DS-624 Quality Management
Chapter 15
F. Process and Performance Capability
DS-624 Quality Management
How are process limits or performance calculated?
What is the traditional definition of natural process limits?
How are process specifications determine?
When do you call a process capable?
What is Process Capability?
What are the different types of process variations?
What are examples of common causes of variation?
What are examples of special causes of variation?
What type of process variation is used to calculate process capability?
When is a Process considered Stable?
What are the steps to perform a process capability study?
What are the two calculations done to identify how capable a process is?
What is the difference between Cp and Cpk?
What are process capability scenarios ( Cp vs Cpk)?
Process Performance vs. Process Specifications
How are process limits or performance calculated?
Natural process limits are calculated from process variation. This is done after all special causes have been removed and the process has achieved statistical stability.
What is the traditional definition of natural process limits?
The traditional definition of natural process limits is +/- 3 sigma
How are process specifications determine?
Specifications, on the other hand, are expectations from the engineering or customer point of view.
Process Performance vs. Process Specifications
When do you call a process capable?
When the process variation is significantly lower than the width of the specification (upper limit – lower limit), then we call the process a capable process.
Process Capabilities Studies
What is Process Capability?
It is the ability of the process to meet the expected specifications.
What are the different types of process variations?
Every process has variation due to common causes and special causes, both internal and external to the process.
Process Capabilities Studies
What are examples of common causes of variation?
Interaction between process steps caused by the way the processes are sequenced.
Manufacturing equipment design.
Natural variation in incoming material supply.
Measuring equipment design.
Process Capabilities Studies
What are examples of special causes of variation?
Significant changes in incoming material quality
Operators with varying skills levels
Changes to process settings
Environmental variations
Equipment drift
Process Capabilities Studies
What type of process variation is used to calculate process capability?
The random process variations caused by common causes influence the ability of the process to meet the expected specifications (process capability)
Assignable (special) causes are investigated and removed before estimating the process capability.
When is a Process considered Stabled?
A process is tabled, when no special causes are present after plotting 20 to 30 subgroups of points.
Steps for Process Capabilities Studies
What are the steps to perform a process capability study?
Measurement System Verification. Once the MSA is performed and sources if variation are identified and removed, the percentage of measurement variation reduces to a small proportion of the overall process variation.
Identify appropriate rational subgrouping of samples for control chart plotting
Verify normality
If data is not normal, convert to normal
If data is normal, use a normal distribution table to estimate process capability
Process Capabilities Calculations
What are the two calculations done to identify how capable a process is?
Cp (Capability Index)
Cpk (Process Performance)
What is the difference between Cp and Cpk?
Cp tells us how capable our process is. If there is tons of variation your process may not be very capable at all. In this case you will want to reduce variation which will improve your Cp.
When you are happy with Cp you can move onto to Cpk which tells us how centered our process is. If your process is capable (good Cp) but is hugging the upper customer specification limit your Cpk will be poor. In to improve Cpk you will need to work on “shifting” the process back towards the mean of the process.
“Cpk is an index (a simple number) which measures how close a process is running to its specification limits,
Process Capabilities Calculations
What are process capability scenarios ( Cp vs Cpk)?
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