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

• ReliabilityT: Failure time, a random variableReliability R(t) is the probability that a system is working at time t.R(t) = P(T>t)

Quality Control

• Series systemA series system works if all the components work.

• Parallel system A parallel system works if at least one of the components works.

EXAMPLESExample 1. Quality Control

Lathe Machining Car Parts:• Quality Characteristic: ThicknessVariable Characteristic Process Mean ThicknessProcess Standard Deviation of ThicknessKnown or EstimatedCompany 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