The Essentials Of Gage Calibration
All gaging equipment must be calibrated periodically to ensure that it’s capable of performing the job for which it’s intended—measuring parts accurately. Establishing and maintaining a regular program of gage calibration is necessary not only to maintain quality, but also for customer requirements.
More and more OEMs demand that suppliers document their quality efforts from start to finish. ISO 9000 is one more manifestation of this trend, and it is forcing companies to examine their calibration programs, identify their weaknesses and improve wherever possible.
Some large companies can justify the cost to hire and train specialists in gage calibration methods and to supply them with the equipment and resources they need to perform virtually all calibration duties in-house. For most machine shops, however, the economical approach is to hire a calibration service.
ISO 9001, which applies to all manufacturing operations, requires suppliers to calibrate “all inspection, measuring and test equipment and devices that can affect product quality at prescribed intervals, or prior to use, against certified equipment having a known valid relationship to nationally recognized standards—where no such standards exist, the basis used for calibration shall be documented.” (ISO9001.4.11) Let’s elaborate on some of these points:
“Prescribed intervals” usually translates to a minimum of once per year. When annual calibration is inadequate to ensure accuracy, a shorter interval must be established.
“Certified equipment having a known, valid relationship” means that the calibration house must have its own equipment certified. In the United States, “nationally recognized standards” implies the National Institute of Standards and Testing (NIST), although other standards, such as DIN, are used to satisfy overseas customers.
“Where no such standards exist,” usually refers to highly specific industries or products. The manufacturers must develop their own standards and test methods—for example, a foam pad of known density used to master a chocolate-pudding-consistency gage.
Calibration houses issue a certificate of calibration for every gage tested. These certificates are essential for users to document their calibration programs. At minimum, they must include:
• A statement referencing the accreditation standards to which the laboratory adheres, the scope of the accreditation, the accreditation body, and its lab code.
• The serial number and description of the gage tested.
• The serial number of the gage(s) used to perform the testing.
• The level of uncertainty of the calibration—in other words, the tolerances of the data. This is an important number—the smaller the number the better. And, it may very well determine who can meet your calibration requirements and who cannot.
• A serial number identifying the NIST test upon which the calibration house’s own standard is based.
• The reference temperature under which the calibration was performed.
• The name of the customer as well as the name and address of the calibration service.
• The date of calibration and signature of the technician.
• Test results—error in the gage—measured at appropriate intervals across its entire range.
• If the gage is adjusted subsequent to testing, it must be recalibrated, with results as above.
Some calibration service providers automatically remind their clients which gages need to be calibrated and when. Most gages can be boxed and shipped to the calibration house, although in the case of large, elaborate gages—such as circular geometry gages, CMMs and others—the mountain must come to Mohammed. The calibration service will come prepared with NIST-traceable gage blocks, precision balls, a thermometer and any other standards needed to perform the job.
Since the mid 1990s, there has been a revolution in setting standards for calibration laboratories. Introduced in 1994, ANSI/NCSL Z540-1 was based on two previous standards: ISO/IEC Guide 25 and MIL-STD 54662A. In 1999, the international community released 17025. ISO/IEC 17025 has been accepted by Mutual Recognition Arrangement as the international standard for accreditation of calibration and testing laboratories.
Laboratories meeting the requirements of ANSI/NCSL Z540-1 do not necessarily meet the requirements of ISO/IEC 17025. For those laboratories that need both accreditations, special additional requirements are needed to issue an accreditation listing both ISO/IEC 17025 and ANSI/NSCL Z540-1.
Getting this accreditation is not a simple task. One has to work through an accreditation body, such as NAVLAP or A2LA, and demonstrate processes and procedures that are well documented and adhered to. The labs are subjected to audits every 2 years, which may include auditors from NIST. Also, the accreditation has a specific scope, which means that a lab may be able to do gage blocks, master rings and discs, but not threads. So be sure to look for a lab’s scope of accreditation to ensure that they can do what you want them to do.
Finally, watch out for labs that say they are “compliant” with ISO/IEC 17025. Compliance really means nothing: Either you are accredited or you are not.