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    Situation in production

    Two operators, same part, different values

    The same part gives different results depending on who measures or with what.

    What happens

    The same part gives different values depending on who measures, with which gauge or on which occasion.

    What should apply

    The variation of the measurement system itself should be small compared with the tolerance and with the process variation.

    Why it is difficult

    • Measurement variation does not show as such. It looks like process variation.
    • The process may be adjusted needlessly, or a real change may be missed.
    • Capability values become misleading when the measurement varies too much.

    The context that is needed

    • Gauge, fixture and measurement method
    • Who measured
    • Repeated measurements of the same part
    • The tolerance for the characteristic
    • The calibration status of the gauge

    How much of the variation you see comes from the measurement?

    Illustration · Example data

    1 · What you measure is wider than the process

    LSLUSLProcess: σ 1.00 · Cp 1.33Measured: σ 1.17 · Cp 1.14

    2 · Variances add, spreads do not

    1.00+0.36=1.36σ² processσ² meas.σ² measuredSide = σ. Area = σ² (variance).0.6 + 1.0 gives 1.17, not 1.6.

    3 · The measurement system variation has two parts

    Same part, five readings per operatorABCaba Within one operator: repeatabilityb Between means: reproducibility

    The variation you see is the process variation plus the measurement system variation. It is the variances that add, so a moderate measurement variation makes the spread a little larger but can still lower the capability index noticeably.

    Source: AIAG, Measurement Systems Analysis Reference Manual, 4th edition, 2010

    Step by step

    • The process spread (σ) is 1.00 and the measurement system's 0.60 in the example.
    • The variances add: 1.00 + 0.36 = 1.36. The measured spread becomes √1.36 ≈ 1.17, not 1.6.
    • With a tolerance of ±4, Cp is 1.33 in reality but 1.14 as measured.
    • The measurement system variation has two parts: repeatability (the spread when the same operator measures the same part several times) and reproducibility (the difference between different operators' means).

    Same situation, different questions

    Who needs to know what

    The same event looks different depending on which decision you have to make.

    The same evidence for everyone

    • Gauge, fixture and measurement method
    • Who measured
    • Repeated measurements of the same part
    • The tolerance for the characteristic
    • The calibration status of the gauge

    • Operator

      Asks

      "Am I measuring the right way?"

      Decides

      How the measurement should be done.

    • Quality

      Asks

      "How much of the variation comes from the measurement?"

      Decides

      Whether a measurement system analysis is needed, and whether the measurement system is fit for purpose.

    • Engineering and planning

      Asks

      "Does the measurement method or the fixture need to change?"

      Decides

      A change of measurement method, fixture or gauge.

    What may follow

    1. 01Run a measurement system analysis (Gage R&R)
    2. 02Change measurement method, fixture or gauge
    3. 03Train people in how to measure
    4. 04Check the calibration of the gauge

    Knowledge

    Concepts that help

    Where Rektron can help

    What is available today

    Measurement system analysis with Gage R&R is available in Eyesight and in Rektron SQC. Gauge register and calibration are available in Eyesight Calibration and in Rektron NetGauge.

    Common in: Plastics and injection moulding

    Similar situations

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    Tell us what it looks like where you are, and we will go through what is needed to handle it.

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