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Chapter 8

Chapter 8. Variable charts for limited data. We will now consider the precontrol chart and the individual X and MR chart. Techniques are similar to the charts we have already discussed. Precontrol chart. Used to monitor a process

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Chapter 8

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  1. Chapter 8

  2. Variable charts for limited data • We will now consider the precontrol chart and the individual X and MR chart. • Techniques are similar to the charts we have already discussed.

  3. Precontrol chart • Used to monitor a process • Process needs to be charted with a variable control chart first • Process needs to be in control and capable • a.k.a Rainbow and Stop Light Charts

  4. Precontrol chart • If you are “out-of-control” on a pre-control chart • Are you capable? • Possible assignable cause? • Fill out a variable control chart • Do not lose your actual data values

  5. Procedure for Precontrol Charts • Is process capable (Cpk>1.33*) and in control? • Divide tolerance by 4 to create zones Based on the normal distribution(Cpk=1): 86% will be in the green zone, 7% in each yellow zone

  6. Rules • Rules for precontrol • Start with 5 consecutive green pieces • Take a sample • Green – keep running • Yellow – check next piece • Green - keep running • Yellow – stop, check, adjust if necessary • Red – stop and adjust if necessary • Do not make any adjustment until the process signals you • Reduce sampling after 25 consistent green pieces

  7. Probability of out-of control • Based on a z=4 process, what is the probability of getting 2 yellows consecutively?

  8. Precontrol exercise notes

  9. Precontrol exercise 2-3 (pg 598)

  10. MS means midpoint specification Modified Precontrol Charts • Up to this point we have seen how precontrol charts are commonly used • Limits based on tolerance • Part acceptance mentality • We can also base the charts on process capability • Limits based on standard deviation • Calculated similarly • Instead of dividing the tolerance by 4 you divide the process width by 4

  11. What is an x and MR chart? • Pronounced “individual x and moving range” • The most common chart used with limited data • Each point on the chart represents an individual value • Used when subgroup samples need to be 1 • Works well with processes that have trends that develop and disappear quickly

  12. 11 step procedure for control charts • Select a process measurement • Stabilize process and decrease obvious variability • Check the gages (10:1, GRR) • Make a sample plan • Setup the charts and process log • Setup the histogram • Take the samples and chart the points – at least 10 measurements before calculations • Calculate the control limits and analyze for control - histogram • Calculate the capability and analyze for capability • Monitor the process ( ) • Continuous Improvement

  13. For the moving range control chart: For the individual control chart: How to calculate control limits estimate s by UCL-LCL 6

  14. IX & MR exercise notes (data pg 598)

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