1. CHAPTER 6 Control Charts for Variables
Quality Control
Source: Besterfield, Dale H., Quality Control.

2. Control Chart Techniques Establish the Revised Central Line and Control Limits

3. Control Chart Techniques Achieve the Objective

4. Control Chart Techniques The Sample Standard Deviation Control Chart
Replace the “R chart” with an “s chart”
s chart is more accurate than an R chart
s charts for subgroup sizes of 10 or more
R charts are sensitive to extreme values

5. Control Chart Techniques The Sample Standard Deviation Control Chart

6. State of Control Process in Control
eliminate assignable causes
develop ideas on how to make the process better
develop ideas to reduce process variation
implement quality improvement ideas
analyse process control charts

7. State of Control Process Out of Control
Out of control is usually thought of as being undesirable
situations where this condition is desirable
out of control  change due to an assignable cause
point (subgroup value) falls outside its control limit
assignable cause of variation is present

8. State of Control Process In Control

9. State of Control Process Out of Control

10. State of Control Analysis of Out-of-Control Condition
Process is out of control
assignable cause must be found
detective work necessary to locate the cause
Types of out-of-control “X bar” and R patterns are
change or jump in level
trend or steady change in level
recurring cycles
two populations
mistakes

11. Specifications Central Limit Theorem
Samples taken from a distribution that is not normal
averages will tend toward normality
provided that the sample size nP4
tendency gets better as sample size increases

12. Specifications Process Capability and Tolerance
Process spread = process capability = 6
difference between specifications = tolerance
tolerance is established by the design engineer
manufacturing must work with design engineer
three situations
process capability < tolerance
process capability = tolerance
process capability > tolerance

13. Specifications Process Capability and Tolerance
6 < USL — LSL
most desirable case

14. Specifications Process Capability and Tolerance
6 = USL — LSL

15. Specifications Process Capability and Tolerance
6 > USL — LSL
undesirable situation exists

16. Process Capability Capability Index

17. Process Capability Capability Index
Which is most desirable?

18. Different Control Charts Charts for Better Operator Understanding
Production may not understand X-bar and R charts
Other charts available
Chart for trends
upward or downward trend
Chart for moving average and moving range
common in the chemical industry
one reading is possible at a time
Chart for median and range
simplified variable control chart
minimizes calculations

19. Process Capability Cpk
Cp does not take target or nominal into account !
Cpk does take target or nominal into account
In-class assignment – find Cpk for the previous example
or work on excel labs

20. SME
Value Stream Mapping 40 min