IE405-Quality Management Review Recitation before MIDTERM 2 9/12/2002

SUMMARY OF SHEWART CONTROL CHARTS

Possible Applications: Determining the type of chart:Attribute or Variable graph Calculating the chart parameters:Centerline, UCL and LCL Evaluating process parameters; i.e What is (are) the best parameter(s) that is(are)representative of the process?

Process Capability ‘Refers to the ability of a process to meet the specifications set by the customer or the designer.’ Cp measures POTENTIAL CAPABILITY Why? Does not take into account where the process mean is located relative to the specifications Cpk measures ACTUAL CAPABILITY It takes process centering into account

Process Capability Ratios POTENTIAL CAPABILITY ACTUAL CAPABILITY

Possible Applications: Calculate the Cp and Cpk ratios Interpret their results: Cp>1Cp>1 Process is in control Cp=1Cp=1 Process is in control and centered at the midpoint of the specifications Cp<1Cp<1 Process is out of control. Cp=CpkCp=Cpk Process is centered Cpk<1Cpk<1 Process producing parts that do not conform to specifications Cpk=0Cpk=0 Process average is equal to one of the specification limits Cpk= -1Cpk= -1 Average is outside specification limits

GAGE AND MEASUREMENT SYSTEM CAPABILITY STUDIES

Characteristics of the X and R charts in gage and measurement system capability studies DISCRIMINATINGX chart shows the DISCRIMINATING POWER of the instrument, ability of the gage to distinguish between units of product. R chart shows the MAGNITUDE of measurement error, or gage capability. R values represent the difference between measurements made on the same unit using the same instrument.

Components of Measurement Error Reproducibility : due to different operators using the gage (different time periods,different environments, or in general different conditions) Repeatibility: as reflecting the basic inherent precision of the gage itself

Precision to Tolerance Ratio P/T OF 0.1 or LESS often implies adequate gage capability

OC Curve - Operating Characteristic Curve The OC curve shows how the probability of acceptance (y-axis) depends on the quality level (bottom axis).