Process Capability Analysis
Introduction
• “Practice makes man perfect”, but not even a skilled person can do the job perfectly without error.
• There is always scope for error, which is inherent to the part/component.
• So there is a term, called as tolerance, which is defined as we go on through the presentation.
Tolerances
• Tolerance is the maximum error, that can be accepted for a particular component.
• They are denoted in % or dimension.
• For example: diameter of a shaft should be (50 +/- 1%)mm. This indicates that the all the shafts in the diameter range (49.5mm,50.5mm) are accepted.
Causes for error
Common causes
SpecialCauses
Causes for error
• Common causes can be reduced by using improved machinery, skilled persons.
• Special causes have to addressed when they occur, they cannot be predicted, so cannot be reduced or eliminated.
Data Collection
• In our search, we have found that food and drugs industry has the most extensive quality control techniques.
• So we approached one company (name not to be revealed).
• They have shown us the process testing and statistics of one of their products.
• The charts are shown, as we are not in liberty to divulge the data.
Cp Index
• A measure of the process potential.
• It is measured using the beside equation.
• Ideal value is infinity, but Cp > 1 is reliable.
• Here “sd” is the standard deviation.
Cp = 𝑈𝑆𝐿 − 𝐿𝑆𝐿
6∗(𝑠𝑑)
Cpk Index
• A measure of the process potential and process centering.
• It is measured using the beside equation.
• Ideal value is infinity, but Cpk > 1 is reliable.
• Here “sd” is the standard deviation.
Cpk =min( 𝑈𝑆𝐿 −𝑚𝑒𝑎𝑛
3∗(𝑠𝑑),𝑚𝑒𝑎𝑛 −𝐿𝑆𝐿
3∗(𝑠𝑑))
CPU CPL
Significance of the indices
• Cpk is taken the minimum value to show where the defects are prominent. If CPU is minimum, then the defects are more towards upper specification side and similarly for CPL
• If Cp = Cpk, then the process is called a “CENTERED PROCESS”.
• Generally we find off-centered processes usually.
Thank You
B S V P Surya Teja, 2012A4PS039HK Rohit, 2012A4PS377H