Discussion -- Metrology, Testing, & Inspection:
Download and read Chapter 9 in Essentials of Quality With Cases and Experiential Exercises. Review the Discussion Questions at the end of the chapter to be sure that you understand what you have read.
Motivation for Maintaining Proper Calibration ProceduresA federal grand jury in Dallas returned a 30-count indictment against 13 analytical chemists, laboratory technicians, and managers of Intertek Testing Services for allegedly manipulating test data to indicate that test equipment was properly calibrated when it was not. If convicted, the defendents could face decades of jail time and fines ranging from $750,000 to $4.75 million.Chemical & Engineering News, Oct. 2, 2000, p. 11.
Metrology is the science of measurement. Measurement systems must be assessed for accuracy and precision. Inaccurate or inprecise measurement systems do not provide a true measure of the dimension being assessed. Accuracy is assessed by calibration; reliability is assessed by guage repeatability and reproducibility (R & R) study.
The use of an uncalibrated measurement system is akin to measuring the size of your foot with a ruler that claims to be 12 inches long but which is actually 14 inches long. You will systematically obtain readings which are too short. The use of an inprecise measurement system is akin to measuring your foot with a rubber ruler. Depending upon how far you stretch it, you can obtain whatever reading you wish. In neither case will you be able to determine the true length of your foot.
The electronic scale in my bathroom gives a different weight each time I step onto it. The weight it shows might vary as much as 5 pounds in two weighings taken just seconds apart. I don't think that I gain and lose 5 pounds within a few seconds, therefore I conclude that the scale is inprecise. It is also inaccurate because when I compare the typical weight that I obtain from my scale with my weight measured in my physician's office, I find a significant difference. Therefore I really don't know what I weigh, and I'm not sure whether my diet is effective or not.
It is very unwise to use a measurement system in an industrial process without being sure that the system is both accurate and precise. You are never sure about whether the readings you obtain have any real meaning. I have seen thousands of feet of wire which met all dimensional specifications discarded because of an inaccurate measurement system (micrometer). The micrometer had a valid calibration sticker but had been dropped. The operator failed to report having dropped it and thus it was not recalibrated. The result was a faulty disposition of hundreds of dollars of wire.
Selection of the appropriate measurement instrument for the job is very important. The Rule of 10 says that a measurement tool should have 10 times more resolution than the tolerance of the dimension. So, if you need to measure a part whose tolerance is expressed in hundredths of an inch (0.01"), you should select a measurement system whose resolution is expressed in thousandths of an inch (0.001"). The following is true for common hand-held dimensional measurement tools. Rulers are quick and easy to use but have poor resolution. Calipers have better resolution. Micrometers have better resolution yet, but generally have a smaller measurement range.
Information about non destructive testing (NDT) can be found at the American Society for Non Destructive Testing web site.
There are a number of types of automated inspection systems. Coordinate measuring machines (CMM) are off-line devices that take measurements according to a program written by the operator. CMM capability is often built in to computer numerically controlled (CNC) machines. This allows measurements to be taken while the part is still in the machine (i.e. on-line measurement). For more information about CMM technology visit the Brown & Sharpe web site.
Other automated testing systems use a variety of contact and non-contact sensors to take measurements on-line. This video shows the SHSU Flexible Manufacturing Cell which is equipped with linear potentiometers to automatically inspect parts for height and diameter. The measurement information is fed directly to an on-line SPC system and plotted on x-bar and R charts.
Click on this picture to see the SHSU Flexible Manufacturing Cell's automatic measurement system in action. This video clip is best viewed using Real Player and your display setting (use Control Panel for this) set at millions of colors.
Answer Discussion Questions 1-11 at the end of Chapter 9 of the text.Experiential Exercise:
Do Problems 1-6 at the end of Chapter 9 of the text.
Do Experiential Exercise 1 at the end of Chapter 9 of the text.Click here to begin Lesson Eleven Presentation.