SPC统计过程控制(英文版).pptx

1、Statistical Process Control,Statistical Process Control,Variability exists in everything-people,machines and naturePurpose-to be sure that processes are performing in an acceptable mannermonitor over timeforecast if will still be well in futuredetermine factors that make it out of control,Statistica

2、l Process Control,Design quality checks into the process-eliminate need for inspectionbuild better product during designmake product withstand variation-Taguchi Method,History,Prior to IR craftsmen were responsible for entire process-pride of workmanshipDuring 1920s W.Shewart of Bell Labs build Cont

3、rol Charts-ASQ formedDuring 1950s Deming introduced QC techniques to Japanese1961 Feigenbaum wrote Total Quality Control,Statistical Process Control,Two main subjectsAcceptance Sampling-test random samples of finished goods and accept or reject whole lotProcess Control-sample process output at diffe

4、rent points in time-see if process is in control-determine cause of out-of-control behavior,Statistical Process Control,Monitor the production process in one of three places-before,during or afterPhysically examine some of the unitsBasic issues;where do we inspecthow much and how often do we inspect

5、do we inspect variables or attributesBalance two costs:inspection and service,Rules of thumb for where to inspect;,1.Inputs-do not pay for goods that do not meet standards2.Finished Goods-protect your reputation3.Before a costly operation-do not waste time and labor4.Before an irreversible operation

6、-rework only up to a certain point5.Before a covering operation-painting or assembly,Types of Sampling,Defn:Attribute Sampling-check for the presence or absence of a product characteristicDefn:Variable Sampling-product characteristics that can be measured on a continuous scale,Statistical Process Co

7、ntrol,used to evaluate the output of a processmost processes exhibit some kind of variationrandom variation-natural variation,uncontrollableassignable variation-can identify causedetermine whether non-random sources of variation are responsible for problems,Control Charts,Prevent production of exces

8、sive number of defects-look for causes of variabilityA time-ordered plot of sample statistics Control limits-determine the difference between random and assignable variation-usually 3 Each new sample statistic is plotted and compared to the control limits,StatisticalProcessControl,Types of Control C

9、harts,1.Variable Control chartsA.Means Chart-monitors central tendency B.Range Chart-monitors dispersion-both can be used to monitor same process-use 20-25 samples to build initial limits-take samples of 5-9 observations-reasonable amount of time and costs less,Types of Control Charts(cont),2.Attrib

10、utes Control ChartsA.P-Chart-count number of defects in sampleB.C-Chart-count number of defects per unit-building control charts adds to the time and cost of production-attribute sampling is easier than variable-need larger sample sizes,Process Capability,Specify ranges for certain product dimension

11、s during designTolerancesMax and min acceptable level of outputBearing for rotating shaft-diameter is 1.25 inches 0.005Process has=0.02 inches-a 3-control chart limits are wider than tolerances,Capability Index,Shows how well parts fit into the range specified by design limitsIf design limits are wi

12、der than process limits,the process can shift and still produce good partsCpk=,Capability Index,Example-light bulbs are produced with an average life of 90 hours.The target design life is 100 hours.The UTL=120 and the LTL=80 hours.If=4.8 what is the Cpk?,4,Acceptance Sampling,PurposesDetermine polic

13、y for accepting or rejecting a production lot of materials or componentsSample quality is used to make decisionAdvantagesLess handling damageFewer inspectorsApplicability to destructive testingEntire lot rejection(motivation for improvement),5,Acceptance Sampling,DisadvantagesRisks of accepting“bad”

14、lots and rejecting“good”lotsAdded planning and documentationSample provides less information than 100-percent inspection,Acceptance Plans,1.Single-Sample Plans-only one sample define n and cOperating Characteristic Curve-graphs the performance of planAcceptable Quality Level-defines good lotsLot Tol

15、erance Percent Defective-defines bad lotsAverage Outgoing Quality-the percent defects in lots leaving inspection,Acceptance Plans(cont),-Producers Risk()-probability of rejecting good lot-Consumers Risk()-probability of accepting a bad lot-can reduce errors by not biasing the sample and taking a lar

16、ger one-use the Poisson distribution to compute probability of acceptance,9,Operating Characteristic Curve,Acceptance Plans,Plans considers the interaction among the above criteria-it considers the objectives of the supplier and consumerThe producer would like low probability of rejecting a good lot

17、 and consumer wants to not accept a bad lotHP and Japanese vendor,Acceptance Plans(cont),2.Double-Sample PlansA.Draw one sampleB.if number of defects is less than a lower limit,accept lot if number of defects is greater than some upper limit reject lotC.if number of defects is between the limits,take another sample,