In a perfect world, there would be just one international standard.
Modern Machine Shop, George Schuetz
From the monthly column: Quality Gaging Tips
In today’s global economy, standards for inspection products or inspection product testing conditions are invaluable to ensure that products perform similarly by different gaging manufacturers in different parts of the world. In a perfect world, there would be just one international standard, and every inspection product designed for that one standard would be the same. If only it were that simple.
Fortunately (or unfortunately, depending on your viewpoint), people in different parts of the world have different ideas and priorities for product standards. Just as there are different languages and different interpretations of identical words—and even differences in agreeing on a common currency or measuring system—it’s difficult for people to agree on a single way to define a product. This results in several different inspection product standards in various areas of the world for the same product concept. Often, these standards are very similar, differing only in a mathematical formula. However, sometimes the standards are based on completely different philosophies for how the apparatus is used. Thus, the specifications in the standard can be significantly different. Let’s take a look at some examples starting with portable surface gaging.
The design philosophy for surface-finish gages has been around for 70 years. It’s pretty simple: A very-fine-tipped sensor is moved across the part at a controlled speed for a controlled distance, and the results are gathered and analyzed. While stylus and drive speed have been agreed upon for the most part, the analyses are often rooted in individual needs, understandings and applications. Thus, as you look at most surface-finish gages today, you will see that there are selections for common standards such as ISO, ASME, JIS and MOTIF. Whether measuring a roughness parameter in Germany, Japan or the United States, it’s important to understand what standard should be referenced. Otherwise, parts can be accepted or rejected based on the wrong analysis method.
The same thing can happen when calibrating dial or digital indicators. As with surface-finish gages, there are ASME standards for calibrating dial indicators and similar standards for DIN/ISO and JIS. However, with dial indicators there is also a slight difference in philosophy about how they are used in the United States compared to the rest of the world. This is reflected in the calibration process.
We shouldn’t be too general here, but if you look at most U.S.-manufactured dial indicators, you will notice that they are typically very short-range indicators often used in comparative gaging applications. On the other hand, DIN and other metric-based indicators have a relatively long range. This difference in application philosophy means there is a slight difference in the way the specs are written for testing the indicators.
U.S.-manufactured indicators are generally governed by an ASME standard, B89.1.10. Because U.S. indicators are shorter range and thought of more as comparators, the specifications are all relative to a zero-based measurement, and specification limits are made relative to the zero position. Thus, the accuracy limits might be specified as ±1 count, meaning all the values must fall within two gradations of zero based on the range being measured.
DIN/ISO is a little different. DIN indicators seen in DIN 878 typically have a longer range for absolute measurement applications, so the DIN/ISO specification calls for an absolute variation between the minimum and maximum deviations seen over its range. Depending on the standard being used, this may be referred to either as the “span of error” or “error of indication.” For example, the specified limit for a DIN indicator might be 5 µm over its entire range.
So it is important to know what style indicator you are working with and what specification you are testing it against. Using the example above, a DIN indicator having a total variation of 4 µm might not pass an ASME test depending on where the zero reference is.
The same is somewhat true for digital indicators. However, digital indicators came a long time after dial indicators, and in some ways the standards are just being brought up to include these indicators. While there is an appendix in the ASME B89.1.10 standard for digital indicators, it is non-mandatory. There currently is no digital indicator standard from DIN/ISO, although it is being worked on and should be available soon. Again, manufacturers in Europe have worked on their own definition of how to test and what the specified limits are for their digital indicators.
There is nothing unusual about this. Manufacturers can and often do define what specifications their products are built to. As a user, it is important to know and understand what the differences are and which specification to test against, whether it’s an International Standard, a National Standard or the manufacturer’s brand-specific specification.