Comparative Gages and Temperature Compensation
The use of electronic temperature compensation in gaging has become a valuable tool in improving accuracy as well as gage repeatability and reproducibility (GR&R) in harsh manufacturing environments. Temperature compensation is needed when the expected errors from environmental temperature variation take up more than 10 percent of the part tolerance.
From where does this expected error come? Virtually all materials expand with temperature. Whether it is aluminum, steel or carbide, they all have known thermal expansion rates of how much the material will grow per degree of temperature increase. There are modifiers to this, of course. For example, all parts have some mass, and the larger the part, the longer it takes to stabilize to the surrounding environment. Although the parts may be steel, their actual composition may vary slightly from the reference steel, and slight variations will be seen. It all comes down to how much error can be accepted.
Comparative gages don’t actually measure something (like a caliper or micrometer), but they do compare a part to a known master. This tells the user whether the part is larger or smaller relative to the master, hence the term “comparative.” These types of gages are usually set to measure a specific feature, making the gage fast, easy to use and more accurate than other portable, handheld measuring devices.
However, a gage—even a comparative one—is only one part of the measurement process. The total process for a comparative measurement consists of the gage, a master, an operator, a part and the measurement environment. The environment certainly affects the master, gage and part because they are metal, and they expand with increased temperature. I would imagine temperature affects the operators as well, but I don’t know about the expansion part.
In any case, with the part, gage and master used in the comparative gage a certain amount of temperature compensation is constantly occurring. When the part, gage and master are all at the same stabilized temperature and the gage is mastered, the measurement is apt to be fairly accurate: certainly less than the 10 percent that might require additional temperature compensation tools. The reason the system is accurate is that if the materials for the part, gage and master are the same and they have had a chance to stabilize to that temperature, then they all have changed the same amount. For instance, the reading is the same whether the temperature of the three items is 65ºF or 85ºF. So, in effect, the gage is always compensating for the environmental changes.
As with most things, there are “buts.” There are two important aspects to consider. The first is when the materials of the part, gage and master are not the same. For example, the parts may be aluminum while the gage and master are steel. In this situation, the comparative gage would not compensate for temperature as well as it would if all three were made of the same material. Mastering on a steel part and measuring aluminum, which has a different coefficient of expansion, means there would be error in the measurements.
The second source of error in this concept is when the three items are not all the same temperature. Two things can happen here. In one scenario, all three components are the same material, but the part just coming off a lathe may be a different temperature than the gage and master. The comparative gage would not be able to compensate for this difference, and some error would be seen. However, this may be manageable. If the temperature difference is minor, the part size is small and the tolerance is not tight, the error budget can still be maintained.
The other scenario occurs with sudden changes in temperature. The part, gage and master are different sizes, so they have different masses and their rate of change with temperature will vary slightly. Therefore, areas where temperature varies quickly in short periods of time—such as near opening and closing doors or under A/C vents—can be problematic to the ability of a comparative gage to adequately compensate.
As you are looking at ways to get the most from your measurement, and you have concerns about what temperature is doing to your process, take the time to analyze the situations with which you are faced. Keep in mind the part tolerance, the materials and the environment. You may be able to make some pretty good measurements under difficult environmental conditions if you use a comparative gage as part of a known measuring process.
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