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46 MoldMaking Technology —— MAY 2017 SKILLS CENTER Once the apprentice understands measurement and tolerance at a concep- tual level, it is time to learn about the unique tools for taking measurements, such as micrometers, calipers, height gages and indicators. Image courtesy of Praeco Skills. Apprentice Training: Precision Inspection and Measurement By Ryan Pohl Explaining to a new apprentice the ±0.00007 tolerances of a medical component that might end up as an implant in a person's spinal cord, and why those tolerances are important, requires a significant amount of effort, as it did for me when I first entered the trade. I recall being 16 years old in my machine tool class with Mr. Rodgers as he was explaining how to use a micrometer. Up until that point in my life, I only knew the accuracy of a tape measure. When Mr. Rodgers demonstrated micrometer accu- racy by measuring a piece of paper, a new world opened up for me. I often remember this experience because I teach so many people who are new to the trade, and it revealed to me how the concepts of precision and accuracy are almost completely unknown. Here are some basic principles concerning inspec- tion and measurement to take into account when developing your related technical instruction (RTI) plan. Measurement. To understand precision, the apprentice must first understand the concept of measurement. This seems rudimentary at first, until you look at measurement as it relates to standards. The numbers used in measurement are not random. Every appren- tice must understand that all measurement connects to a standard. For example, impe- rial or metric, and how these are used in different regions or industries. Even in ancient times, there were standards. The pyramids were built according to the measurement standard of the cubit, which was the length from a person's elbow to his fingertips. Having a solid understanding of mea- surement and standards leads to a better long-term ability to understand quality and quality control. Part and process quality are all rooted in the ability to take accurate measurements. There are many good books, arti- cles and online courses that can teach the theory of measure- ment and help the apprentice learn these concepts throughout his or her RTI. Tolerance. Depending on how much time an apprentice has spent building material goods, the concept of tolerance may also be new. Many apprentices create a lot of scrap because they do not fully understand acceptable part variation. For example, oftentimes when using basic machining concepts to teach the value of tolerance, I have the apprentice build a simple set of 1-2-3 blocks. When the apprentice goes to the trouble of milling, drilling, heat treating and grinding the 1-2-3 block, and it ends up as a 0.998-1.999-2.9975 block, making it scrap, an important light bulb goes off. Suddenly, the appren- tice understands how the concept of measurement is tied to the concept of tolerance. His or her inability to properly mea- sure resulted in an out-of-tolerance part. When this happens, the term "quality" takes on a whole new meaning. A 1-2-3 block must be exactly 1 inch by 2 inches by 3 inches, nothing bigger or smaller. This will lead to a proper understanding of the vari- ous tolerances that exist, such as dimensional, fit, assembly, location, form and orientation. Common measurement tools. Once the apprentice under- stands measurement and tolerance at a conceptual level, it is time to learn about the unique tools for taking measurements, such as micrometers, calipers, height gages and indicators. These are not tools typically found in the average person's garage, so it may take time for the apprentice to reach a work- able comfort level. In addition, there is still a lack of general knowledge about some of the lesser-used measurement tools, such as optical comparators, gage blocks, sine bars and sine plates. These must also be taught. On top of that, explaining the proper role and care of sur- face plates is another important facet of measurement, which should also include an overview of the variety of gages, such as bore and thread gages. The best way to teach an apprentice The goal of a good precision measurement and inspection course is to teach the apprentice how to properly use pre- cise tools and techniques to determine the quality of a part or process.