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Statistical Process Control

Statistical Process Control

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Statistical Process Control

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  1. Statistical Process Control Quality Series

  2. Outline • What is Statistical Process Control • Variability • Control Charts • Process Capability Cp • Measuring Instruments • Repeatability & Reproducibility

  3. Statistical Process Control (SPC) • Invented by Walter Shewhart at Western Electric • Distinguishes between • common cause variability (random) • special cause variability (assignable) • Based on repeated samples from a process

  4. Even very rare outcomes are possible (probability > 0) Even very rare outcomes are possible (probability > 0) Fewer in the “tails” (upper) Fewer in the “tails” (lower) Most outcomes occur in the middle Variability The world tends to be bell-shaped

  5. Variability Even outcomes that are equally likely (like dice), when you add them up, become bell shaped Here is why:

  6. “Normal” bell shaped curve Add up about 30 of most things and you start to be “normal” Normal distributions are divide up into 3 standard deviations on each side of the mean Once your that, you know a lot about what is going on

  7. Causes of Variability • Common Causes: • Random variation (usual) • No pattern • Inherent in process • adjusting the process increases its variation • Special Causes • Non-random variation (unusual) • May exhibit a pattern • Assignable, explainable, controllable • adjusting the process decreases its variation

  8. Samplingdistribution Processdistribution Mean Lowercontrollimit Uppercontrollimit Control Limits

  9. Abnormal variationdue to assignable sources Out ofcontrol UCL Mean Normal variationdue to chance LCL Abnormal variationdue to assignable sources 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Sample number Control Chart

  10. Process vs. control limits Distribution of averages • Variance of averages < variance of individual items Control limits Specification limits Distribution of individuals Process limits

  11. Shift in Process Average

  12. Process Capability • The ratio of process variability to design specifications Natural data spread The natural spread of the data is 6σ -3σ -2σ -1σ µ +1σ +2σ +3σ Lower Spec Upper Spec

  13. © 2005 Superfactory™. All Rights Reserved. Repeatability and Reproducibility Studies • Quantify and evaluate the capability of a measurement system • Select m operators and n parts • Calibrate the measuring instrument • Randomly measure each part by each operator for r trials • Compute key statistics to quantify repeatability and reproducibility