Specification Limits, Tolerances

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Specification Limits, Tolerances

• Specification limit conformance boundary
  specified for a characteristic.
• Statistical tolerance limits limits of the
  interval for which it can be stated with a given
  level of confidence that it contains at least a
  specified proportion of the population

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Process Specification Conflicts

• Type I conflict the inherent variability of the
  process is too great or the specification limits
  are too narrow (Two sided specification limits
  and less than 6?).
• Type II conflict one-sided specification limits
  and less than 3 ?.
• Type III conflict the specification limits are
  too wide for acceptable quality.

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Actions in the case of Type I Specification
Conflict

• Action 1 - Changing the process (reduce the
  spread of the distribution)
• Action 2 try to avoid setting the specification
  limits tighter than necessary.
• Action 3 setting up an inspection/sorting
  operation (an interim procedure, not a permanent
  solution)
• Action 4 adjusting the center

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Actions in the Case of Type II Specification
Conflict

• Action 1- Attempt to center the process at a
  value far enough from the specification limit to
  avoid individual items outside of the limit (use
  xbar chart)
• Action 2 Changing the specification limit
• Action 3 Setting up an inspection/sorting
  operation to find and remove or repair the units
  of product that fall outside the specification
  limit.

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Actions in the Case of Type III Specification
Conflict

• Action 1 Conduct more formal programs of
  process experimentation to determine what range
  of values cause non-conformities.
• Action 2 Make specification limits more tight.
• Action 3 Establish specification limits on a
  temporary basis until final values are determined.

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The Process Capability Index Cp

• Cp (U-L)/6?
• If Cp 1, then the natural spread of the process
  equals the width of specification limits
• If Cp lt 1 a type I specification conflict occurs
• The most typical Cp 1.33 (U-L 8)

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