Two decades ago, when checking a workpiece dimension on the shop floor, you probably never fully believed what your gage was telling you. Instead, you would send the piece off and hold your breath while waiting to see if it would be accepted or rejected by someone in your inspection room or laboratory.
Today, things are very different. There are probably documents in your company that cover the history of a certain gage, its operation, the training of the operators using it and the standards it was checked against. Gage doubting has been replaced by hard facts and figures that document gage performance.
Another change is that today’s industry cannot afford to wait for critical dimensional gaging results to come back from a lab. Gaging used to be one of the most critical and least-understood operations in machine shops. Today, it’s more important than ever—so important that accurate gaging has become everyone’s job.
The metrology industry has also become more sophisticated in the use of the term “accuracy.” There is no one condition that determines the accuracy of a gage or system. Today, the term “accuracy” is considered so nebulous that it is more often used as a general gage qualifier, or as a concept, than as a specification.
You will no longer see a simple accuracy statement relative to a gage’s performance. To get an understanding of how well a gage will perform, you must evaluate several different performance characteristics. It is acceptable to say that, based on specific performance characteristics, one gage has better performance than another. From that, you may deduce that one gage is generally more accurate than the other. However, you will not see a single number tied to accuracy.
A number of specifications have been standardized to help define gage characteristics. Combined, they give an indication of accuracy. First are two terms that describe the physical characteristics of most gages or gage systems:
Discrimination: The ability to perceive differences between things. In gaging, discrimination is the largest change that an indicating device can undergo before registering a change on the indicator of the instrument.
Resolution: This is the gage’s ability to distinguish beyond its discrimination limit. On an analog gage, it is the smallest difference between indications, or gradations, on the device’s display that can be distinguished meaningfully. On a digital gage, this value is called the digital step, and it is equal to the least-significant digit.
The following terms are related to the performance of a gage or gaging system. When combined, they determine whether one gage has better accuracy than another.
• Repeatability (precision) is the ability of a gage or gaging system to provide similar indications for repeated applications on the same part and under the same conditions in a short period of time. These conditions include the measuring procedure, operator and the environment in which the tests are conducted.
• Error (of indication) is the difference between what the gage is reading and the true value of the corresponding input—typically determined with a reference standard.
• Calibration of the measuring equipment is a set of operations that, under specific conditions, establishes the relationship between the quantities indicated by the measuring system and the corresponding values realized by the reference standards.
• Hysteresis is a property of the measuring equipment in which the indication of the equipment depends on the direction relative to the preceding input. Also commonly known as “lag,” it is seen when the response to a decrease in an input variable is different from the response to an increase.
Whether the gaging instrument or system consists of a dial, digital indicator, bench amplifier, surface or form system, its specifications should call out most, if not all, of these performance characteristics. Only by knowing and comparing them can we begin to make decisions regarding relative accuracy.