Quality Assurance ---process Control

Quality Assurance - Process Control-tools & techniques

Srinivasan Nenmeli-K

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Q A through Process Control ◆ ◆ ◆

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Process control is the key for QA Identify the key processes & their parameters Set limits to the parameters-higher and lower; sometimes only one of these. Suppose you make cakes---take baking step as a key process Set lower and higher limits for baking temperature--say 50 degrees and 60 degrees centigrade--the "tolerance" What are the other key processes in baking?-list now! QA-2 NKS 2

Tolerance -What it means? ◆

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Based on experience or testing, you fix the tolerance. As a rule,.closer the tolerance, costlier and difficult is the process Suppose you make shafts for automobiles rear axle Suppose you set the length of shaft as: 120 mm with tolerance + or - 3mm.This may be suitable for a farm tractor. For passenger cars ,you may set 120 mm with tolerance + or - 1mm For smooth-riding,expensive car [Rolls-Royce QA-2 NKS ,say] you may set 120 mm with tolerance + or -

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Quality Control--Old Style ◆ ◆

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Quality control and inspectors go together! What did old style inspectors do for the shafts mentioned? Suppose the length is 120 mm ,tolerance + or - 1mm Upper specification limit [USL]is 121 mmLower specification limit [LSL] is 119 mm------Tolerance is: USL-LSL=2mm-Inspectors checked a shaft with two gages of 121 and 119 mm-- 'gages' called 'GoNoG0' gages QA-2 NKS

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QC- Old style ◆

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Inspectors passed if the length was within specs: between 121 mm and 119 mm. Others were rejected. Do you want most of the shafts close to 120 mm? Do you check for variation of length between 121 mm and 119 mm? How many are close to the mean,120 mm? QA-2 NKS

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Variability of parameters ◆ ◆

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Statistics is a science of variations. If all of us are of same height at certain age,there is no need for statistical analysis. How do we approach the variations? Suppose your auto-plant makes 1000 shafts a day.You take a sample of 100 shafts and measure their lengths. Most probably, you will find the average or mean value of the lengths.Suppose the mean is 120 mm .Are you satisfied? The old-style inspectors found the number falling within 121mm and `119 mm--let us say 68 shafts QA-2 NKS were within the limits and accepted.

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Mean and Sigma ◆

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The old style inspector would reject the remaining 32 shafts--outside the limits--32% rejection! Suppose you find the "standard deviation" or sigma from the 100 measurements of length. {Find the formula for sigma--most calculators and computer programs give this} sigma squared = sigma x sigma = variance ---> remember this Now our Engineer Tom finds that the sigma value turns out to be 0.98 mm, take it as 1mm--> sigma = 1mm Aha! you have got the clue now: QA-2 NKS 7

Tolerance and Sigma ◆ ◆

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Tom found that tolerance was mean + /- sigma. Tom talked to his Stats Professor, Prof. Variant, and he told that if you take those pieces with this tolerance ,you will get only 68.2% acceptable within tolerance.The rejection will be 32% [or nearly 2/3rd fall within tolerance , 1/3rd outside the limits---easy to remember] Prof Variant told that the lengths of the shafts followed "Normal Distribution" [the bell -shaped curve]with mean as 120 mm and sigma or Standard Deviation as 1mm. How to reduce rejections with the same QA-2 NKS tolerance?No use looking at the mean.The sigma 8

Process Control-sigma ◆ ◆ ◆

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QA means "process control". Process control means "reduce sigma to the desired level" This is the great lesson the Japanese learnt from Deming,Juran and others...control variability--reduce sigma. Tom improved the process of making shafts . Now Tom finds sigma = 0.5 mm for 100 shafts . Then tolerance limits are: 121 = 120 + 2x0.5 mm and 119 =120 - 2x0.5 mm.Within 2sigma above and below the mean. Prof Variant told now: Normal distibution tells that nearly 95 % of shafts will pass .Tom is sure that only 5% of shafts will be rejected.Great improvement! but not sufficient. QA-2 NKS 9

Sigma control ◆ ◆

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Reducing sigma or control of variance is the key to QA From 1930's and particularly since World War II [say 1945] in most industries,many learnt this lesson..The Japanese applied to almost all manufacturetoys,electronics,optics,steel and the automobiles.Some did not learn and lost to competition their business. A standard rule was developed:

Tolerance/sigma =6 ◆ ◆ ◆ ◆

'Maintain this -dummies '---told the Quality gurus. Performance Index = tolerance/ 6 (sigma) PI should be greater that 1 [can be 1.2 or more] Suppose tolerance = 2mm sigma should be 2/6=0.33mm QA-2 NKS or less!

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Taguchi and Quality Cost ◆ ◆ ◆

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Taguchi was a great quality guru. No use inspecting using tolerance only!; Any deviation from the mean entails cost-cost to customers & cost to society To make it quantitative,Taguchi gave a simple cost equation: If the mean is xo and the actual value is x, the cost is : Cost = k( x - xo)^2 where k is a constant. The cost increases as the square of the deviation from mean --- a parabolic curve or quadratic QA-2 NKS 11 equation

How to reduce sigma ? ➢

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QA involves reducing sigma for critical parameters Reduction of sigma applies to both manufacturing and service industry--examples follow Suppose you run a transportation company.The delivery time from New York to Chicago may vary with mean 32 hours ,with sigma +/-2 hours.Your effort should be to reduce the sigma from 2 to 1 hour. In process control, first you reduce sigma ,then you may shift the mean to lower or higher value--QA-2 NKS 12 first you control the process ,that is, reduce

Find sources for variation ◆

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What are the sources of variation? What factors increase sigma? In manufacturing: sigma squared = sum of sigma squareds for material, machine,labor,tools and instruments. Find which sigma-squareds are high.If you are using poor quality materials and worn-out tools,their sigmas will be large. This is called variance analysisor Anova to separate out the causes of variation or sigma. QA-2 NKS

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Simple QA tools ◆

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Use check-lists as often as you can--for simplest of tasks...checklist is a powerful QA tool.! Develop internal company-wide standards--they should be written down,discussed and reviewed periodically 'Auditing'--What is this? It is "checking against specifications" --Encourage auditing and review audit reports;train internal auditors; Do "quality audit" at least once a quarter[3 months] Use the magnifient "Seven Q C tools" QA-2 NKS

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Process Flow Chart ◆

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Draw a process flow chart--Can you simplify the chart? Making cake: measure ingredients-->mix-->make dough-->put in trays-->bake in oven For each process step,identify the critical parameters Can you avoid some steps?Can you reduce waiting times between steps? Involve your staff in making and analyzing process flowcharts Can you combine steps?--example: your QA-2 NKS 15 receptionist combines reception and phone-

Measurements and process control ◆ ◆

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" What you can measure,You can control" Measure critical parameters of processes--not just productivity Measurements relating to:1 materials,2machines,3methods & tool,4men & women 5 Mother Nature--environment [humidity for instance in some industries] Metrics is the word used for measurements and their analysis QA-2 NKS

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Process Ownership ◆

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Give process ownership to your staff/managers Do they follow QC/QA concepts? Are they willing to follow Quality Audits? Do they write standards for their processes? Do they perform defect measurement and analysis? Do they measure critical parameters? QA-2 NKS

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Q A for service industry ◆

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Several service industries apply QA --health care,information technology,hospitality,telecom and education Measure mean and sigma for service parameters and reduce the sigma value.The most important service parameters are often 'time-related factors'.We saw the example of transportation earlier. Response time to fix customer complaints/problems is an important parameter to control---find its mean & sigma.Can you reduce QA-2 NKS 18 this?

Practical means to reduce sigma ●

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See my other presentation : "QA- 5 tips"---Learn about the Deming cycle or Plan-Do-Check-Act cycle Training of employees at all levels is the potent method to reduce sigma.Without sufficient training ,they cannot control processes. Develop the culture of 'Continuous Improvement'--Quality is not a quick-fix program. Control the sigma of incoming raw material or products-this is very essential for product industries---Control the process sigma of equipment and tools used. In service industry, improve documentation and logging of records.The documents serve as 'raw materials' to control the sigma. QA-2 NKS 19 Above all, apply the 'magnificent seven' QC tool

Summary ◆

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See the connection between quality and tolerance and sigma values of process parameters Practice "Continuous Improvement" Identify key processes and their critical parameters Use PDCA cycle and the seven QC tools to reduce sigma of these parameters Emphasize training for all employees

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Suggested Reading

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Dale Besterfield et al --- Quality control Dale Besterfield et al --- Total Quality Management Peter Pande -- The Six Sigma way

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Contact Contact me for further presentations and info: email ID: [email protected]

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