Measurement System Analysis (MSA)

What is MSA?

 - MSA is a experimental and mathematical method of determining the amount of variation that exists within a measurement process.

 - Variation in the measurement process can directly contribute to overall process variability.

 - MSA used to certify the measurement system for use by evaluating the system’s accuracy, precision and stability.

 - It measures the quantification of particular characteristics of any object, It includes combination of gauges, fixtures, software's and personnel required to measure the characteristics of any object.

Basics of MSA

 - The sources of variation in measurement process include:

1) Process - Test methods and specifications

2) Personnel - Operators, their skill level etc.

3) Equipment's - Gauges, fixtures, test equipment's and their calibration system.

4) Object to be measured - the part and sampling plan.

5) Environmental factors - Temperature, humidity etc.

 - While selecting equipment following basic points need to be considered:

1) Range of equipment

2) Accuracy

3) Least count and other relevant factors (Least count of the instrument should be 1/10th of the tolerance range.)

Elements of MSA

1) Bias

 - Bias means difference between measured value and true value (Actual value/Metrological value) of a part. If true value is unknown then it can be calculated by averaging several measurements with the most accurate measuring equipment.

2) Linearity

 - Linearity means consistency of instrument over the different range of measurement.
 i.e. if we measure one object (30 mm width) with Vernier it shows 30 mm and another object (130 mm width) with same Vernier and it shows 130.1 mm. then it is not linear as 0.1mm bias is observed.

3) Repeatability

 - Repeatability means variation when same operator measures the same part repeatedly with the same instrument.

4) Reproducibility

 - Reproducibility means variation when different operator measures the same part with same instrument.

5) Stability

 - Stability means capacity of a system to produce the same values over time when measuring the same sample.

6) Attribute Data

 - Data that can be counted for recording and analysis (True-false, Go-No-go etc.)

7) Variable Data

 - Data that can be measured. Data that has a value and vary from one sample to the next.

Why Perform MSA?

 - To understand the impact of these system on operations.

 - To make effective measurements, they must be timely, accurate and precise.

 - We can get good quality products reducing errors in system.

Measurement Systems Analysis Fundamentals

 - Determine the total appraisers, quantity of sample parts, and the number of repeat readings. more 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.

 - Use appraisers who normally perform the measurements and familiar with the equipment and procedures.

 - Make sure there is a set and a documented measurement procedure that is followed by all appraisers.

 - 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.

 - If applicable, mark the exact measurement location on each part to minimize the impact of part variation (e.g. out-of-round).

 - Ensure that the measurement device has adequate resolution (ability to detect small changes).

 - 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 there to record the measurements, the appraiser, the trial number, and the number for each part on a table.

Variable Gauge R&R Study

Gauge Repeatability and Reproducibility (Gauge R & R) can be performed to evaluate the level of uncertainty within a measurement system. To perform a Gage R & R, perform the following steps:

 - Obtain at least 10 random samples of parts manufactured during a regular production.

 - Choose three operators or inspectors that regularly perform the particular inspection.

 - Each operator measure the sample parts and record the data.

 - Repeat the measurement process three times with each operator for the same parts.

 - Calculate the average (mean) readings and the range of the trial averages for each of the operators.

 - Calculate the difference of each operator’s averages, average range and the range of measurements for each sample part used in the study.

 - Calculate repeatability to determine the amount of equipment variation.

 - Calculate reproducibility to determine the amount of variation introduced by the operators.

 - Calculate the variation in the parts and total variation percentages.

Guidelines for making determination for accepting the gauge are :

 - The measurement system is acceptable if the Gauge R & R score falls below 10%

 - The measurement system may be determined acceptable depending upon the relative importance of the application or other factors if the Gauge R & R falls between 10% to 20%

 - Any measurement system with Gauge R & R greater than 30% requires action to improve and should be evaluated for effectiveness.

 - If the repeatability value is large in comparison to the reproducibility value, it would indicate a possible issue with the gauge used for the study. The gauge may need to be replaced or re-calibrated.

 - If the reproducibility value is large in comparison with the repeatability value, it would indicate the variation is operator related. The operator may need additional training on the proper use of the gauge or a fixture may be required to assist the operator in using the gauge.

Attribute Gauge R&R Study

 - Determine the gauge to be studied.

 - Obtain 10 random samples from a regular production.

 - Select 3 different operators who perform the particular inspection activity regularly

 - Have the operators perform the inspection three times for each of the sample parts and record the data.

 - Next, calculate the kappa value.

 - When the kappa value is greater than 0.6, the gauge is deemed acceptable

 - If not, the gauge may need to be replaced or calibrated

Gauge R&R Study shall be conducted under following circumstances:

1) When a new or different measurement system is introduced.

2) Following any improvement activities performed on the current measurement system due to the results of a previous Gauge R&R study.

3) Annually in alignment with set calibration schedule of the gauge.

Sample MSA report is linked to this text.