PP, or Process Capability, is calculated using a specific formula that compares the voice of the customer (specification limits) to the voice of the process (standard deviation). This formula determines if a process is capable of producing output within customer specifications.
PP Calculation Formula
The formula for calculating PP is:
Pp = (USL – LSL) / (6 * s)
Where:
- USL = Upper Specification Limit
- LSL = Lower Specification Limit
- s = Standard Deviation (of the process)
Understanding the Components
Let's break down each component of the formula:
- USL (Upper Specification Limit): This is the highest acceptable value defined by the customer or design requirements. Any output exceeding this limit is considered non-conforming.
- LSL (Lower Specification Limit): This is the lowest acceptable value defined by the customer or design requirements. Any output falling below this limit is also considered non-conforming.
- Standard Deviation (s): The standard deviation represents the dispersion or fatness of the bell curve representing your process's output. A smaller standard deviation indicates that the data points are clustered closely around the mean, signifying a more consistent process. A larger standard deviation indicates that the data points are more spread out, indicating a less consistent process. It is a measure of the variability within the process. It is critical to calculating PP.
PP Interpretation
The resulting PP value indicates the potential capability of a process.
- A higher PP value generally indicates a more capable process.
- A PP of 1.0 means the process is barely capable; the process's natural variation is just fitting within the specification limits.
- A PP of less than 1.0 indicates that the process is not capable of meeting specifications.
- A PP greater than 1.0 suggests the process is capable, with higher values indicating greater capability. Common benchmark values are 1.33 and 1.67, depending on the industry and criticality of the application.
Example Calculation
Let's say you are manufacturing bolts. The specification for the length of the bolt is:
- USL = 10.5 mm
- LSL = 9.5 mm
After measuring a sample of bolts, you find the standard deviation (s) of the process is 0.1 mm.
Using the formula:
Pp = (10.5 - 9.5) / (6 * 0.1)
Pp = 1 / 0.6
Pp = 1.67
This PP value of 1.67 suggests that the process is capable of producing bolts within the specified limits.
Practical Insights
- PP only considers the potential capability of the process. It doesn't account for process centering (how well the process mean aligns with the target value).
- PP is calculated using short-term variation. It does not factor in long-term shifts or drifts in the process. Cpk is used to measure process capability by factoring in centering.
- Accurate data collection is crucial for reliable PP calculations. Ensure the data used to calculate the standard deviation is representative of the process.
