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