Metrology
Metrology
The Foundation of Dimensional Measurement
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The Foundation of Dimensional Measurement
George Schuetz - Mahr Federal Inc.


We have talked about getting down to basics with dimensional measurement many times. Nothing is more basic than that big rock on the shop floor or in the center of the calibration lab.

Actually, the use of rock for the foundation of measurement is relatively new. Until World War II, iron was the material of choice for creating a flat surface. But with the war going on, iron was hard to find and that created the need for a substitute that was really flat. Thus was created the first granite surface plate. As it turned out, granite plates were an improvement over the old iron plates in that they didn't rust, corrode, or warp, and if accidentally struck, there were no "nicking and humping" issues to be compensated for. Granite provides longer life and has a smaller co-efficient of thermal expansion. A whole new industry was created out of the simple shortage of iron!

In order to manufacture a surface plate out of stone, you have to have a way of determining its flatness. From this need came gages like the Planekator, Repeat-O-Meter, electronic levels, and optics for use with lasers—all for checking and eventually mapping surface plate flatness.

Flatness and repeatability are the two specifications that help define the accuracy of a surface plate. The most common specification used by manufacturers is GGG-P-463c (granite surface plates). A new specification, ASME B-89.3.7, will soon be published that incorporates most of the elements of the GGG standard.

Flatness is the first specification most people look at when judging a surface plate. It basically says that all points on the surface of the plate will fall between two parallel planes separated by the flatness tolerance. This can be thought of as a "Total Indicator Run-out Reading" as with a dial indicator. The highest and lowest points are found, and the difference should not exceed the flatness specification. The three grades for surface plates as defined in the specification are:

• Laboratory Grade AA: (40 + diagonal [in inches] of surface plate squared / 25) x 0.000001 inches.
• Inspection Grade A: Laboratory Grade AA x 2
• Inspection Grade B: Laboratory Grade AA x 4

Tolerance is the total deviation. You may see some specifications expressed as "unilateral" or "bi-lateral" tolerances, but they all mean the same. For example, one plate may be labeled with a total deviation of 200 microinches while another may be labeled +/-100 microinches; these are both the same.

While the overall flatness of the plate is important, it's also important to not have all the flatness variation happen in one localized spot. This is where repeatability comes into play. The repeatability test simulates placing a height gage, gage block, or part on the surface plate. You wouldn't want a large variation in the plate "tipping" the height gage. We have seen in the past how the error of this small variation gets magnified the farther away from the plate one measures. Some pretty big errors could be seen with a 40" height gage. That is why this specification is much tighter than the flatness specification.

                                                           
The test for repeatability is done with a gage that is zeroed on the center of the plate and then slid over the plate, checking to see that there are no local variations that exceed the repeatability specification. For a plate with a diagonal measurement of 30 to 60 inches, the specification of repeat is:

• Laboratory grade AA: 45 microinches
• Inspection grade A: 70 microinches
• Tool room grade B: 120 microinches

Besides the specification of accuracy that is manufactured into the plate, the other issue to consider is the actual material of the rock itself. Since granite is a material found in nature, all granite is not created equal. Based on local conditions, granite can have varying degrees of quartz content, with percentages ranging from 22% to 32%. The material that you choose is dependent on your use, among other factors. A little less quartz may mean slightly less wear resistance, while a little more may mean a stiffer plate. This could result in a thinner plate with less weight.

Discussing these differences with the manufacturer will provide the best plate for the application. You should also consider this as a long-term purchase. There is a reason granite is also used for tombstones, and your plate is apt to be around long after you purchase it and have moved on to bigger and better career opportunities. In other words, the choice you make in a granite plate is likely to be a legacy that will long be remembered.