Example of Normal Capability Analysis

An engine manufacturer uses a forging process to make piston rings. The quality engineers want to assess the process capability. They collect 25 subgroups of five piston rings and measure the diameters. The specification limits for piston ring diameter are 74.0 mm ± 0.05 mm.

The engineers perform normal capability analysis to evaluate the diameters of the piston rings in relation to the specification limits.

  1. Open the sample data, PistonRingDiameter.MTW.
  2. Choose Stat > Quality Tools > Capability Analysis > Normal.
  3. In Single column, enter Diameter.
  4. In Subgroup size, enter 5.
  5. In Lower spec, enter 73.95.
  6. In Upper spec, enter 74.05.
  7. Click Options.
  8. In Target (adds Cpm to table), enter 74.
  9. Click OK in each dialog box.

Interpret the results

All the measurements are within the specification limits. The process is on target and the measurements are approximately centered between the specification limits. The capability indices Cpk, Ppk, and Cpm are all greater than 1.33, which is a generally accepted minimum value for a capable process. Therefore, the engineers conclude that the forging process meets the requirements for the diameters of the piston rings.

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