Tech Directions - March 2009 - (Page 18) Statistics to the Rescue! Using Data to Evaluate a Manufacturing Process By Michael G. Keithley mgk28539@msn.com HE use of statistics and process controls is not new, but it’s a useful tool that is often overlooked in educating students. This article describes an activity appropriate for high school students who have a background in material processing. It gives them a chance to advance their knowledge by determining whether or not a manufacturing process works well. It’s important to understand fundamentals of producing an item, but having a controlled process for high-volume production is key. As a quality assurance manager and technology education teacher, I believe there’s a real need to cover this topic in the current curriculum. The activity follows a demonstration in Minitab® Statistical Software (www.minitab.com) functions. Students measure samples and use the resulting data to determine process capability. Once they have calculated and evaluated the data, they interpret it in a report. T Usefulness of Statistics Software like Minitab Statistical Software allows users to simplify investigations with capability analysis. According to the statistics entry in Wikipedia (2008): “Statistics is a mathematical science pertaining to the collection, analysis, interpretation or explanation, and presentation of data. It is applicable to a wide variety of academic disciplines, from the natural and social sciences to the humanities, and to government and business.” Before students take part in this activity, they need to understand the fundamentals of basic manufacturing processes used today. In many cases, students taking advanced technology classes have tackled a mass-produced project using jigs and/or fixtures to build a product from raw materials. A statistical analysis activity offers a real-life experience by challenging students to evaluate an existing process. Once students understand the basics of material processing, the teacher can harness their knowledge and put their work to the test. Did the students design a process The Importance of Process Control Ensuring that a process has the ability to produce products within tolerances is critically important in real-world manufacturing because it reduces potential defects and overall cost. A company cannot be profitable or competitive if its manufacturing methods are not robust. Evaluating defects on a parts-per-million basis helps in identifying manufacturing risks. Risk is the amount of opportunities for a lot (i.e., multiple manufactured products) to be rejected during inspection sampling. A rejected lot could cost a company in terms of materials, labor, and late fees. In some cases, an engineer will design and build equipment but fail to test its ability to produce products efficiently. Many people erroneously assume that if equipment will make one good part, it must be ready to make 10,000. Conducting a capability analysis upfront is analogous tuning up a car—sometimes it needs adjustment to run its best. Michael G. Keithley is a certified technology education teacher and a senior quality assurance manager at L-3 Communications, Columbia, PA. Terminology Process Capability Index (CP)—Estimates what the process would be capable of producing if the process could be centered. Assumes process output is approximately normally distributed (Process Capability Index, 2008). Process Capability (Cpk)—Estimates what the process is capable of producing if the process target is centered between the specification limits. Cpk is an index (a simple number) which measures how close a process is running to its specification limits, relative to the natural variability of the process. The larger the index, the less likely it is that any item will be outside the specs (Process Capability, 2009) Parts-Per-Million (PPM)—Particularly in low-value (high-ratio) proportions at the parts-per-million (Parts per Million, 2008). 18 techdirections ◆ MARCH 2009 http://www.minitab.com
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