Special Characteristics Ppk Tracker Worksheet
A PPAP package for a part with many special characteristics needs one consolidated view of capability. This worksheet gives each characteristic a row, computes Pp, Ppk, PPM and the gap to 1.67, and flags the weakest one so you know where to spend effort first.
| Characteristic | Class | USL | LSL | Mean | Overall std dev | Pp | Ppk | Ppk minus 1.67 | Expected PPM | |
|---|---|---|---|---|---|---|---|---|---|---|
1.6667 | 1.6667 | -0.0033 | 1 | |||||||
1.4815 | 1.4074 | -0.2626 | 14 | |||||||
1.8182 | 1.697 | 0.027 | 0 | |||||||
2.0833 | 2.0417 | 0.3717 | 0 | |||||||
1.5152 | 1.4545 | -0.2155 | 8 |
Enter the overall standard deviation of all readings per characteristic. Rows with a negative gap column are below 1.67 and need an action plan or customer agreement. Use signed values for position or runout only if the customer accepts bilateral treatment; many treat them as one-sided.
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How to use the worksheet
Replace the example rows with your own characteristics from the control plan. For each, enter the specification limits, the mean and the overall standard deviation from the initial process study. Pp, Ppk, the gap to 1.67 and expected PPM calculate automatically. Add or remove rows as needed, sort in the exported Excel file, and attach the result to the PPAP package as a capability summary. Update the means and deviations after any process change and resubmit as needed.
What each column means
Pp measures how the process spread compares to the tolerance without regard to centering. Ppk adds centering by using the nearer limit. The gap column subtracts 1.67 from Ppk, so a negative value is a shortfall against a common initial study target. Expected PPM uses the normal distribution to estimate the fraction outside both limits. Class is for your internal sorting; use the names your customer defines, since their symbols for special characteristics differ.
Using the summary
The weakest Ppk is where the action plan starts. If several characteristics are below target, rank them by class first and then by gap. For marginal values, look at centering: a large difference between Pp and Ppk means moving the mean is the fastest improvement. For low Pp, the work is on variation, which usually means fixturing, tooling wear, material consistency or measurement error, and takes longer.
Common mistakes
Entering subgroup standard deviation instead of overall deviation overstates Ppk. Using different units for limits and data silently ruins the numbers. Treating one-sided characteristics such as flatness as bilateral by giving them an LSL of zero produces misleading results, since the data are not normal near zero. Finally, copying means from a trial run on a different cavity or setup than the one submitted.
Frequently Asked Questions
Can I export this for my PPAP package?
Yes. The worksheet exports to Excel, CSV and PDF. Attach it as a capability summary, and confirm the customer's required format before submission.
Why 1.67 as the gap reference?
1.67 is a common initial study target for special characteristics. If your customer requires a different value, subtract it from Ppk yourself or edit the exported sheet.
What about one-sided specs?
For one-sided limits, use only the relevant index. Enter a far-away dummy limit on the open side only if you understand it will drive that side to a very large number, and then read Ppk accordingly.
Does this replace statistical software?
No. It is a fast summary for reporting. Use statistical software for normality tests, control charts and non-normal capability methods, and bring the results here.
Netray can help feed inspection results from your ERP or MES into a tracker like this, so the capability summary is built from records, not re-keyed numbers.
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