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Chapter 10 – Quality Control

Chapter 10 – Quality Control. Control process, statistical process control (SPC): X-bar, R, p, c, process capability. SPC – The Control Process. Define Measure Compare to a standard Evaluate Take corrective action (if necessary) Evaluate corrective action. Variation.

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Chapter 10 – Quality Control

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  1. Chapter 10 – Quality Control Control process, statistical process control (SPC): X-bar, R, p, c, process capability SJSU Bus 140 - David Bentley

  2. SPC – The Control Process • Define • Measure • Compare to a standard • Evaluate • Take corrective action (if necessary) • Evaluate corrective action SJSU Bus 140 - David Bentley

  3. Variation • Random, chance, or common cause (natural) • Assignable, non-random, or special cause (due to some specific change) • Sampling used to detect non-random • Sampling assumes a normal distribution • Sample statistics calculated: • Mean • Range SJSU Bus 140 - David Bentley

  4. Control Charts • Control limits • When is a process out of control? • Charts for variables • Mean (X-bar) charts • Range (R) charts • Charts for attributes • Proportion or percentage (p) charts • Number [or defects per unit] (c) charts SJSU Bus 140 - David Bentley

  5. Type I and Type II Errors • Type I • Conclusion: non-randomness is present and the process is out of control • Reality: randomness is present and the process is in control • Type II • Conclusion: randomness is present and the process is in control • Reality: non-randomness is present and the process is out of control SJSU Bus 140 - David Bentley

  6. Mean & Range Control Charts • Take required number of samples • Mean (X-bar) charts (see Table 10-2) • Calculate mean (X-bar) for each sample • Calculate grand mean (X-double-bar) • Calculate range (R) for each sample • Calculate mean of all sample ranges (R-bar) • Calculate UCL and LCL for means • Plot grand mean and control limits on X-bar chart SJSU Bus 140 - David Bentley

  7. Mean (X-bar) Chart Control Limits • UCLX-bar = X-double-bar + A2 (R-bar) • LCLX-bar = X-double-bar - A2 (R-bar) Where X-double-bar = the grand mean, And R-bar = the mean of the sample ranges And A2 = the value in Table 10.2 for n SJSU Bus 140 - David Bentley

  8. Range (R) Chart Control Limits • UCLR= D4 (R-bar) • LCLR= D3 (R-bar) Where R-bar = the mean of the sample ranges, and D4 and D3 = the values in Table 10.2 for n SJSU Bus 140 - David Bentley

  9. Mean & Range Control Charts • Range (R) charts (see Table 10-2) • Calculate UCL and LCL for ranges • Plot range mean and control limits on R- chart • Plot additional samples and determine if within range limits • Note: factors based on the size of the sample, not the number of samples! SJSU Bus 140 - David Bentley

  10. Run tests and charts • Counting above/below the median • Counting “ups” and “downs” SJSU Bus 140 - David Bentley

  11. Process Capability Ratios Non-centered process (general case): choose cpk = the lower of: Upper spec – process mean cpu = ---------------------------------- or 3 Process mean – lower spec cpl = ---------------------------------- 3 SJSU Bus 140 - David Bentley

  12. Process Capability Ratios Centered process (special case): specification width cp = ---------------------------- process width Upper spec limit– lower spec limit = ----------------------------------------- 6 SJSU Bus 140 - David Bentley

  13. Process Capability Requirements • Process must be normally distributed • Process must be in control • Process capability result: • > 1.34 = capable • < 1.33 = not capable • = 1.33 = barely capable • > 5 or 10 is “overkill”, excessive resource use SJSU Bus 140 - David Bentley

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