Ch 8 Process Capability

Ch 8 Process Capability - INDE 321 Statistical Quality...

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1 INDE 321 Statistical Quality Control Instructor : Linda Boyle Industrial and Systems Engineering Chapter 8 Process and Measurement System Capability Analysis
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2 Process Capability Analysis What is Process Capability Analysis? A study set up to estimate your process capability. Observe the dynamic behavior of the process What is Process Capability? To compare the performance of a process against its specifications To observe how uniform is the process.
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3 Estimating PCR - Review Quantitative way of expressing process capability For a quality characteristic with upper and lower specification limits: σ - = 6 LSL USL C p Provides an estimate of the fraction of nonconforming items for any process where specification limits are involved
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4 Process Capability Ratios, Review The percentage of the specification band that the process uses up is denoted by The C p statistic assumes that the process mean is centered at the midpoint of the specification band – it measures potential capability. % 100 C 1 P ˆ p =
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5 Process Capability Analysis How is uniformity measured? Uniformity of output is measured by looking at the variability in the process. What kind of variability am I interested in? 1. Natural or inherent variability (instantaneous) 2. Variability over time
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6 Natural Tolerance Limits Upper and lower natural tolerance limits of the process are: UNTL = μ + 3 σ LNTL = μ - 3 σ This is what your process should be capable of If the process involves a quality characteristic that follows a normal distribution with mean μ , and standard deviation, σ .
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7 Process capability analysis Major uses of data 1. To predict how well your process hold the tolerances. 2. Help product developers/designers in selecting or modifying a process. 3. Help establish an interval between sampling for process monitoring.
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8 Process Capability Analysis Major uses of data (cont.) 1. Help specify performance requirements for new equipment . 2. Help you select between competing vendors . 3. Planning the sequence of production processes when there is an interactive effect of processes on tolerances. 4. Reducing the variability in a manufacturing process.
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9 Process Capability Analysis Techniques to use 1. Histograms or probability plots (Ch 3) 2. Control Charts (Ch. 6, 7, 9 and 10) 3. Designed Experiments (Ch. 13, INDE 316) 4. Process Optimization (Ch 14, INDE 316)
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10 Histogram and Probability Plots The histogram, sample mean, and sample standard deviation provides information about process capability. The process capability can be estimated as The shape of the histogram can be determined Histograms provide immediate, visual impression of process performance. x ± 3 s
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11 Frequency Distributions and Histograms Histogram 0 1 2 3 4 5 6 209 219 229 239 249 Coffee weight (ml) Frequency 02 . 40 8 . 224 3 x : as estimated be can capability Process 34 . 13 8 . 224 ± = ± = = S S x
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12 Probability Plotting Probability plotting is useful for determining Shape, center, and spread of distribution
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