Cpk知识资料-香港新科、光宝课件

Our greatest weakness lies in giving up The most certain way to succeed is always to try just one more time Our greatest weakness lies in giving up The most certain way to succeed is always to try just one more time Our greatest weakness lies in giving up The most certain way to succeed is always to try just one more time Definition of Terms Capability The ability of a process to produce parts within specification A process is said to be capable when nearly all parts produced by the process are within specification Process Capability Indices Measures of the capability of a process X The average of a sample of data The sample standard deviation S USL LSL Upper and Lower specification x x n i s xx n i 2 1 Cp USLLSL s6 Cpk X s X s minimum USL 3 LSL 3 Our greatest weakness lies in giving up The most certain way to succeed is always to try just one more time Our greatest weakness lies in giving up The most certain way to succeed is always to try just one more time Our greatest weakness lies in giving up The most certain way to succeed is always to try just one more time Why use Cpk Cpk 5 75 91 01 151 301 331 523 or ppm13 2 4 0 7 2700 ppm 561 ppm 96 ppm 66 ppm 7 ppm 2 ppb 1 ppb Single number summary of process performance Can compare Performance across Processes Motivates variance reduction and process centering Distinguishes among in spec processes Cpk can be converted to an estimated defect rate for normally distributed processes These numbers assume a stable centered process Our greatest weakness lies in giving up The most certain way to succeed is always to try just one more time Proper Use of Cpk Assumptions 1 The process is statistically stable 2 Data is distributed approximately normally 3 Specifications represent customer needs 4 Process target is centered between specs 5 Measurement variation is small Our greatest weakness lies in giving up The most certain way to succeed is always to try just one more time More On Assumptions 1 The data collected should exhibit stability when plotted on a control chart Cpk of the unstable process may be representative of the historical performance of the process and not necessarily indicative of the ability of te process to meet the specs in the longer term 2 Because Cpk for the non normal process misrepresents the process due to the effect that non normality has some estimation 3 Specification limits used for calculating Cpk may be arbitrary Cpk may be good quality of product could be bad Cpk may be bad quality of product could be good Specs may be best guesses or based on process Changing specs causes Cpk to change while the performance of the process in unchanged Our greatest weakness lies in giving up The most certain way to succeed is always to try just one more time More on Assumptions 4 The target needs to be the center of the spec limits If the target is not the center of the spec limits then the process mean must be located in the interval target spec range 6 to target spec range 6 5 The measurements variation in the values used for calculating the Cpk must be small relative to the specific process capability for the parameter being measured If the measurement variation is comparatively large then the Cpk may not be indicative Because Cpk will be including this variation which is not related to the ability of thew process to meet the specifications Our greatest weakness lies in giving up The most certain way to succeed is always to try just one more time Proper Use of Cpk Calculating and Interpreting Cpk Collect at least 100 measurements over an extended time period 2 4 weeks or more Validate the assumptions above Calculate Cpk using the overall standard deviation of the individual data Don t use subgroup averages or pooled standard deviation Calculate a confidence interval for Cpk especially if n 100 Make a capability plot histogram with overlaid spec limits Resolve any differences Our greatest weakness lies in giving up The most certain way to succeed is always to try just one more time Top 10 List of Misuse and Confusion 1 People think Cpk is the only measure that matters 2 There is no agreement on which Cpk value is good 1 1 15 1 33 1 5 2 etc 3 Specification limits used for Cpk may be arbitrary 4 There are too many variations on the Cpk theme 5 People don t graph their data when interpreting Cpk 6 The scale of Cpk is non characteristic 7 Cpk is inappropriately used as a control parameter 8 The Cpk estimate for small samples is regarded as the truth 9 Cpk misrepresents non normal or unstable processes 10 The use of Cpk is mandated without proper training Our greatest weakness lies in giving up The most certain way to succeed is always to try just one more time Reason 10 The use of Cpk is mandated without proper training Used in situations where it doesn t fit Calculated just for management without any benefit to those who manage the process Our greatest weakness lies in giving up The most certain way to succeed is always to try just one more time Our greates