Session 5 - Quality Management Tools 2 Flashcards
What is the Process Capability Index?
Cp
The process capability index, or process capability ratio, is a statistical measure of process capability: the ability of an engineering process to produce an output within specification limits.
Can tell whether a process is capable.
What are the the measures used in find the Cp?
1- UCL
2- LCL
3- Process Mean (µ)
4- LSL
5- USL
How do you find Cp?
Cp = ( USL - LSL ) / 6σ
can also be worded as =
New Requirement / Current Process
What are the rules when it comes to whether the process is capable in Cp?
All of the process variation has to be capable for the entire process to be capable
Cp < 1 , process “not capable” of fulfilling the requirements
Cp = 1 , process is “borderline capable” of fulfilling the requirements
Cp > 1 , the process is capable of fulfilling the requirements
How far are the UCL and LCL from the process mean (µ)?
3σ
- 3 standard deviations
What happens when the process is not capable and Cp < 1?
In this case, the new specifications are stricter than the current process variations
- There are “Higher” level of requirements from the new spec
In order for the current process to be capable of and meet the new specs, what must a firm do?
Needs to redesign the process in order to meet the new requirements
What is the rule when it comes to meeting the new specification requirements when the process was initially not capable?
USL - LSL < 6σ
What happens when the process was barely capable?
Cp = 1
In this case, the current process variation “exactly meets” the new specification and the current process is borderline/barely capable of meeting the new specs
What is the general rule when Cp = 1?
USL - LCL = 6σ
What happens when the process is capable?
Cp > 1
The current process is stricter than new specifications
It means any process variations will fall within the new spec limits
What can a firm do when the current process is capable of meeting the new specification?
The firm can fulfill the new requirements
What is the general rule when Cp > 1?
USL - LSL > 6σ
How do you find whether a process is capable according to Cp?
Cp = ( USL - LSL ) / 6σ
What happens when you increase process variance?
It reduces process capability
What is a limitation of Cp (when Cp =1) when the process mean is the same as the mean of the new spec?
The process is barely capable according to the Cp value
What is a limitation of Cp (when Cp =1) when the process mean is not in line with the mean of the new specs?
aka the process mean is lower than the spec mean
The process is not capable even when the Cp =1
What is another way to measure process capability other than Cp?
Cpk
What does Cpk do?
Takes into account distances between process mean (µ) and two spec limits (LSL and USL) separately
What is the rule when calculating Cpk?
Cpk < 1 , Process is not capable
Cpk = 1 , Process is borderline capable
Cpk > 1 , Process is capable
How do you calculate Cpk?
Cpk = min { (USL - µ / 3σ) , (µ - LSL / 3σ) }
it’s the minimum between the upper spec limit minus the process mean divided by 3 times process standard deviation and the process mean minus the lower spec limit divided by 3 times process standard deviation
What is a summary for Cp?
is a good process capability measure only when the process mean does not skew from the center of the specification limits
What is a summary for Cpk?
Takes into account the process mean, thereby is not susceptible to a skewed process mean (in relation to the center of the spec limits)
What is Six Sigma used for?
To achieve near perfect products
