ments by severity and risk of occur- rence. Some FMEAs have hundreds of line items.” FMEA shows you what could go
wrong in your process. Te control plan outlines for your customer how you will control the process so it doesn’t go wrong. PPAP requires measurement system analysis (MSA) studies to use statistics to prove your process is in control. “Measuring systems have bias, so it is important to know what the bias is and whether it is going to cause what you are measuring to appear different than it actually is,” Hartigan said. If your customer calls out critical
dimensions or properties such as tensile strength and hardness, the statistical tools are additional ways the customer can verify the castings are delivered as specified. Like the rest of the PPAP documentation, the statistical evidence requires sig- nificant effort. Additional training may be needed to properly perform the measurements and calculate the statistical data. St. Paul Brass & Aluminum brought in an outside consultant and set aside two full days to train its entire team on the process. “Statistics do apply to castings,”
Hartigan said. “Even though we have a wide band of tolerances, error can be introduced in the process in many places. Te supplier has to figure out whether it is consistently hitting a certain point in the dimension or showing too much variability.” Caterpillar provides feedback to
its casting supplier when it sees red flags such as too much variability in the PPAP. It may reject the PPAP or interim approve it for a short time to give the supplier time to correct a concern. “Te bottom line is, the PPAP is a time stamp of what the supplier did when it made this part and it is a living document that is reviewed and updated periodically,” Helgeson said. “For instance, when an engineer changes a part, he can go back and look at the history of the PPAP and see what the issues were that might be a concern to him, as well.”
September 2013 MODERN CASTING | 43
Caterpillar will review the PPAP if it sees a dimensional trend in the castings it receives, where a dimen- sion is drifting and the measure- ments are different than expected. In the review, the supplier and Caterpil- lar may find if control of a process is slipping or if tool wear is occurring. “Te PPAP is not just done once and
then put away,” Helgeson said. Dimensional results aren’t the
only measurement examined. Cater- pillar pays close attention to metal- lurgical results, as well. Tese may be addressed in PPAP documentation. “Due to the additional strength
requirements of the applications being developed, the metallurgical
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