Quality Control All activities undertaken to control materials, processes and products in order to...
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![Page 1: Quality Control All activities undertaken to control materials, processes and products in order to ensure quality of conformance Detects defects before.](https://reader036.fdocuments.in/reader036/viewer/2022062401/5a4d1b527f8b9ab0599a80c1/html5/thumbnails/1.jpg)
Quality Control• All activities undertaken to control materials, processes and products in order to ensure quality of conformance
• Detects defects before further failure costs are incurred
• Determines if processes are under control and deliver consistent results
• Determines if processes deliver the quality customers expect
• Identifies how quality of conformance can be improved
![Page 2: Quality Control All activities undertaken to control materials, processes and products in order to ensure quality of conformance Detects defects before.](https://reader036.fdocuments.in/reader036/viewer/2022062401/5a4d1b527f8b9ab0599a80c1/html5/thumbnails/2.jpg)
Quality Control Tools• Acceptance Sampling
• Inspection of goods before, during or after production
• What/where to inspect?
• How many to sample and how often to sample (rigor)?
• Tradeoff between appraisal costs and failure costs
• Statistical Process Control
• Cause-and-effect (fishbone) diagram
• Pareto chart
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Statistical Process Control (SPC)• Variability in the inputs (labor, material, equipment) will
cause variability in the output
• SPC detects variability in outputs by
• Measuring and controlling it
• Establishing systems to flag situations where quality exceeds defined bounds
• SPC reduces variability in outputs by improving product and/or process design (including inputs and process execution)
![Page 4: Quality Control All activities undertaken to control materials, processes and products in order to ensure quality of conformance Detects defects before.](https://reader036.fdocuments.in/reader036/viewer/2022062401/5a4d1b527f8b9ab0599a80c1/html5/thumbnails/4.jpg)
SPC Focus• For a chosen variable (a product or a process attribute),
SPC is concerned with the variable’s mean and variability.
• The process mean and standard deviation is estimated by calculating sample means and measures of sample variation for different samples taken over time.
• SPC measures, manages and
• Reduces the dispersion (variability) as needed
• Ensures that the mean does not shift
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![Page 6: Quality Control All activities undertaken to control materials, processes and products in order to ensure quality of conformance Detects defects before.](https://reader036.fdocuments.in/reader036/viewer/2022062401/5a4d1b527f8b9ab0599a80c1/html5/thumbnails/6.jpg)
Four Key Concepts for SPC• All processes have an inherent variation (sometimes
referred to as common, natural or noise variation)• Variability that is not inherent in a process is called
assignable or special variation. It arises when something has changed in the process itself and can be assigned to a specific cause.
• A process is in control (stable) when it exhibits only inherent variation. A process is out of control when it exhibits assignable variation.
• A process is capable if it consistently meets design and/or customer specifications (tolerances). A process that is in control is not necessarily capable.
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![Page 8: Quality Control All activities undertaken to control materials, processes and products in order to ensure quality of conformance Detects defects before.](https://reader036.fdocuments.in/reader036/viewer/2022062401/5a4d1b527f8b9ab0599a80c1/html5/thumbnails/8.jpg)
![Page 9: Quality Control All activities undertaken to control materials, processes and products in order to ensure quality of conformance Detects defects before.](https://reader036.fdocuments.in/reader036/viewer/2022062401/5a4d1b527f8b9ab0599a80c1/html5/thumbnails/9.jpg)
Control Limits • Mean Chart
• UCL = + z * s = + * • LCL = - z * s = - *
Where s = with d2 given in table for small n
or with d2 or given in table for small n & assuming z=3
• Range Chart• UCL(R) = * with D4 given in table for n• LCL(R) = * with D3 given in table for n
X
XX
X
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![Page 11: Quality Control All activities undertaken to control materials, processes and products in order to ensure quality of conformance Detects defects before.](https://reader036.fdocuments.in/reader036/viewer/2022062401/5a4d1b527f8b9ab0599a80c1/html5/thumbnails/11.jpg)
Process Capability• 2 common measures used to determine if a process is
capable:• Process Capability Ratio =
where USL is the Upper Specification Limit and LSL is the Lower Specification Limit as defined by the design requirements or customer needs
If > 1, then the process is capable.
• Process Capability Index = minimum( , )
If > 1, then the process is capable.
When the process mean is not centered between LSL and USL, must be used to test capability.