 We will now consider the precontrol chart and the individual X and MR chart.  Techniques are similar to the charts we have already discussed.

 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

 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

Is process capable (Cpk>1.33*) and in control? 2.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

Rules for precontrol 1.Start with 5 consecutive green pieces 2.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 3.Do not make any adjustment until the process signals you 4.Reduce sampling after 25 consistent green pieces

 Based on a z=4 process, what is the probability of getting 2 yellows consecutively?

10  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 MS means midpoint specification

11  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 1. Select a process measurement 2. Stabilize process and decrease obvious variability 3. Check the gages (10:1, GRR) 4. Make a sample plan 5. Setup the charts and process log 6. Setup the histogram 7. Take the samples and chart the points – at least 10 measurements before calculations 8. Calculate the control limits and analyze for control - histogram 9. Calculate the capability and analyze for capability 10. Monitor the process ( ) 11. Continuous Improvement

13 For the moving range control chart: For the individual control chart: estimate s by UCL-LCL 6

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