【正文】 The Bell Shaped (Normal) Distribution ? Symmetrical shape with a peak in the middle of the range of the data. ? Indicates that the input variables (X39。 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 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 outes 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 outes, but arise because of specific circumstances –Special causes can be identified using Statistical Process Control (SPC) USL LSL Nominal value Defect USL LSL nominal value 9 169。 NOKIA 20xx Section 1 Variation, Tolerances and Dimensional Control 5 169。ren Lundsfryd Approved for Global Use NOTE – All ments and improvements should be addressed to the creator of this document. 2 169。1 169。 NOKIA 20xx 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 3 169。 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。 NOKIA 20xx Tolerances LSL (lower specification limit) 10,7 USL (upper specification limit) 10,9 Acceptable part Rejected Part Rejected Product Nominal 10,8?0,1 Rejected Part A tolerance is a allowed maximum variation of a dimension. 10 169。 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。s) to the process are randomly influenced. 17 169。 (m252。 NOKIA 20xx d2* values for SST Where: N = no. of subgroups, n = no. of samples in each subgroup d2* d2 Typical: N=20 amp。 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。 NOKIA 20xx Defining Cp and Pp Sample mean Process variation 6*s stp sL SLU SLC*6? USLLSL LSL USL Nominal dim ltp sL SLU SLP*6?The tolerance area divided by the total process variation, irrespective of process centring. 30 169。R) 33 16