Requirements:
Following are general requirements of all capable measurement systems:
1. Statistical stability over time.
2. Variability small compared to the process variability.
3. Variability small compared to the specification limits (tolerance).
4. The resolution or discrimination of the measurement device must be small relative to the smaller of either the specification tolerance or the process spread (variation). As a rule of thumb, the measurement system should have resolution of at least 1/10th the smaller of either the specification tolerance or the process spread. If the resolution is not fine enough, process variability will not be recognized by the measurement system, thus blunting its effectiveness.
Measurement Systems Analysis Fundamentals:
a) Determine the number of appraisers, number of sample parts, and the number of repeat readings. Larger numbers of parts and repeat readings give results with a higher confidence level, but the numbers should be balanced against the time, cost, and disruption involved.
b) Use appraisers who normally perform the measurement and who are familiar with the equipment and procedures.
c) Make sure there is a set, documented measurement procedure that is followed by all appraisers.
d) Select the sample parts to represent the entire process spread. This is a critical point. If the process spread is not fully represented, the degree of measurement error may be overstated.
e) If applicable, mark the exact measurement location on each part to minimize the impact of within-part variation (e.g. out-of-round).
f) Ensure that the measurement device has adequate discrimination/resolution, as discussed in the Requirements section.
g) Parts should be numbered, and the measurements should be taken in random order so that the appraisers do not know the number assigned to each part or any previous measurement value for that part. A third party should record the measurements, the appraiser, the trial number, and the number for each part on a table.
Stability Assessment
1. Select a part from the middle of the process spread and determine its reference value relative to a traceable standard. If a traceable standard is not available, measure the part ten times in a controlled environment and average the values to determine the Reference Value. This part/sample will be designated as the Master Sample .
2. Over at least twenty periods (days/weeks), measure the master sample 3 to 5 times. Keep the number of repeats fixed. Take readings throughout the period to capture the natural environmental variation.
3. Plot the data on an x̄ & R chart - consult the Statistical Process Control section of the Toolbox and calculate control limits.
4. Evaluate the control chart for statistical control. Again, consult the Statistical Process Control section of the Toolbox for assistance with this assessment.
Bias Assessment
1. Referring to the & R chart, subtract the Reference Value from to yield the Bias:
Bias = x̄ - Reference Value
Process Variation = 6 Standard Deviations (Sigma)
2. Calculate the Bias percentage:
Bias Percentage = Bias / Process Variation
3. Analyze the results. If there is a relatively high value, examine the following potential root causes:
a. Appraisers not following the measurement procedure
b. An error in measuring the Reference Value
c. Instability in the measurement. If the SPC chart shows a trend, the measurement device could be wearing or calibration could be drifting.
