ASME's Dimensional Standards are relatively stable for a long period of time. This is good because change is a hard thing to deal with, especially in the metrology world. The United States Gage Block Standard has not changed much since 1970, when Federal Specification GGG-G-15B was introduced.
However, there have been many changes over the years concerning the use of gage blocks. There have been changes in block materials and in the internationalization of manufacturing, and changes have also resulted from the influence of new measuring principles, such as uncertainty and traceability. Thus, in 1990, ASME B89.1.9 started the ball rolling towards updating the standard by introducing more international influence. Finally, in ASME B89.1.9 2002, the Gage Block Standard was revised to bring it closer to ISO 3650, while at the same time incorporating our desire for inch units, square blocks and some of the grade requirements ingrained in North America.
While most of the changes in the new specification don't affect the everyday user of the blocks, they may influence how gage blocks are identified (or graded), how they may be measured and how the information is presented. The standard has already affected the way gage block manufacturers specify their products.
Actually, ASME B89.1.9 isn't bad reading. Besides the normal terms, definitions and tables, it presents some good information about referencing from the old standards to the new, the handling of gage blocks and typical set configurations. It even lists the differences between previous standards and the new one to clear up different terminology and to match the different grading conventions, old and new.
In regards to grading, there are a couple of important items to point out. The first is that grading is now modeled much closer to the ISO method, as follows:
Former Federal Grade New ASME Grade
(GGG-G-15C) (ASME B89.1.9)
There are additional categories for calibration grades of blocks, designated as Grade K or 00, that are used as reference standards.
The other noteworthy change is in the actual tolerance of the blocks. Gone are the unbalanced tolerances such as +16, -8 microinches as was seen for a grade 3, 2-inch block. Today, that tolerance is ±16 microinches for the equivalent grade AS-1.
Finally, a change was made to add a specification for length variation of the block. This was previously referred to as parallelism. Today, each grade has a length variation tolerance that is dependent on the grade of the block. What is also important is that this length variation can be as small as 2 microinches and the standard specifies where it must be measured. The new requirement is that all blocks be measured at the gage length (which is the center of the measuring face) and at all four corners. Corner measurements must be taken at approximately 0.06 inch from each of the side faces, and all of the points measured must not only be within the length tolerance for the grade, but must also be within a specific variation range for the length tolerance. For example, if the grade length tolerance is ±8 microinches and the tolerance on length variation is 4 microinches, then all your measurements must fall within the ±8 microinches and also not vary from each other by 4 microinches.
This may not seem to be much of a chore, but it increases the burden for the gage block calibration people who do the actual measurements. Previously, they would make four measurements somewhere on the face of a rectangular block, simply moving the block around with tweezers (precision tongs). Now that the spec requires measuring at an exact location, measurement is much more difficult and time-consuming for the operator.
To ease this situation, special gage block manipulators need to be incorporated into gage block comparators to position the blocks to the exact location in each corner, every time and very repeatably. Remember this last statement, because with gage blocks, we are in the world of microinches. Like real estate, the key to measuring millionths is location, location, location. The better you can position the gage block, the more you increase repeatability and reduce uncertainty.
Eventually, all the tools used to calibrate gage blocks will need to incorporate gage block manipulators if calibration facilities are to improve throughput and maintain tight uncertainty levels.blog comments powered by Disqus