Whether you’re a small machine shop or a large multinational manufacturing giant, you no doubt use any number of dimensional gages to maintain the standards of quality in your production. The accuracy of these instruments must be periodically checked to ensure that they are correctly performing the job for which they are intended. And small shop or large, more and more requirements are being placed on you to document this performance and ensure that all gaging is under control.
This points out the two critical elements of the gage calibration process. The first is the physical part, the actual making-the-measurement portion of the process. The second is documentation. Both must go hand in hand, like air in a tire, in order for you to produce and document good parts.
If you are a manufacturer without expertise in calibration, how do you select a gage calibration 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 such a manual, and if it’s unavailable, look elsewhere.
Another key point is to look at the master or reference standard on a calibration certificate issued by the calibration lab in question. You’ll likely see a statement that describes the uncertainty of the measurement as being within a certain range. In this case, smaller is better. This statement is the result of a carefully calculated measurement analysis. To get to these numbers, the lab had to take into account all aspects of the measurement—the standards, the calibration technicians, the equipment and the environment—and come up with a list of potential errors and the resulting uncertainty.
More and more labs are also participating in organizations as a way to document their measuring capabilities and procedures. Calibration labs that work with A2LA or NVLAP are taking steps to demonstrate their adherence to proper procedures, training and documentation in an effort to assure their customers that they are working to their documented level of expertise.
But probably the best way to confirm that a calibration service can best meet your needs is to make a visit to the facility. A good service will welcome you with open arms. Don’t be afraid to ask questions—lots of them. What are its areas of expertise? How are the technicians trained, and what is their level of experience? What test equipment is used, and to what standards can test methods be certified (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 to the facility is the best way to get answers to these questions.
Calibration providers issue a certificate of calibration for every gage tested. These are essential for users to document the proper calibration of their devices. At minimum, they must include:
• The serial number and description of the gage tested, as well as the serial number of the gages 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), and a serial number identifying the NIST test upon which the calibration house’s own standard is based.
• The accreditation body that the calibration provider is part of and its certification number.
• The reference temperature under which the calibration was performed.
• The name of the customer, and name and address of the calibration service, as well as the date of calibration and signature of the technician.
• Test results, such as error in the gage, measured at appropriate intervals across its entire range.
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 such as circular geometry gages and CMMs, the calibration technician will do the service on site. The calibration service will come prepared with NIST-traceable gage blocks, precision balls, a thermometer and any other standards needed to perform the job. The field service calibration should be documented and controlled under the same organization that would be used for the provider’s lab—something to check for the next time your provider comes in to check out the measuring instruments on your floor.
Besides reminding you of when your gages need to be calibrated, some calibration providers offer other services that may be important to you. These services may include storing your records for online access/recovery; keeping histories of your gages and masters, including calibration and issues tracking; and online status of calibration services.
The key is to remember that there are two steps to the calibration of your gages and standards: the calibration process itself and the resulting documentation of the results of that process. Both are equally important. If a provider can’t meet your documentation needs, keep looking for a lab that can.