Process Capability IE 4291 Tutorial 4

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Process Capability IE 4291 Tutorial 4

• He Bin
• Dept. of ISE

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The correct way to calculate S-bar

• You should use
• Not to use

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Question 1.

• Plastic strips that are used in a sensitive
  electronic device are manufactured to a maximum
  specification of 305.70mm and a minimum
  specification of 304.55mm. If the strips are less
  that the minimum specification, they are
  scrapped if greater than the maximum
  specification, they are reworked. The part
  dimensions are normally distributed with a
  population mean of 305.20mm and a standard
  deviation of 0.25mm. What percentage of the
  product is scrap? What percentage is rework? How
  can the process be centred to eliminate all but
  0.1 of the scrap? What is the rework percentage
  now?

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Question 1. (cont.)

• We are given
• USL 305.70 LSL 304.55
• 305.20 0.25

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Question 1. (cont.)

• Percentage of the scrapped P(XltLSL)
• P(Xlt304.55) P(Zlt-2.6)
• 0.4661
• Percentage of the reworked P(XgtUSL)
• P(Xgt305.70) P(Zgt2.0)
• 2.275

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Question 1. (cont.)

• s.t. P(XltLSL) 0.1
• P(XltLSL)
• Percentage of the reworked P(XgtUSL)
• P(Xgt305.70) P(Zgt1.5096)
• 6.557

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• The calculation of process capability indexes
  should be based on the estimates of process
  parameters from stable/in-control process.
• It is unsafe to estimate process capability in an
  out-of-control process.
• The process must be stable to produce a reliable
  estimate of process capability.
• In the case of an out-of-control process, the
  first objective is finding and eliminating the
  assignable causes to bring the process into an
  in-control state.

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Question 2.

• Compute the process capability indices for
  Problem 1 of the Tutorial Control Charts for
  Variables.
• From the results, we have
• USL 0.90 LSL 0.50 m 0.70

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Question 2. (cont.)
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Question 3.

• For Problem 4 of the Tutorial Control Charts for
  Variables, assume m 550, LSL 450 and USL
  650, compute the process capability indices.

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Question 3. (cont.)