Adam
Metrology is the science of measurement. Although its history goes back much further, the beginning of modern metrology is the invention of the decimal-based metric system in 1795. Of course, measuring things is key to all aspects of manufacturing. “Part A must be able to fit into Part B,” is just one example. Here are a couple of ways metrology can improve manufacturing operations.
1. Tolerance
Tolerance is the measure of how much a certain part is allowed to deviate from the ideal part size. In some cases, the tolerance is quite loose, allowing for some play in the two pieces fitting together. In other cases, such as with pipe fitting or the mating of a bearing with a shaft, the tolerance will be exceptionally tight. Machinists talk in 1,000ths of an inch, or “thou.” Tight tolerances are generally less than 10/1,000 of an inch, or “10 thou.” Some tools and parts have tolerances that are measured in microns, and a micron is 1/1,000,000 of a meter, which is also 1/1,000 of a millimeter.
2. Accuracy and Precision
While tolerance is all about how single pieces fit together, accuracy and precision are all about how close to the ideal every member of a group of items is. Accuracy is how close the mean of the deviation from the ideal is to the actual idea. Precision is how close the items produced are to the mean.
Let’s say that a certain pipe that’s used in the construction of refineries has a tolerance of 2 thou on its diameter of 5 cm. To simplify the example, let’s say that the refinery needs four pipes. The actual pipes are 1 thou, 2 thou, 2 thou, and 1.5 thou from the ideal. The mean of the difference is 1.625 thou. So, the accuracy is 1.625 thou on a reference diameter of 5 cm.
Precision is expressed as standard deviation. The standard deviation of our sample of four pipes is 0.415. A general rule in machining and manufacturing is that parts should be within two standard deviations of the ideal. Two standard deviations is 0.830, which means that all four of our sample pipes are precise enough for use. Usually, the quality-control person needs to be 95% certain that all items are within those two standard deviations.
3. Uncertainty
When calculating accuracy and precision, it’s not possible to be 100% certain of anything. The tool used for measuring the produced parts must be at least a couple of degrees more accurate than the required tolerance. In our example, the tolerance was 0.002 inches. Therefore, the measuring tool should be able to determine measurements of 10 x 0.0002 inches or better. This reduces the uncertainty of just how close to 2 thou the machinist was able to get when producing the items, such as the pipes in our example.
These three pillars of metrology improve manufacturing operations simply by existing. If all of the personnel involved in the manufacturing are following these precepts, then the processes will be far better than if they did not.
