【正文】 p k . LC p k . USmall sample sizes gives wide confidence intervals 55 169。 NOKIA 20xx Data Collection Sheet () 47 169。 NOKIA 20xx Estimate Cp and Cpk? The width of the normal distributions shown include 177。R) 33 169。 NOKIA 20xx The difference between the standard deviations sLT and sST gives an indication of how much better one can do when using appropriate production control, like Statistical Process Control (SPC). s x x x x x xNLTN? ? ? ? ? ? ??( ) ( ) . . . ( )1 2 2 2 21s R N d R dST jjN????? ???? ???12 2* *Shortterm standard deviation : Longterm standard deviation : The difference between sST and sLT 26 169。 (m252。 NOKIA 20xx Why Make Process Capability Studies LSL (lower specification limit) 10,7 USL (upper specification limit) 10,9 Nominal 10,8?0,1 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 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 14 169。 NOKIA 20xx Two Types of Product Characteristics Variable: A characteristic measured in physical units, . millimetres, volts, amps, decibel and seconds. ON OFF Attribute: A characteristic that by parison to some standard is judged “good” or “bad”, . free from scratches (visual quality). 6 169。1 169。 NOKIA 20xx Section 1 Variation, Tolerances and Dimensional Control 5 169。 NOKIA 20xx Process Capability What is it? ? Process Capability is a measure of the inherent capability of a manufacturing process to be able to consistently produce ponents 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 ponents 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 13 169。 NOKIA 20xx What Measurements Can Be Used to Describe a Process or System ? x x x xN N? ? ? ?1 2 . . .Example: x1 = 5 x2 = 7 x3 = 4 x4 = 2 x5 = 6 62475 ???????x? mean (average) or describes the location of the distribution x? 181。 NOKIA 20xx The Difference Between SST and sLT !! mean Time Dimension Short term Standard Deviation Long term Standard Deviation Subgroup size n = 5 Number of subgroups N = 7 Subgroup No. 1 25 169。 NOKIA 20xx What Do These Indexes Tell Us ?? ? Simple numerical values to describe the quality of the process The higher the number the better ?Requirement for Cp and Cpk is min. ?Remendation for Pp and Ppk is min. ? This leaves us some space for the variation, . a safety margin ? Are we able to improve our process by using SPC? ? If index is low, following things should be given a thought: ? Is the product design OK? ? Are tolerance limits set correctly? ? Too tight? ? Is the process capable of producing good quality products? Process variation? DOE required? ? Is the measuring system capable? (See Gage Ramp。 NOKIA 20xx Exercise 2: Cp and Cpk ? Calculate Cp and Cpk for the 100 measurements in the file Data exercise ? Determine the approximate Cp and Cpk for the 4 sample populations on the following page ? Should actions be made to improve these processes. If yes, which? 39 169。 NOKIA 20xx Example of how to Collect Data 1. Run in and stabilise process 2. Note the main parameters for reference 3. When the process is stable run the tool for 10 hours 3. Take 5 parts out from each cavity every half hour and mark them with time, date and cavity. Total 20 sets of 5 parts from each cavity