some useful data.
What next? How do you know if the manufacturer’s recommended calibration frequency is valid for your situation?
Without any historical data of your own, you do not have any way of being sure. If it is a new system, you can compare the manufacturer’s final inspection calibration data to the “as found” data at the time of installation. That review will give you the worst-case requirements for calibration frequency-but there is an easier way.
The Golden PartMy parts have tolerances in thousandths. How do I know if errors are from my parts or from the measuring machine?
This is where a golden part can be extremely helpful for determining system changes and using that information to establish the appropriate calibration frequency. A golden part is a reference sample of the production parts that has been thoroughly measured for all the important dimensions. It is simply a part that closely mimics the dimensional requirements you inspect.
After your system has been installed and calibrated, the first step is to measure one of your representative parts with the system. Make measurements typical of those you will be taking on a daily basis, ensuring that you include the most critical features. You have just created your initial golden part data. Store this data in a golden part folder, in a convenient, protected and accessible place-preferably near your system. Make sure you note the part number and material, temperature and temperature change, and conditions of the setup. This is the start of your system history file.
The importance of your golden part is that it embodies all the dimensions that are important for manufacturing like parts. Rather than interpret how any deviations from a calibration artifact might affect a manufacturing decision, deviations in measurements from those of the golden part have obvious, direct implications.
By remeasuring the golden part periodically, you will be able to see how the system is functioning by comparing the measurement data and environmental conditions to your original run. This will enable you to realize when your system is changing, alerting you to the possible need for a recalibration. Over time you will see how often to use the golden part. Its measurements will help establish a calibration frequency initiative.
Unscheduled CalibrationAs important as a regular, scheduled calibration process is, there are times when a calibration may be necessary before it is scheduled. An obvious example of this is when there is a “crash.” If a probe or sensor is driven into a part, a stage runs at high speed into its end of travel or the system is subjected to any abrupt operation that is out of the ordinary, recalibration may be necessary.
Unfortunately, it may not always be as obvious as these examples. A system operator may inadvertently drive a probe into a part, recover and continue measuring as though nothing happened. The indication whether something is wrong should become apparent the next time your golden part is measured. Using a golden part in events such as this can help process and manufacturing engineers identify the real source of the change in measurement results.