# Process Capability Studies - PowerPoint PPT Presentation

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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