Xbar and R (range) control charts

This guide will walk you through configuring a measurement template designed for statistical process control. An Xbar and R chart is for monitoring process stability and capability when using small, consistent subgroups.

Part 1: Configuring the Measure Template (The Setup)

Before you begin collecting data, you must define the "container" for your measurements. This ensures the math remains consistent throughout the life of the process.

Defining Observations per Date (Subgroups)

For an Xbar and R chart, data is collected in "subgroups." To align with your specific requirement:

  • Observations (Rows): Set your template to require the number of per entry (e.g. 4 entries per entry).

Setting Initial Targets and Tolerances

The software uses these values to calculate whether your process is "capable" of meeting customer requirements.

  • Target (Nominal): Enter your primary engineering goal (e.g., 0.0201).
  • Tolerances: Define your Tolerance (e.g., ±0.0010), and this will determine the Upper and Lower Specification Limits (USL/LSL)


Part 2: Recording Measurements

Once the template is saved, a Measure can be created for the data entry phase.

Adding Observation Values

When you enter a new date or time entry, the system will present empty fields (your Y-axis readings).

  • Ensure measurements are entered in the order they were produced.
  • The system will automatically calculate the Mean (Xbar), Standard Deviation and the Range (R) for that specific set of entries.


Part 3: Managing Performance and Capability

After data entry begins, you can monitor the health of your process through three key indices: Cp, Cpk, and Ppk.

Editing Targets and Tolerances

If there is an engineering change (ECR), you can update the Target or Tolerance within the measure settings.


Understanding the Indices

The software automatically generates these values to satisfy PPAP (Production Part Approval Process) requirements:

  • Cp (Potential Capability): This tells you if your process is capable of fitting within the tolerance if it were perfectly centered. It ignores where the process is currently aimed and looks only at the "spread."
  • Cpk (Process Capability): This tells you how the process is actually doing. It accounts for both the spread and how well you are hitting the target.
  • Ppk (Process Performance): While Cpk looks at "short-term" variation within your groups of readings, Ppk looks at the "long-term" performance of all readings combined.

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