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 Change History : Issue Date Handled By Comments 1.0 21st Dec01 Jim Christy & Sren Lundsfryd Approved for Global Use NOTE All comments and improvements should be addressed to the creator of this document.,Contents,Section Heading / Description Page 1 Variation, Tolerances and Dimensional Control 4 2 Population, Sample and Normal Distribution 15 3 Cp and Cpk Concept 28 4 Use of the NMP Data Collection Spreadsheet 44 5 Confidence of Cpk 52,Process Capability - Evaluating Manufacturing Variation,Acknowledgements Benny Matthiassen (NMP CMT, Copenhagen, Denmark) Frank Adler (NMP Alliance, Dallas, USA) Joni Laakso (NMP R&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,The Sources of Process/System Variation,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,Unstable 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 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 Statistical 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 most 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 quantity, 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,Process 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 Process Capability Studies,A process capability study would reveal that the tool should not be accepted,When a dimension needs to be kept 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,Section 2. Population, Sample and Normal Distribution,The Bell Shaped (Normal) Distribution,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 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,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 of all values in the population. When only a sample of the population is being described, mean is more properly denoted as (x-bar) :,The most simple measure of variability is the range. The range of a sample is defined by as the difference between the largest and the smallest observation from samples in a sub-group, e.g. 5 consecutive parts from the manufacturing process.,What Measurements Can Be Used to Describe Process variation?,sST - often notated as or sigma, is another measure of dispersion or variability and stands for “short-term standard deviation”, which measures the variability of a process or system using “rational” sub-grouping.,