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Achieving Acceptable Cp and Cpk Values in Sheetmetal Stampings
ISSN: 0148-7191, e-ISSN: 2688-3627
Published September 28, 1999 by SAE International in United States
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Detail parts are approved during several different phases of the prototype build cycle. There is much pressure at all stages to meet strategic body quality targets. Parts stamped for assembly must meet a process capability requirement of Cpk>1.33. For final PSO (process sign off), as called out in the PPAP (Production Part Approval Process) manual, the requirement can be increased to meeting a Cpk>1.67.
During the 2000 Neon part approval process, the PPAP requirements provided the guideline necessary for consistent buy-offs. However, on some critical parts the Cpk requirement made part approvals difficult to accomplish. Occasionally this caused resources to be focused in the wrong place. This paper will discuss how a requirement of Cpk>1.33 can make part approvals more difficult to achieve and change the entire application of a tolerance. This paper will outline Dimensional Control’s goals for part specifications and how capability and build requirements can be achieved without sacrificing those goals.
This paper will show 2000 Neon parts presented to DaimlerChrysler for approval. It will discuss Dimensional Control’s strategy for meeting capability requirements on all these parts while meeting all of the body in white dimensional integrity.
Using example parts from the 2000 Neon program, it will show the strategy and options used by Dimensional Control and Product Engineering to achieve acceptable Cp and Cpk values, as well as body dimensional objectives. This paper will discuss the advantages and disadvantages for four different options that are available on three different types of parts.