Cpk Software at DataLyzer.com
Cpk and Ppk Benchmarking
As an example of legal requirement for statistical benchmarks, automotive supplier sales cycle requires the supplier to initially submit documentation of compliance to Production Part Approval Process (PPAP – See AIAG PPAP Manual 3rd Edition, item I.184.108.40.206.1- I.220.127.116.11). In general, this process requires production sample parts to be run by the supplier through his production process (equivalent to 1-8 hours of processing, min. 300 pieces), and then a subset of samples are measured and analyzed (100+ consecutive pieces and could be more depending on the subgroup sample size required by the supplier’s control plan). If Cpk or Ppk values are high enough (1.67+), the supplier’s products may be considered for purchase in the future. As a condition of purchase, however, the supplier must warrant the Cpk or Ppk values of all future production parts supplied shall meet or exceed the Cpk or Ppk values certified in the original PPAP. The customer typically dictates whether Cpk or Ppk shall be used. We do not recommend using both statistics for PPAP validation. (Cpk should only be considered if process stability can be demonstrated).
Cpk/Ppk and Datalyzer® Spectrum
Cpk and Ppk benchmark settings are available in certain Datalyzer® Spectrum software platforms. Global Cpk/Ppk benchmark values can be set from the DataLyzer® Spectrum main menu. Cpk/Ppk benchmarks can also be set by characteristic. Cpk and Ppk can be selected to appear in both control chart and Capability Histogram on the screen and in printed reports.
The declared Cpk or Ppk values are used as reference in several areas:
1) During data entry at the HUB or Satellite, all characteristics with benchmark Cpk/Ppk values are monitored. If the last 25 points (or number matching your personal settings) of the control chart produce a Cpk/Ppk lower than the benchmark setting, a dark red flag is produced in the status area, and users can enter a note.
2) In the Performance report, all characteristics’ current Cpk/Ppk values below the benchmark value can be reported.
3) In the Capability History report, a line representing the benchmark Cpk/Ppk appears on the report for reference to the current and historical values. The capability history report can display capabilities in a bar chart, bat chart, or dot chart.
4) User defined reports are available to customize a report for your needs.
5) In the system notes, an internal warning note is saved each time the Cpk/Ppk goes below the benchmark value. When viewing the notes report, it is possible to view each time a characteristic fell below the benchmark value.
6) Control charts and histograms can display the current Cpk and Ppk values, as explained above.
The following graph is an example of a daily benchmark Ppk bar graph. The red line represents the benchmark Ppk value. The first bar on the graph (labeled “All”) represents the capability of the entire process.
The following graph is an example of a daily benchmark Ppk dot graph. The red line represents the benchmark Ppk value. The first bar on the graph (labeled “All”) represents the capability of the entire process.
Datalyzer® Spectrum by Stephen Computer Services, Inc. offers the latest in CpK and Ppk statistics analysis reports.
Calculations included in the reports: Cpk, CpIndex, CpRatio calculations are done instantly; Ppk, PpIndex, PpRatio.
Capability Study/Histogram Report
Statistics include: Process Performance values (Ppk, PpRatio, PpIndex), Process Capability values (Cpk, CpRatio, CpIndex), Sample size, mean, sigma, skewness, kurtosis, three, four, five, and six sigma limits, "Best-fit" frequency distribution analysis (Normal, Gamma, Beta, 2nd Approximation of Normal), percent below lower specification, percent above upper specification, upper and lower Z values, PPM.
Cpk Software at DataLyzer.com