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1、Process Capability (Cp / Cpk / Pp / Ppk)Global Training Material,Creator : Global Mechanics Process Manager Function : Mechanics Approver : Gary Bradley / Global Process Team Document ID : DMT00018-EN Version / Status : V.1.0 / Approved Location : Notes : NMP DOCMANR4 PCP PC Process Library DocMan C
2、hange History : IssueDateHandled ByComments 1.021st Dec01Jim Christy & Sren LundsfrydApproved for Global Use NOTE All comments and improvements should be addressed to the creator of this document.,Contents,SectionHeading / DescriptionPage 1Variation, Tolerances and Dimensional Control4 2Population,
3、Sample and Normal Distribution15 3Cp and Cpk Concept28 4Use of the NMP Data Collection Spreadsheet44 5Confidence of Cpk52,Process Capability - Evaluating Manufacturing Variation,Acknowledgements Benny Matthiassen(NMP CMT, Copenhagen, Denmark) Frank Adler(NMP Alliance, Dallas, USA) Joni Laakso(NMP R&
4、D, Salo, Finland) Jim Christy(NMP SRC, Southwood, UK),Section 1 Variation, Tolerances and Dimensional Control,Two Types of Product Characteristics,Variable: A characteristic measured in physical units, e.g. millimetres, volts, amps, decibel and seconds.,In this training we deal with variables only,T
5、he Sources of Process/System Variation,Methods,Operators,Customer Satisfaction,Material,Environment,Equipment,Process,Two Types of Processes,All processes have: Natural (random) variability= due to common causes,Stable Process:A process in which variation in outcomes arises only from common causes,U
6、nstable Process: A process in which variation is a result of both common and special causes,Unnatural variability = due to special causes,Shewhart (1931),The Two Causes of Variation,Common Causes: Causes that are implemented in the process due to the design of the process, and affect all outcomes of
7、 the process Identifying these types of causes requires methods such as Design of Experiment (DOE), etc. Special Causes: Causes that are not present in the process all the time and do not affect all outcomes, but arise because of specific circumstances Special causes can be identified using Statisti
8、cal Process Control (SPC),Defect,USL,LSL,nominal value,Tolerances,LSL (lower specification limit) 10,7,USL (upper specification limit) 10,9,Acceptable part,Rejected Part,Rejected Product,Nominal 10,80,1,Rejected Part,A tolerance is a allowed maximum variation of a dimension.,Measurement Report,In mo
9、st cases we measure only one part per cavity for measurement report,Example of Capability Analysis Data,For some critical dimensions we need to measure more than 1 part For capability data we usually measure 5 pcs 2 times/hour=100 pcs (but sampling plan needs to be made on the basis of production qu
10、antity, run duration and cycle time),Process Capability - What is it?,Process Capability is a measure of the inherent capability of a manufacturing process to be able to consistently produce components that meet the required design specifications,Process Capability is designated by Cp and Cpk,Proces
11、s Performance is a measure of the performance of a process to be able to consistently produce components that meet the required design specifications. Process Performance includes special causes of variation not present in Process Capability,Process Performance is designated Pp and Ppk,Why Make Proc
12、ess Capability Studies,This part is within spec. The tool would be approved if only this part was measured,These parts are out of spec and could be approved if only one good part was measured,A process capability study would reveal that the tool should not be accepted,When a dimension needs to be ke
13、pt properly within spec, we must study the process capability . but still this is no guarantee for the actual performance of the process as it is only an initial capability study,The Nokia Process Verification Process,10 parts/cavity measured for measurement report,Capability study: Requirement: Cp
14、and Cpk 1.67 by E3. Quantities to be agreed with supplier. Minimum 5 parts pr 1/2 hour in 10 hours measured for each cavity = 100 parts. Can vary depending on tool capacity, e.g. stamped parts (see DMY00019-EN),Section 2. Population, Sample and Normal Distribution,The Bell Shaped (Normal) Distributi
15、on,Symmetrical shape with a peak in the middle of the range of the data. Indicates that the input variables (Xs) to the process are randomly influenced.,“Population Parameters” = Population mean = Population standard deviation,Population versus Sample,Population An entire group of objects that have
16、been made or will be made containing a characteristic of interest,Sample The group of objects actually measured in a statistical study A sample is usually a subset of the population of interest,Population,Sample,“Sample Statistics” x = Sample mean s = Sample standard deviation,The Normal Distribution,What Measurements Can Be Used to Describe a Process or System ?,mean (average) or describes the location of the distribution, (m), a measure of central tendency, is the mean or average
