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Quality Control IENG102 Prof. Dr. Alagar Rangan Quality Control • Quality is conformance to specifications • Quality characteristic – Variable – Attribute • Variable: Quality characteristic that can be measured on a numerical scale length, diameter, density, etc. • Attribute: That cannot be measured on a numerical scale, Yes, No; Good, Ok, Bad; etc. • Quality Control: A system that is used to maintain designed Quality in a product or service. • Father of Quality Control: Edward Deming. Quality Control • comprehensive approach to improving Quality is TQM (Total Quality Management). • Graphical Methods: Histograms, Cause and effect Diagrams, etc. Quality Control • Control charts: CL, UCL, and LCL. • Average and Variations. • Average – Mean. • Variation – Range. Quality Control • Control Charts based on Normal distribution. • Normal distribution is symmetric. • In a production process, measurements of a Quality are normally distributed. Quality Control • Reliability T: Failure time, a random variable Reliability R(t) is the probability that a system is working at time t. R(t) = P(T>t) Quality Control • Series system A series system works if all the components work. • Parallel system A parallel system works if at least one of the components works. EXAMPLES Example 1. Quality Control Lathe Machining Car Parts: • Quality Characteristic: Thickness Variable Characteristic Process Mean Thickness Process Standard Deviation of Thickness Known or Estimated Company wishes to control the thickness of parts • • • • • • Take samples of size 5 parts every hour Calculate sample mean (x1+x2+...+x5)/5 These means are normally distributed Using this idea, draw control lines Upper and lower control line If every value of sample mean falls within UCL & LCL then process is under control • If not, stop production • Check for assignable causes • Then restart production • Improves quality, saves cost, reduces lead time Example 2. Reliability • Failure Time of a Solid State Power Unit • Failure times are random variables • Reliability : The power unit operates continuously for a fixed period without failure • Expected time to failure • Design Life: How to design power units to have a required reliability • Rate of failure of systems Thank you