Tech Directions - March 2009 - (Page 19) Fig. 1—Snapshot of Minitab screen used to enter subgroup size and specification limits that was robust enough for production or was it marginally acceptable? The teacher will decide if the actual project tolerances should be evaluated or samples from multiple processes will be used to give the students a better feel for process variation. For this activity, I have selected samples from three different manufacturing processes used in industry today. could cause serious injury or death to the user. Students use a digital micrometer to measure the outside diameter of the parts with a tolerance of 0.500+0.005. Once they’ve entered the data, students start to run a normal capability analysis by setting the product tolerance and number of samples. (See Fig. 1.) Minitab will create a capability chart that summarizes the process. In this activity, only process capability and parts per million values of the data are reviewed. Questions that students must answer include: • Is the process capable? • Is the process shifted to one side of the tolerance? If so, why? • Is more information needed to determine process capability? • Does the process always need to have a high process capability to be capable? Example Samples Samples measured in a first example group, injectionmolded samples, are depicted in Fig. 2. They show an unacceptable process capability of 0.46. Even though none of the samples were out of tolerance, the parts-per-million nonconformance projections located at the bottom of the chart predict a nonconformance. This particular process is experiencing dimensional variation that may allow for nonconformances. The Activity The class will break down into teams of two or three students. One student from each team will measure parts, while the other student(s) enter the data into Minitab. (Using only one person to collect variable data reduces operator variation.) The challenge starts with the instructor handing out three groups of 100 samples. Each group represents a different material and manufacturing process. On completion of this activity, students will be able to collect meaningful data, perform statistical capability evaluations, and interpret results. • Sample group 1 consists of injection-molded parts from a single cavity die. • Sample group 2 consists of random die cast parts from a four-cavity die. • Sample group 3 consists of machined aluminum parts produced using a modern CNC machine. The teacher will explain the dimensional tolerances for each group and their importance before students perform their inspections. Important characteristics typically require a minimum process capability of 1.33, and 2.00 for critical features. In this activity, the characteristics will be considered important. Important characteristics are ones that can affect form, fit, or function of the product. Critical features are ones that if found to be nonconforming Fig. 2—Process capability analysis of 100 injection molded parts If the process capability index (assumed centered process capability) were above 1.33 but one sided, it may have been explained. In many cases production tooling www.techdirections.com MANUFACTURING 19 http://www.techdirections.com
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