This has always been necessary for the purpose of maintaining quality, but there are now additional, external reasons to establish and maintain a regular program of gage calibration: customers' 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 them wherever possible.
Some large companies with thousands of gages can cost-justify hiring or training specialists in gage calibration methods and supplying them with equipment and resources to perform virtually all calibration duties in-house. For most machine shops, however, the economical approach is to hire a calibration service.
ISO 9002, 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." (ISO9002.4.10.b) Let's elaborate on some of these points:
"(P)rescribed intervals" usually translates into a minimum of once per year. Where annual calibration is inadequate to ensure accuracy, a shorter interval must be established. "(C)ertified equipment having a known, valid relationship" means that the calibration house must have its own equipment certified. In the U.S., "nationally recognized standards" implies the National Institute of Standards and Testing (NIST), although other standards, such as DIN, may be used to satisfy overseas customers. "(W)here no such standards exist," usually refers to highly specific industries or products, where the manufacturer must develop his own standards and test methods -- (say, 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:
- 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.
- A statement of traceability to NIST (or other standard).
- A serial number identifying the NIST test upon which the calibration house's own standard is based.
- Reference temperature under which the calibration was performed.
- Name of the customer; name and address of calibration service.
- Date of calibration and signature of the technician.
- Test results: i.e., 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 providers automatically remind their clients which gages need to be calibrated, and when. Most gages can simply be boxed and shipped to the calibration house, although in the case of large, elaborate gages (e.g., circular geometry gages, CMMs), the calibration service will come prepared with NIST-traceable gage blocks, precision balls, a thermometer, and any other standards needed to perform the job.
How can a machine shop without expertise in calibration intelligently select a provider? Naturally, cost and turnaround time are important, but don't sacrifice quality for convenience. Above all, ISO 9000 requires that consistent procedures be applied, and any professional calibration house should be able to document its methods in a procedures manual. Ask to see it, and if it's unavailable, look elsewhere.
Surprisingly, there are no certification standards for calibration labs themselves, so a supplier's reputation is important. Don't be afraid to ask questions -- lots of them. What are his areas of expertise? How are his technicians trained, and what is their level of experience? What test equipment is used, and to what standards can test methods be certified (e.g., MIL, GGG, ANSI)? What quality control methods are in place? What is the physical design of the facility, what are the control tolerances on temperature and humidity, and how is the equipment protected from the effects of outside vibration? A visit may be well worthwhile.
