Quality & InspectionFree worksheetPDF, Excel, Word exportDiscrete Manufacturing

Gage Stability Study Worksheet (X-bar and R Limits)

Stability is the change in a measurement system over time when measuring the same master. This worksheet turns repeated readings into subgroup means and ranges and calculates X-bar and R control limits so you can see drift early.

Download:

Date	Reading 1	Reading 2	Reading 3	Subgroup mean	Subgroup range
				25.002	0.002
				25.0017	0.001
				25.0027	0.001
				25.003	0.002
				25.004	0.002
				25.0027	0.001

Grand mean

25.0027

Average range

0.0015

X-bar upper control limit

25.0042

X-bar lower control limit

25.0011

R upper control limit

0.0039

Limits use the standard constants for subgroups of three (A2 = 1.023, D4 = 2.574, D3 = 0). Collect at least 20 subgroups before treating the limits as stable. Compare each row's mean and range to the limits and look for trends and runs.

Automate stability monitoring

Optional review of how to trend gage checks from your ERP or QMS data.

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How to set up the study

Pick a master part or reference standard near the middle of the gage range. At a fixed interval, such as weekly, have a routine appraiser take three readings and enter them with the date. The worksheet computes the subgroup mean and range for each row and updates the grand mean, average range and control limits as you add data.

Use the same master, appraiser method and environment each time
Plan for at least 20 subgroups before trusting the limits
Store the master under controlled conditions between uses

The control limit formulas

For subgroups of size three, the X-bar limits are the grand mean plus or minus A2 times the average range, with A2 equal to 1.023. The upper range limit is D4 times the average range, with D4 equal to 2.574 and no lower limit for this subgroup size. Points outside the limits, runs on one side of the mean or steady trends indicate the measurement system is not stable.

Acting on the result

An unstable gage should not be used for decisions until the cause is found. Typical causes are wear, contamination, thermal change, a damaged master or an appraiser technique change. Record the investigation and corrective action in your calibration or CAPA system, and shorten the calibration interval if drift is real. A stable chart supports keeping or extending an interval.

Frequently Asked Questions

Why subgroups of three?

Three readings per session is a practical balance between effort and a useful range estimate. You can use other sizes, but the control chart constants change, so adjust the formulas in the exported Excel file.

How often should I run a stability check?

Many shops check critical gages weekly or each shift and others monthly. Choose a frequency based on how fast the gage can drift, how critical the feature is and your history of out-of-tolerance findings.

What if a point is out of control?

Stop using the gage for critical decisions, verify the master, repeat the readings, and investigate. If the gage is confirmed out of control, follow your out-of-tolerance procedure and assess previously measured product.

Does stability replace calibration?

No. Stability monitoring shows drift between calibrations and supports interval decisions. Calibration against traceable standards is still required.

Netray can help connect gage and inspection results from your ERP or MES to recurring stability reviews, with the data staying on your own systems.

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