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

Control Chart Techniques Establish the Revised Central Line and Control Limits

Control Chart Techniques Achieve the Objective

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

Control Chart Techniques The Sample Standard Deviation Control Chart

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

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

State of Control Process In Control

State of Control Process Out of Control

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

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

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

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

Specifications Process Capability and Tolerance 6 = USL — LSL

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

Process Capability Capability Index

Process Capability Capability Index Which is most desirable?

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

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

SME Value Stream Mapping 40 min