Six Sigma Training

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Six Sigma Training

• Dr. Robert O. Neidigh
• Dr. Robert Setaputra

2
Process Out of Control

• Point above UCL or below LCL
• 9 consecutive points on one side of center line,
  indicates that process mean has shifted
• 6 consecutive points increasing or decreasing,
  signals a drift in the process mean
• 14 consecutive points alternating up and down,
  indicates that two systematically alternating
  causes are producing different results
• 2 out of 3 consecutive points in Zone A or
  beyond, early warning of a process shift,
  probability of a Type II Error is approximately
  2

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Process Out of Control cont.

• 4 out of 5 consecutive points in Zone B or
  beyond, early warning of a process shift,
  probability of a Type II Error is approximately
  2
• 15 points in a row in Zone C, indicates less
  variability than expected, would like to
  determine why
• 8 consecutive points in Zones B and A or beyond
  on either side of the center line, indicates that
  different samples are affected by different
  factors resulting in a bimodal distribution of
  means

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Causes of Variation

• Common causes causes of variation that are
  inherent in the process, responsibility of
  management to change the policies and procedures
  that define the process, qualitative tools can be
  used to help make changes
• Assignable causes causes of variation that are
  not inherent in the process, responsibility of
  the workers and engineers, control charts are
  used extensively

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Process Capability

• Compares the output of an in-control process to
  the specification limits by using capability
  indices.
• Capability indices are ratios between the spread
  of the specification limits and the spread of the
  process.
• Two capability indices
• Cp
• Cpk

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Cp

• s known
• Cp (USL LSL)/6s
• s unknown
• Cp (USL LSL)/6S
• The higher, the better!

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Cp

• USL LSL 6s 8s 10s 12s
• Cp 1.00 1.33 1.66 2.00
• Rejects 0.27 64 ppm .6 ppm 2 ppb
• of spec 100 75 60 50
• Assumes mean is centered in specification limits

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Cpk

• s known
• Cpk min(USL µ)/3s, (µ LSL)/3s
• s unknown
• Cpk min(USL x-bar)/3S, (x-bar LSL)/3S
• The higher, the better!

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Qualitative Tools

• Cause and Effect Diagram
• Root Cause Analysis 5 Whys
• Failure Mode and Effects Analysis (FMEA)

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Cause and Effect Diagram

• Used to organize possible sources of variation in
  a CTQ
• Fishbone diagram
• Page 151

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Root Cause Analysis

• Ask what causes the problem
• Write down the answer
• If answer does not identify root cause of the
  problem, ask why again
• Continue until root cause is identified
• 5 Whys is rule of thumb

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FMEA

• Considers at causes of failures
• Redesign process to eliminate or minimize future
  failures
• Assigns risk priority number (RPN) based upon
  scales of severity, likelihood of occurrence, and
  detection
• The higher the RPN, the more important the
  failure
• After redesign, reassign RPN
• Pages 156 - 160

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What does Six Sigma Represent?

• Voice of the Process (VoP) should take up no more
  than half of the Voice of the Customer (VoC).
• Plus or minus 3s should take up no more than half
  of the specification limits. Thus, the
  specification limits should be no less than plus
  or minus 6s. If the mean is centered, than the
  defect rate is 2 ppb!!!
• It is common for process variation to increase
  periodically and then drop back to the normal
  level. Studies have shown that this increase is
  equivalent to a 1.5s shift in the mean. When this
  occurs, the defect rate is still only 3.4 ppm.
  This is why quality in six sigma is defined as
  3.4 ppm!

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Analyze Step and Six Sigma

• The tools provided in this section are used to
  decrease process variation, properly center the
  process mean, and to eliminate waste or
  unnecessary steps.
• Also, by listening to the VoC the process
  specifications can be properly determined.
• Control charts are not just tools to control a
  process, but more importantly are used to
  decrease variation through the detection and
  elimination of assignable causes of variation.