CONTROL CHART BASIS. BPT2423 – STATISTICAL PROCESS CONTROL. CHAPTER OUTLINE. Control Chart Functions Variation Basic Principles Choice of Control Limits Upper Control Limit (UCL) Lower Control Limit (LCL) Sample Size and Sampling Frequency Rational Subgroups Analysis of Patterns.
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BPT2423 – STATISTICAL PROCESS CONTROL
Provide an economic basis for making a decision as to whether to investigate for potential problems; to adjust the process or to leave the process alone
Point out where improvement is needed and help to provide a statistical basis on which to formulate improvement actions
Stable and Unstable Variation
The Chart contains:
Upper Control Limit (UCL) = µw+ 3σw
Center Line = µw
Lower Control Limit (LCL) = µw– 3σw
ARL = 1/p
where p is the probability that any point exceeds the control limits
Chart with 3σ limits, p=0.0027 : the probability that a single point falls outside the limits when the process is in control
ARL = 1/p = 1/0.0027 = 370
which means an out-of-control signal will be generated every 370 samples, on the average
Some guidelines to be followed
A control chart exhibits a state of control when:
A process that is not under control or is unstable, displays patterns of variation - the process need to be investigate and determine if an assignable cause can be found for the variation
Patterns in Control Chart:
Example : Change / Jump / Shift
The process begins at one level and jump quickly to another level as the process continues to operate. Causes for sudden shifts in level tend to reflect some new and significant difference in the process.
Possible causes :
New operator, new batches of raw material or changes to the process settings