“Our Cp is good, still rejection is coming.”
This is a very common problem in production and quality departments.
The real reason is simple:
Cp shows only potential. Cpk shows reality.
Let us understand this clearly with practical meaning.
What is Cp – Process Potential
Cp tells us how wide the process spread is.
It checks only one thing:
Is the process variation inside specification limits or not?
Important points about Cp:
It assumes the process is perfectly centered
It does not care where the mean is
It shows best possible capability
Mostly used for comparison or study purpose
Example:
Your drawing limit is wide and machine variation is small.
Cp will look good even if the process is shifted.
So Cp can give a false confidence.
What is Cpk – Actual Process Capability
Cpk tells us how the process is actually running on the shop floor.
It checks two things:
Process spread
Process centering
Important points about Cpk:
It shows how close the mean is to specification limits
It reflects real production performance
It directly links to rejection and customer complaints
Customers always look at Cpk, not Cp
Example:
Even if variation is low, but the mean is near USL or LSL,
Cpk will drop and rejection will increase.
This is real life.
Why Cp Can Be Good but Cpk Is Poor
This happens when:
Tool setting is not correct
Process drifts over time
Operator adjustment is inconsistent
No centering control
No reaction plan for mean shift
In short, process is capable but not controlled.
Simple Shop-Floor Truth
Cp = What process can do
Cpk = What process is doing
Customers do not pay for potential.
They pay for consistent output.
Key Takeaway
Cp is useful for study and comparison
Cpk is critical for daily production control
Focus on centering, not only variation
Improve Cpk to reduce rejection and complaints
Strong processes are not wide.
They are centered and stable.
