Ch 19: Process Capability

Throughput Yield (TPY)

= (Output / Input) x 100%

Defects per Million Opportunities (DPMO)

= (Total number of defects / Total number of opportunities) x 1,000,000

Percentage Defective

= (total number of defective units / total number of units) x 100%

Defects per Unit (DPU)

= Total number of defects / Total number of units

Process Capability Index

A dimensionless number that is used to represent the ability to meet customer specifications. The common indices are Cp, Cpk, and Cpm.

Process Performance Index

A dimensionless number that is used to represent the ability to meet specification limits on a characteristic of interest. Three basic process performance indices are Pp, Ppk, Ppm.

Process performance

A statistical measure of the outcome of a characteristic from a process that may not have been demonstrated to be in a state of statistical control.

Cpm

Also know as the process capability index of the mean, is an index that accounts for the location of the process average relative to a target value.

Long-Term Capability

Are the process performance indices Pp and Ppk process capability measures. This is because the standard deviation used in their calculation is calculated using the Root Mean Square Error Method. Which is inflated by shifts in the process mean

Conducting a Process Capability Study: Step 2 of 8

Confirm the measurement system used to acquire the process data. Large measurement system variation can obscure the "true" process variation and will produce an unreliable estimate of process capability.

Cp Formula

Cp = (USL - LSL) / 6σ

Cp Index

Describes process capability in relation to the specified tolerance of a characteristic divided by the natural process variation for a process in a stat of statistical control.

Conducting a Process Capability Study: Step 7 of 8

Determine process capability indices and interpret them. If the process in not capable , work to improve it.

Cpk

Determines the proximity of the process average to the nearest specification limit.

Conducting a Process Capability Study: Step 3 of 8

Gather the data. Ideally, tracked on a control chart.

Conducting a Process Capability Study: Step 6 of 8

Obtain the process specification. The process specifications may be either one- or two-sided specifications.

Short-Term Capability

Process performance Indices Cp and Cpk process capability measures. The standard deviation used in their calculation is calculated using the Range Method which is not affected by changes to the process mean.

Specification Limits

Refers to the customer determined acceptable levels of process performance.

Natural Process Limits

Refers to the ±3σ limits around the process average.

Rolled Throughput Yield (RTY)

Represents the percentage of units of product passing defect free through an entire process. It is determined by multiplying throughput yields from each sub-process of the total process.

Conducting a Process Capability Study: Step 1 of 8

Select a quality characteristic of a specific process for study. Ideally, a capability study should be performed for every critical quality characteristic.

Process Capability

The inherent variability of the characteristics of a process.

Parts per Million (ppm)

The number of times a defective part will occur in a million parts produced.

Conducting a Process Capability Study: Step 8 of 8

Update the process control plan, Once the process has achieved a desired level of capability, update your control plan to ensure that the gains are maintained. If no control plan is present, create one.

Conducting a Process Capability Study: Step 4 of 8

Verify process stability and ensure that the process is in control. This can easily be accomplished using control charts.

Conducting a Process Capability Study: Step 5 of 8

Verify that the individual data are normally distributed. Plot the data in a histogram and test for normality.