A small demonstration of process capabitlity index in quality control

1. Process Capability Analysis

2. 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.

3. 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.

4. Causes
for error
Common
causes
Special
Causes

5. 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.

6. 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.

9. 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)

10. 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

11. 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.

12. ThankYou
B S V P Surya Teja, 2012A4PS039H
K Rohit, 2012A4PS377H