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Process Capability Studies

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Natural Limits - indicate what a process is capable of producing. ... Upper Natural Limit (UNL) = process centerline 3(process standard deviation) ... – PowerPoint PPT presentation

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Title: Process Capability Studies


1
Process Capability Studies
  • Before performing a process capability study the
    process must be stable.
  • The natural behavior of a process is termed its
    process capability.
  • Once the process capability study is completed,
    process improvement studies can start (PDSA)
    cycle.
  • For attributes, process capability studies are
    carried out with control charts and rational
    subgrouping as discussed previously.
  • We will concentrate on Variables process
    capability studies

2
  • Natural Limits - indicate what a process is
    capable of producing. That is they provide an
    indication of the variability that is inherent in
    the process.
  • They are calculated as follows
  • Upper Natural Limit (UNL) process centerline
    3(process standard deviation)
  • Lower Natural Limit (LNL) process centerline -
    3(process standard deviation)
  • Note it is assumed that the measurements follow
    a normal distribution

3
  • Example
  • A process is stable, centered, and distributed
    normally with an average of 30 lbs. And an
    average range of 4.8 lbs. Find the natural
    limits(assume n 5)

4
Process capability indices
  • Process capability indices are used to state a
    processs ability to meet specifications in one
    summary statistic.
  • Assumptions
  • Process is stable
  • Variables data
  • Process is normally distributed
  • Cp Used to summarize the processes ability to
    meet a two sided specification limit.
  • It is assumed that the process average (x bar
    bar) is centered on nominal

5
  • CPU Used to summarize a processs ability to
    meet a one sided upper specification limit.
  • CPL Used to summarize a processs capability to
    meet a one sided lower specification limit.

6
  • Cpk Used to summarize a processs ability to meet
    a two-sided specification without assuming the
    process is centered on nominal.
  • Note
  • If a process is unstable process capability
    studies are meaningless
  • If a process is not normally distributed,
    out-of-spec fraction can be incorrectly
    calculated.

7
  • Example (Problem 2 chapter 11)
  • The ABC Co. produces steel tubes. The steel
    tube process is a stable cut-to-length operation
    that generates tubes that are distributed
    normally with a mean of 12.00 inches and a
    standard deviation of 0.10 inch.
  • The LMN Co. wishes to but tubes from the ABC Co.
    The LMN Co. requires steel tubes 11.95 inches
    long with a tolerance of .30 inches.
  • Compute Cp, CPU, CPL and Cpk
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