One Spec, Two Spec, Three Spec, More

One Spec, Two Spec, Three Spec, More
George Schuetz, Mahr Inc.

 In today's global economy, Standards for Inspection Products or Inspection Product Testing Conditions are invaluable in assuring that products made by different gaging manufacturers in different parts of the world perform similarly. 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 on what inspection product Standards should be. 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. The result is that there are different inspection product Standards written for the same product concept in various areas of the world. Often these Standards are very similar and may differ only in a mathematical formula. But sometimes the Standards are based on completely different philosophies on how the apparatus is used, and thus, the specifications in the Standard can be significantly different. Let's take a look at some examples.

 Take 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. The results are gathered and analyzed. And while, for the most part, stylus and drive speed have been agreed upon, 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. When measuring a roughness parameter in Germany, Japan or the US, it's important to understand what Standard should be referenced. Otherwise there is the potential for accepting or rejecting parts based on the wrong analysis method.

 The same could happen when trying to calibrate dial or digital indicators. Just as with surface finish gages, there are ASME Standards for calibrating dial indicators and similar Standards for DIN/ISO and JIS. But with dial indicators, there is also a slight difference in philosophy on how they are used in the US compared to the rest of the world, and this is reflected in the calibration process.

 We shouldn't be too general here, but if you look at most US manufactured dial indicators you will notice that they are typically very short range indicators often used in comparative gaging applications. On the other hand, most DIN and other metric-based indicators have a relatively long range. Because of this difference in application philosophy, there is a slight difference in the way the specs are written for testing the indicators.

 US manufactured indicators are generally governed by an ASME Standard, B89.1.10. Because US 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. Since DIN indicators seen in DIN 878 typically have a longer range for absolute measurement applications, the DIN/ISO specification calls for an absolute variation between the min and max 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 or might not pass an ASME test depending on where the zero reference is.

 The same is somewhat true for digital indicators. But 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. And there is no digital indicator Standard yet from DIN/ISO, although it is being worked on and should be available soon. But 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.

 When selecting the test scheme to calibrate an inspection product, ensure that the correct Standard is referenced.