Section 2 of the PPAP requires a design record, in- cluding a blueprint or 3D model, geo- metric dimension- ing and toleranc- ing, and additional specifi cations.
have varying degrees of requirements, depending on the application of the part and how critical it is to the whole. T e process was fi rst developed by the automotive industry to avoid critical failure that could result in death. “FMEA is basically asking what
can go wrong, and if it does, how badly can it aff ect things,” he said. “You can prioritize the control documents by severity and risk of occurrence. Some FMEAs have hundreds of line items.” It is the job of the casting supplier to use FMEA to show you what could go wrong in its process. T e control plan shows how it can control the process so it doesn’t go wrong. PPAP requires measurement system analysis (MSA) studies to use statistics to prove your supplier’s 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 diff erent than it actually is,” Hartigan said. If you are calling out critical di-
mensions or properties such as tensile strength and hardness, the statistical tools provide another step to ensure the casting supplier is delivering the part as specifi ed. Like the rest of the PPAP documentation, the statistical evidence requires signifi cant eff ort from the supplier. If you are working with a supplier who is new to PPAP recordkeeping or you are asking for brand new line items, often additional training will be needed to properly perform the measurements and calcu- late 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. T e supplier has to fi gure out whether it is consistently hitting a certain point in the dimension or showing too much variability.” As the customer, Caterpillar pro-
vides feedback to its casting supplier when it sees red fl ags 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.
34 | METAL CASTING DESIGN & PURCHASING | Jul/Aug 2013
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