We are going to start a series talking about the use and type of measuring tools commonly used by the aerospace technician. In this part, we will cover the basics that are needed to keep in mind when selecting and using measuring tools.
There is an old phrase that says, “When the only tool you have in your tool box is a hammer, then every problem looks like a nail.” The same goes with measuring tools. If all you have is a ruler, then you have the tendency to measure everything with it. As we all know, various things such as surfaces, threads, thickness, etc. need to be measured which require a variety of measuring tools, from rulers, dial calipers, to gages.
Though accuracy is needed in the aerospace field, accuracy can actually be too much. When you have need of knowing the measurement of a part within 0.01 of accuracy, you do not need to use a tool that measures within 0.000001 accuracy. That much detail is wasteful and not needed for the task. “The rule of thumb is to select a measuring tool that is ten times more accurate than the total tolerance to be measured, or the tool can discriminate to one-tenth of the total part tolerance.” (Griffith, 2003) This is called the 10 % or 10-1 rule. So if you wish to measure something within 0.01 accuracy as called for by the tolerence listed on the process, you only need to measure ten times that accurate or 0.001 in order to follow the 10-1 rule.
When I took a chemistry lab in college, I had the instructor once comment that I was being too accurate in my measurements. I had taken that as a compliment at the time not understanding that his “tolerance” did not require as much accuracy as I was using in measuring my chemical compounds for the experiment. Though my outcomes were more inline with what the lab book had said we would come up with in the results of the experiment, I was always last to leave the class, and I missed the point that he wanted us to grasp the concept of the experiment, not the technique.
Know your measuring tool! What does it measure? What is the fixed end (reference) and what is the measured surface (movable)? Do you know how to read the measurement displayed? What are the divisons on it and what do they represent? You must be proficient with your tool in order to use and read the correct measurment.
Strive for accuracy and precision. “Accuracy is the difference between the average of several measurements made on a part and the true value of that part. Precision means getting the consistent results repeatedly.” (Griffith, 2003) If you are using the wrong tool during a measurement or reading it wrong, but getting consistent results, you have precision but not accuracy. That is because you are getting the same results each time, but the measurment process is inaccurate. But if you are using the right tool and reading it correctly, then you will get accuracy and precision.
Pay attention to the pressure used when measuring. If you use too much pressure or too little, you can get an less precise reading (especially if the pressure changes each time you measure) and therefore an inaccurate reading. Don’t use a micrometer like a C-clamp to hold a part in place. That is too much pressure and will affect the reading. Same goes for using too little pressure. If contact with the part is too light, then the reading is skewed. Always use consistent pressure.
Take care of your measuring tools. First of all, inspect the tool. Is there a calibration sticker on it stating when it was calibrated and when the calibration expires? Is the tool in good condition? Does all the parts of it work as expected? Is the tool clean of debris and dirt that could affect its use or the measurement? Has the tool been dropped? Have you tested the tool on a part that already has had its measurment determined? Has the tool been stored properly to prevent damage? Measuring tools should not be piled on each other and usually should be kept in individual cases. Is the tool showing any sign of wear, especially on the measuring and fixed surfaces?
Learning about and becoming proficient with your measuring tools will ensure that your work will be more consistant and error free. And, don’t forget the old adage when doing your work; “Measure twice and cut once!”
Sources: The Quality Technician’s Handbook by Griffith
Technology of Machine Tools by Krar and Check