Process Capability and Performance

Process Capability and Performance- Process capability is a predictable pattern of statistically stable behavior where the chance causes of variation are compared to the engineering specifications.

Process Capability Studies
Process Capability and Performance- A process capability study attempts to quantify whether a process can consistently meet the standards set by internal or external customers.

Study Procedure – It includes various steps, as

• Select a process to study which is critical and can be selected using several techniques like a Pareto analysis or a cause-and-effect diagram.
  Verify or define the process parameters. Verification of what the process entails, its boundaries, and gain agreement on the process’s definition. Many of these steps are completed when developing a process map.
• Conduct a measurement systems analysis to ensure that the measurement methods produce sound data.
• Select a process capability analysis method like Cpk, Cp, Ppk and Pp.
• Obtain the data and conduct an analysis.
• Develop an estimate of the process capability. This estimate can be compared to the standards set by internal or external customers.

Identifying Characteristics

Characteristics selected to be part of a process capability study should meet certain requirements

Identifying Specifications/Tolerances
The process specifications or tolerances are determined either by customer requirements, industry standards, or the organization’s engineering department.

Developing Sampling Plans
For new processes, for example for a project proposal, a pilot run may be used to estimate the process capability.

Specification Limits
Specification limits are set by the customer, and result from either customer requirements or industry standards.

Verifying Stability and Normality
If only common causes of variation are present in a process, then the output of the process forms a distribution that is stable over time and is predictable.

Process performance vs. specification
The performance metric indices establish a controlled process, and then maintain that process over time.

Process Limits
A stable process can be monitored to determine if changes that occur are due to factors other than random variation.

Process Performance Metric
It is a measure of an organization’s activities and performance and includes metrics like percentage defective which is defined as the (Total number of defective parts)/(Total number of parts) X 100.

Process capability indices
Process capability indices includes Cp and Cpk, who identify the current state of the process and provide statistical evidence for comparing after-adjustment results to the starting point.

• Cp – It measures the ratio between the specification tolerance (USL-LSL) and process spread.
• Cpk – It measures the absolute distance of the mean to the nearest specification limit.
• Cpm – It is also referred to as the Taguchi index. It is more accurate and reliable than the other indices. It focuses on reducing the variation from a target value (T).

Sigma and Process Capability
When means and variances wander over time, a standard deviation (symbolized by the Greek letter σ) is the most common way to describe how data in a sample varies from its mean.

Process Performance Indices
The most used process performance indices are Pp, Ppk, and Cpm which depict the present status of the process and also act as an important tool for improvement of the process. These metrics have a common purpose as process capability indices but, they differ in their approach.

• Pp – It measures the ratio between the specification tolerance and process spread.
• Ppk – It measures the absolute distance of the mean to the nearest specification limit. It provides an initial measurement to center on specification limits.

Short-term vs. long-term capability
Short-term capability is measured over a very short time period since it focuses on the machine’s ability based on design and quality of construction. By focusing on one machine with one operator during one shift, it limits the influence of other outside long-term factors, including operator, environmental conditions such as temperature and humidity, machine wear and different material lots.

Process capability for attributes data
The control chart represents the process capability, once special causes have been identified and removed from the process.

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