Understanding Control Charts in Statistical Process Control

1. Chapter 10

2. 50% 50% -3s -2s -1s 0 +1s +2s +3s ± ± ± s s s 2 3 1 .9973 .9546 .6826 Normal Distribution Zones A B C C B A z value = distance from the center measured in standard deviations

3. Normal Patterns • The process creating the data on the control chart is operating under statistical control • Produces a graphic that will have a high center, and sloping sides • The points tend to cluster around the center of the chart, show random variation, with only a few points spreading out toward the control limits • Points look random • Here is an example of a process running in statistical control:

4. Normal Patterns

5. Non-normal Patterns • Data that fluctuates excessively and fails to center itself around the centerline is characteristic of assignable or non-normal variation • Several of these patterns have been classified • The next few pages describe the most common patterns seen in processes • Not necessarily a bad thing • Heading in right direction • Result of improvement

6. Random Parts Out of Control - Freaks • A random part located outside of the control limits (1 point outside of zone A) • Occurs for a number of reasons • Any reason requires investigation before continuing to run the job • Reasons to occur:   • An incorrect machine adjustment that is immediately noticed and fixed • Errors in measurement or plotting • A cutting tool that “caught a chip” • May be normal variation

7. Random Parts Out of Control - Freaks

8. Sudden Shift in Level - Shifts • Set of seven or more consecutive points that are all on one side of the center line indicating the center has changed (7 points in zone C or beyond, all on one side of the center line) • Usually temporary / sudden • A sudden shift in the level of parts shown on a chart can be good or bad • Good: if the shift is bringing the parts back to split limit • Bad; if the shift is taking the parts away from split limit. • Reasons to occur:    • A change of material • New operator or inspector • An offset change • Two or more machines/suppliers on one chart

9. Sudden Shift in Level - Shifts

10. Trends - Runs • Trends can also form in the output of a product • Defined as consecutive points on a control chart that are steadily increasing or decreasing in value. (7 consecutive points that either increase or decrease in value – also, 10 out of 11 consecutive points that either increase or decrease in value ) • Usually gradual • Trends can be caused by:   • Air, coolant, or part temperatures that are steadily increasing or decreasing • Tool wear that allows a part to steadily increase or decrease in size • A fixture that is constantly wearing, causing the parts to steadily increase or decrease in size • Operator fatigue

11. Trends - Runs

12. Cycles • There are no number rules to identify cycles • Cycles are defined as repeated patterns in a process • Cycles can be caused by: • Machines that are continually heating up and cooling down • Air temperatures in the shop that rise to a certain point, then are reduced quickly as cooling systems are activated • Tool wear that allows a part to increase or decrease in size until an offset is made • Seasonal

13. Cycles

14. Grouping • Occurs when the points occur in clusters • Can be done visually • Can be done statistically (2 of 3 points in zone A or beyond – 4 of 5 points in zone B or beyond) • Can be caused by:   • Differences in setups • Tools moving • Method problems

15. Grouping .7510 .7505 . .7500 .7495 .7490 Grouping

16. Mixtures - Instability • Can be identified by looking for a majority of parts falling very close to the control limits, with very few in the center of the chart (5 or more consecutive points outside zone C) • Will have a "sawtooth" look to it • Typically, this type of situation is actually a combination of two separate distributions within a process, one at high limit, and one at low limit • Can be caused by:   • Two different gages being used • Output from two or more machines mixed together on the same chart

17. Stable mixtures • Can be identified by looking for erratic ups and downs, but not near the center line (5 or more consecutive points oiutside zone C) • Similar to stratification • Will have a "sawtooth" look to it • Can be caused by:   • Gaging concerns (rule of 10s) • Honest reporting?

18. Stratification • Can be identified by looking for a majority of parts hugging the center line (14 or more consecutive points inside zone C) • Will have a "sawtooth" look to it • Can be caused by:   • Gaging concerns (rule of 10s) • Honest reporting?

19. Mixtures / Stratification

20. Class exercise • Let’s finish the exercise on pg. 594/602

21. Class exercise (pg 595)