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UNCLASSIFIED / FOUO UNCLASSIFIED / FOUO National Guard Black Belt Training Module 24 Measurement System Analysis (MSA) Continuous Data UNCLASSIFIED / FOUO UNCLASSIFIED / FOUO
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UNCLASSIFIED / FOUOCPI Roadmap – Measure 8-STEP PROCESS 6. See 1.Validate 2. Identify 3. Set 4. Determine 5. Develop 7. Confirm 8. Standardize Counter- the Performance Improvement Root Counter- Results Successful Measures Problem Gaps Targets Cause Measures & Process Processes Through Define Measure Analyze Improve Control TOOLS •Process Mapping ACTIVITIES • Map Current Process / Go & See •Process Cycle Efficiency/TOC • Identify Key Input, Process, Output Metrics •Little’s Law • Develop Operational Definitions •Operational Definitions • Develop Data Collection Plan •Data Collection Plan • Validate Measurement System •Statistical Sampling • Collect Baseline Data •Measurement System Analysis • Identify Performance Gaps •TPM • Estimate Financial/Operational Benefits •Generic Pull • Determine Process Stability/Capability •Setup Reduction • Complete Measure Tollgate •Control Charts •Histograms •Constraint Identification •Process Capability Note: Activities and tools vary by project. Lists provided here are not necessarily all-inclusive. UNCLASSIFIED / FOUO
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UNCLASSIFIED / FOUO Learning Objective Understand how to conduct and interpret a measurement system analysis using Continuous Data Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 3
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UNCLASSIFIED / FOUO Acceptable Measurement Systems Properties that all acceptable measurement systems must have: The measurement system must be in control (only common cause variation) Variability of the measurement system must be small in relation to the process variation Variability of the measurement system must be small compared with the specification limits (the tolerance) The increments of the measurement must be small relative to the smaller of: the process variability or the specification limits Rule of thumb: increments are to be no greater than 1/10th of the smaller of process variability or specification limits Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 4
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UNCLASSIFIED / FOUO Measurement System Study- Prep Plan the approach: Select number of appraisers, number of samples and number of repeat measures Use at least 2 appraisers and 5 samples, where each appraiser measures each sample at least twice (all using same device) Select appraisers who normally do the measurement Select samples from the process that represent its entire operating range. Label each sample discretely so the label is not visible to the operator. Check that the instrument has a discrimination that is equal to or less than 1/10 of the expected process variability or specification limits Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 5
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UNCLASSIFIED / FOUO Measurement Study – Prep (cont.) Assure that the gage/instrument has been maintained and calibrated to traceable standards Parts are selected specifically to represent the full process variation Parts should come from both outside the specs (high side and low side) and from within the specification range Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 6
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UNCLASSIFIED / FOUO Running the Measurement Study In order to run the MSA: Each sample should be measured 2-3 times by each operator Make sure the parts are marked for ease of data collection but remain “blind”(unidentifiable) to the operators Be there for the study and record any unplanned influences. Randomize the parts continuously during the study to preclude operators influencing the test The first time evaluating a given measurement process, let the process run as it would normally run Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 7
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UNCLASSIFIED / FOUO Running the Study – Guidelines Because in many cases we are unsure of how “noise” can affect our measurement system, we recommend the following procedure: Have the first operator measure all the samples once in random order Have the second operator measure all the samples once in random order Continue until all operators have measured the samples once (this is Trial 1) Repeat the previous two steps each time for the required number of trials Use a form to collect information Analyze results Determine follow-up action, if any Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 8
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UNCLASSIFIED / FOUO Exercise: Run MSA in Minitab Can we trust our measurement system? The maintenance function at an ANG airlift wing is evaluating a vendor’s non-destructive testing (NDT) method that claims to be better, faster and less expensive Faster NDT reduces overall cycle time for inspections of airframe, hence an Upper Specification Limit Faster is better, but too fast an NDT cycle time might mean an inadequate time for the penetration of the dyes into hairline fractures, hence the Lower Specification Limit USL minus LSL = Tolerance SL minus Mean Response = One Sided Tolerance This MSA evaluates the ability of the measurement system to detect changes in overall NDT inspection cycle time Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 9
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UNCLASSIFIED / FOUO MSA Example in Minitab Ten parts were selected that represent the expected range of the part type variation. Three inspectors measured the ten parts, three times per part, in a random order. This data set is Gage3.mtw. Column Name Description C1 Part Part Number C2 Operator Test Operator number C3 Response Cycle Time for inspection Above is the description of the data from Minitab Is it short form? Long form? Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 10
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UNCLASSIFIED / FOUOData Set = Gage3.mtw Stat>Quality Tools>Gage Study>Gage R&R Study (Crossed) Note: Gage R&R Study (Crossed) is the most commonly used method for Variables (Continuous Data). It is used when the same parts can be tested multiple times, i.e. NON DESTRUCTIVE TESTS. GR&R (Nested) MSA is for DESTRUCTIVE TESTING. . Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 11
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UNCLASSIFIED / FOUOGage R&R in Minitab Enter the variables (circled fields) in the above dialogue box and keep the ANOVA method of analysis checked. The main difference between ANOVA and Xbar and R is that ANOVA will estimate an operator by part interaction. The ANOVA method is the preferred method. Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 12
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UNCLASSIFIED / FOUOGage R&R in Minitab (Cont.) Gage R&R Study (Crossed) dialog box After entering the variables in this dialog box, click on Options to view the options dialog box Options dialog box Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 13
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UNCLASSIFIED / FOUOGage R&R in Minitab – Options Options dialog box. 6.0 is the default for the number of sd in the Study variation. This is the Z value range that calculates a 99.73% potential Study Variation based on the calculated Standard Deviation of the variation seen in the parts chosen for the study. Alternatively, you may see texts use 5.15 sd, that corresponds to 99%. The Spec Limits for the process are 10.75 as the USL and 8.75 as the LSL. You can either enter these in the appropriate boxes (be sure to click on Enter at least one specification limit), OR you can enter the Process tolerance (Upper spec – Lower spec = 10.75 – 8.75 = 2.0) by clicking and entering 2.0 in Upper spec – Lower spec. (Either way gives the same results.) The Process Sigma has been 0.195. Enter .195 in the Dialog Box for Historical standard deviation. Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 14
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UNCLASSIFIED / FOUO Interpreting Acceptability If Process Tolerance and Historical Sigma values are not used in Minitab, a critical assumption is then made that the sample parts chosen for the study, truthfully exhibit the true process variation. In this case, the acceptability of the measurement system is based upon comparison only to the part variation seen in the study. This can be a valid assumption if care is taken in selecting the study sample parts. One element of criteria whether a measurement system is acceptable to analyze a process is the percentage of the part tolerance or the operational process variation that is consumed by measurement system variation. Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 15
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UNCLASSIFIED / FOUOMinitab Gage R&R - ‘Six-Pack’ Gage R&R (ANOVA) for Response Let’s look at these six Reported by : G age name: D ate of study : charts one at a time Tolerance: M isc: Components of Variation Response by Part 100 % Contribution % Study Var 10.00 Percent % Process % Tolerance 9.75 50 9.50 0 Gage R&R Repeat Reprod Part-to-Part 1 2 3 4 5 6 7 8 9 10 Part R Chart by Operator 1 2 3 Response by Operator UCL=0.1073 0.10 10.00 Sample Range _ 9.75 0.05 R=0.0417 9.50 0.00 LCL=0 1 2 3 Operator Xbar Chart by Operator 1 2 3 Operator * Part Interaction 10.00 10.00 O perator Sample Mean 1 _ _ Average UCL=9.8422 2 X=9.7996 9.75 9.75 LCL=9.7569 3 9.50 9.50 1 2 3 4 5 6 7 8 9 10 Part Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 16
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UNCLASSIFIED / FOUO Gage R&R - Relationships A measurement process is said to be consistent when the results for operators are Repeatable and the results between operators are Reproducible A gage is valid to detect part-to-part variation when the variability of operator measurements is small relative to process variability or the tolerance range The percent of process variation consumed by the measurement (% R&R) is then determined once the measurement process is consistent and can detect part-to-part variation Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 17
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UNCLASSIFIED / FOUO Six Pack – #1 Components of Variation Focus on the 3 Bars to the right in each cluster. These represent the % of total variance contributed from the data. Gage R&R is the total variation in our measurement system broken into repeatability and reproducibility. The part to part Study Variation bar is an estimate of our Total Gage R&R process variation. Operator + Between Inspectors Equipment/Gage Or Insp. to Insp. Remember why we Operator measure? An estimate of Process Within the Gage (or Part) Variation Or one Inspector unless the Historical Equipment/Gage Sigma is entered Parts Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 18
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UNCLASSIFIED / FOUO Six Pack – #2 R Chart By Operator Repeatability is checked by using a special Range Chart where the differences in the measurements by each operator on each part is charted. If the difference between the largest value of a measured part and the smallest value of the same part does not exceed the UCL, then that gage and operator are considered to be Repeatable. Repeatability is indicated when virtually all of the range points lie under the upper control limit on the range chart. Any points that fall above the limit need to be investigated. Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 19
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UNCLASSIFIED / FOUOSix Pack – #3 X bar Chart By Operator Reproducibility is best determined analytically using the tabulation analysis in the Minitab Session (discussed in following slides). Graphically it might be seen if there are significant differences in the operator patterns generated by each operator measuring the same samples. Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 20
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UNCLASSIFIED / FOUO X bar Chart By Operator (Cont.) It is desirable to see plots that consistently go outside the UCL and LCL because limits are determined by gage variance and these plots should show that gage variance is much smaller than variability within the parts If the samples chosen do not represent the total variability of the process, the gage (repeatability) variance may be larger than the part variance and invalidate the distinct categories calculation If the patterns of the operators are not comparable, there may be significant operator and part interactions (discussed on another slide) On this chart you want At Least 50% of the points to be Outside the Control Limits Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 21
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UNCLASSIFIED / FOUOGage R&R - Six Pack (Cont.) What do these control limits represent in terms of our Measurement System? Is the Range (R) Chart in control? Where do the limits on the Xbar Chart and the R Chart come from? Do we want the R Chart and the Xbar Chart in or out of control? Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 22
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UNCLASSIFIED / FOUO Gage R&R – Six-Pack Charts (Cont.) This graph shows the data for the ten parts for all operators plotted together. It displays the raw data and highlights the average of those measurements. Part Issues Similar to the top graph but the data is presented by each operator instead of by part. This graph will help identify Operator Issues. This graph shows the data for each operator for all ten parts. It is the easiest to use to uncover Operator & Part Interaction. Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 23
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UNCLASSIFIED / FOUO Six Pack – #4 Response By Part This graph shows the data for all ten parts for all operators plotted together. It should show plots that vary from the smallest dimensions for the parts made by the process to the largest dimensions for the same parts. Parts should be both in tolerance and out of tolerance if the process makes them. If a part shows a large spread, it might be a poor candidate for the test because the feature may not be clear on that part. Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 24
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UNCLASSIFIED / FOUO Six Pack – #5 Response By Operator This graph shows the data for all ten parts for plotted by each operator. The red line connecting the averages of all 10 parts measured by each operator should be horizontal. Any significant slope is an indication that this operator has a general bias to measure large or small when compared to the other operators Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 25
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UNCLASSIFIED / FOUO Six Pack – #6 Operator * Part Interaction Operator Influence: If the lines connecting the plotted average points diverge significantly, then there is a relationship between the operator making the measurements and the part that the operator is measuring. This is not good and needs to be investigated. Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 26
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UNCLASSIFIED / FOUO Minitab Gage R&R - Six-Pack (Cont.) Questions on the graphical output? Gage R&R (ANOVA) for Response Reported by : G age name: Tolerance: D ate of study : M isc: Components of Variation Response by Part 100 % Contribution % Study Var 10.00 Percent % Process % Tolerance 9.75 50 9.50 0 Gage R&R Repeat Reprod Part-to-Part 1 2 3 4 5 6 7 8 9 10 Part R Chart by Operator 1 2 3 Response by Operator UCL=0.1073 0.10 10.00 Sample Range _ 9.75 0.05 R=0.0417 9.50 0.00 LCL=0 1 2 3 Operator Xbar Chart by Operator 1 2 3 Operator * Part Interaction 10.00 10.00 O perator Sample Mean 1 _ _ Average UCL=9.8422 2 X=9.7996 9.75 9.75 LCL=9.7569 3 9.50 9.50 1 2 3 4 5 6 7 8 9 10 Part Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 27
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UNCLASSIFIED / FOUOGage R&R – Session Window Let’s take this output one chunk at a time. These 3 Values should all be Less Than 30% for process improvement efforts These values may not add to 100% Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 28
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UNCLASSIFIED / FOUOGage R&R - The Numerical Output (Cont.) We would like this to be less than 9% This table is from the Minitab Session window. It is an easy-to- understand tabulation of the amount of MSA variation from each source. The first column represents the source of variation, the second column is an estimate of the actual variation for each source (factor). The third column is the linear % that each represents of the total variation. It is depicted as the black bar on the Pareto in the six- pack diagram. Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 29
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UNCLASSIFIED / FOUOGage R&R - The Numerical Output (Cont.) These should all be Less Than 30% This should be 4 or More This tabulation from Minitab builds the % of Study Variation that each source contributes to a calculated potential Total Variation seen in the study. The 6.0 * SD is how statistically 99.73% of the Total Variation is calculated and this is assumed to equal 99.73% of the true process variation unless the Historical Sigma is input into Minitab. The percentages are used to grade the validity of the measurement system to perform measurement analysis using percentages already taught. If the process is performing well, the %Tolerance is then important. The sum of the percentages might add to more than 100% due to the math. The Number of Distinct Categories represents the number of non-overlapping confidence intervals that this measurement system can reliably distinguish in the product variation. We would like that number to be 5 or higher. Four is marginal. Fewer than 4 implies that the measurement system can only work with attribute data. DC= (s parts/s GRR total)* 2 Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 30
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UNCLASSIFIED / FOUO Interpreting Acceptability in Session Window Category Acceptable Marginal Not Acceptable (Green) (Yellow) (Red) % Contribution < 1% 1% to 9% > 9% % Study Var < 10% 10% to 30% > 30% % Tolerance < 10% 10% to 30% > 30% % Process < 10% 10% to 30% > 30% Number of Distinct Categories >5 4 <4 Marginal: Might be acceptable based upon the risk of the application, cost of measurement device, cost of repair, etc. Not Acceptable: Every effort should be made to improve the measurement system Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 31
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UNCLASSIFIED / FOUOGage R&R - Conclusions Gage R&R (ANOVA) for Response Is thisby : Reported Measurement G age name: Tolerance: D ate of study : M isc: System OK ? Components of Variation Response by Part 100 % Contribution % Study Var 10.00 Percent % Process % Tolerance 9.75 50 9.50 0 Gage R&R Repeat Reprod Part-to-Part 1 2 3 4 5 6 7 8 9 10 Part R Chart by Operator 1 2 3 Response by Operator UCL=0.1073 0.10 10.00 Sample Range _ 9.75 0.05 R=0.0417 9.50 0.00 LCL=0 1 2 3 Operator Xbar Chart by Operator 1 2 3 Operator * Part Interaction 10.00 10.00 Operator Sample Mean 1 _ _ Average UCL=9.8422 2 X=9.7996 9.75 9.75 LCL=9.7569 3 9.50 9.50 1 2 3 4 5 6 7 8 9 10 Part Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 32
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UNCLASSIFIED / FOUO Let’s Do It Again Three parts were selected that represent the expected range of the process variation. Three operators measured the three parts, three times per part, in a random order. No History of the process is available and Tolerances are not established Go to exercise set: Gage2.mtw This data set is used to illustrate Gage R&R Study and Gage Run Chart Column Name Count Description C1 Part 27 Part number C2 Operator 27 Operator number C3 Response 27 Measurement value C4 Trial 27 Trial number Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 33
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UNCLASSIFIED / FOUOData Set = Gage2.mtw Stat>Quality Tools>Gage Study>Gage R&R Study (Crossed) Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 34
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UNCLASSIFIED / FOUOFilling in the Dialog Boxes 1. Set cursor in Part numbers box and double click on C-1 Part 2. Set cursor in Operators box and double click on C-2 Operator 3. Set cursor in Measurement data box and double click on C-3 Response 4. Make sure ANOVA is selected and click on OK Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 35
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UNCLASSIFIED / FOUO How Does This Measurement System Look? Why is this study unacceptable? Gage R&R (ANOVA) for Response Reported by : G age name: Tolerance: D ate of study : M isc: Components of Variation Response by Part 100 % Contribution 600 % Study Var Percent 400 50 200 0 Gage R&R Repeat Reprod Part-to-Part 1 2 3 Part R Chart by Operator 1 2 3 Response by Operator 400 UCL=376.5 600 Sample Range 400 200 _ R=146.3 200 0 LCL=0 1 2 3 Operator Xbar Chart by Operator 1 2 3 Operator * Part Interaction UCL=555.8 O perator 500 Sample Mean 1 450 Average _ _ 2 3 400 X=406.2 400 300 350 LCL=256.5 1 2 3 Part Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 36
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UNCLASSIFIED / FOUOGage2.mtw - Results This should be less than 30% for process improvement efforts What does this tell you? Remember this? What does this mean ? Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 37
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UNCLASSIFIED / FOUO Gage2.mtw – Conclusions What needs to be addressed first? Where do we begin improving this measurement system? Gage R&R (ANOVA) for Response Reported by : G age name: Tolerance: D ate of study : M isc: Components of Variation Response by Part 100 % Contribution 600 % Study Var Percent 400 50 200 0 Gage R&R Repeat Reprod Part-to-Part 1 2 3 Part R Chart by Operator 1 2 3 Response by Operator 400 UCL=376.5 600 Sample Range 400 200 _ R=146.3 200 0 LCL=0 1 2 3 Operator Xbar Chart by Operator 1 2 3 Operator * Part Interaction UCL=555.8 O perator 500 Sample Mean 1 450 Average _ _ 2 3 400 X=406.2 400 300 350 LCL=256.5 1 2 3 Part Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 38
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UNCLASSIFIED / FOUO One-Sided Specifications Typically, in the transactions environment, customer specifications are only one-sided. For example, most of the time an upper specification alone given on cycle time… faster is always better. How does Minitab analyze and report findings for a GR&R for a one-sided specification? If only one specification limit is given, percent tolerance is the one-sided process variation (OPV) divided by the one-sided tolerance, OST. The one-sided process variation is Study Var divided by 2. The one-sided tolerance (OST) is the absolute value of the given specification limit subtracted from the average of all the measurements. So, if for example, the USL was 10 and the mean for response was 5, then the OST equals 10-5 or 5 Measurement System Analysis UNCLASSIFIED / FOUO 39
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UNCLASSIFIED / FOUO Takeaways It is important to be able to rely on the accuracy of the measurement system to make good decisions Understand the various types of measurement system variation Eliminate as much of the variation in the measurement system as possible to focus on and improve the true cause of variation in process performance Conduct a Gage R&R analysis to assess the measurement system for continuous data Measurement System Analysis UNCLASSIFIED / FOUO 40
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UNCLASSIFIED / FOUO What other comments or questions do you have? UNCLASSIFIED / FOUO
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UNCLASSIFIED / FOUO UNCLASSIFIED / FOUO National Guard Black Belt Training APPENDIX UNCLASSIFIED / FOUO UNCLASSIFIED / FOUO
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UNCLASSIFIED / FOUO Bias Evaluation (Percent Accuracy) Typically,metrology is responsible for the accuracy of the measurement devices. Calibration typically addresses accuracy. Percent accuracy compared to a tolerance: Average Value - Master Value (100) Tolerance Rule of Thumb for Accuracy Acceptance: < 1% of process variation or tolerance is considered to be adequate accuracy > 1 % of tolerance may warrant corrective action A typical Measurement study will not address accuracy issues unless it is specifically set up to do so (uses a standard instead of parts) Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 43
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UNCLASSIFIED / FOUO Measurement Variation vs. Tolerance Precision to Tolerance Ratio 515 * s MS . Usually P/T expressed as a Tolerance percent Tolerance = USL - LSL Addresses what percent of the Tolerance is taken up by measurement error Note: 5.15 standard Best case: less than 10% deviations accounts for 99% of MS Acceptable: up to 30% variation Includes both repeatability and reproducibility: The use of 5.15 is an industry standard Operator x Unit x Trial experiment P/T Ratios are required by certain customers Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 44
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UNCLASSIFIED / FOUOMeasurement Variation vs. Process Percent Repeatability & Reproducibility (%R&R) s MS ´ % R& R = 100 Usually expressed s Total as a percent Addresses what percent of the Total Variation is taken up by measurement error Best case: less than 10% Acceptable: up to 30% Includes both repeatability and reproducibility Operator x Unit x Trial experiment Again, the stability in the repeated measurements as well as the degree of discrimination could affect the validity of the SMS calculation %R&R is required by certain customers Another Analytical measure is the Discrimination Index defined by: sP D. I. = ´ 2 The D.I. Is similar to the “Number of Distinct Categories” on the Gage R&R Statistics output s MS Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 45
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UNCLASSIFIED / FOUO Gage R&R Relationships If the number of distinct categories is less than two, the measurement system is of no value in controlling the process If the number of categories is two, it would mean that the data can be divided into only high and low groups The number of categories must be at least five for the measurement system to be acceptable for the analysis of the process Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 46
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UNCLASSIFIED / FOUO Gage R&R Statistics – Discrimination Index Source % Contribution % Study Var % Tolerance This “discrimination index” Total Gage R&R 70.62 84.04 225.35 represents the ability of the Repeatability 6.89 26.25 70.40 measurement system to discriminate between one Reproducibility 63.73 79.83 214.07 item and another. We Operator 29.55 54.36 145.76 typically want this number to Oper*Part 34.18 58.47 156.78 be 5 or more!! Part-To-Part 29.38 54.20 145.34 A “discrimination index” of 2 or 3 indicates a measurement Total Variation 100 100 268.16 system that is only useable for Attribute Inspection Number of Distinct Categories = 1 Note: The Discrimination Index is entirely different from the Measurement Unit Discrimination discussed earlier The measurement UNIT discrimination, evaluated in the range chart, determines if the units being used are sufficiently small enough to detect variation (are we using a unit of time such as “days” when we need to be using “minutes” The Discrimination Index looks at Measurement Variation vs. Product Variation to determine if the measurement system is capable of discriminating from item to item Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 47
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UNCLASSIFIED / FOUO Discrimination – Using Control Charts Evaluate measurement unit discrimination by considering the range chart (and the raw data) With the 1st insert R Chart: There are only two layers of measurement resolution under the UCL. We should see 5. Therefore: NOT OK. Subgroup Minimum Number of Size Measurement Units When the unit of measurement is larger 2 4 than the estimated standard deviation, the 3 5 control limits are unreliable 4 5 5 5 6 6 Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 48
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UNCLASSIFIED / FOUO MSA Effect on Capability Indexes We Know that: USL LSL CpAct where s Act s2 s2 Obs MS 6s Act Therefore: USL LSL CpAct 6 s2 s2 Obs MS Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 49
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UNCLASSIFIED / FOUO MSA Effect on Capability Indexes Cpk To include the effects of process centering, we know: USL X Act X Act LSL Be careful of the Cpk Act MIN or direction of the bias 3s Act 3s Act (the sign of the XMS) s Act s2 s2 Obs MS X Act XObs X MS Where and Therefore: USL X Obs X MS X Obs X MS LSL CpkAct Min or 3 s2 s2 3 s2 s2 Obs MS Obs MS Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 50
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UNCLASSIFIED / FOUO AIAG Gage R&R Standards The Automotive Industry Action Group (AIAG) has two recognized standards for Gage R&R: Short Form – Five samples measured two times by two different individuals Long Form – Ten samples measured three times each by three different individuals For good insight into Gage R&R, go to [www.aiag.org] Remember that the Measurement System is acceptable if the Gage R&R variability is small compared to the process variability or specification limits Measurement System Analysis (MSA) - Continuous UNCLASSIFIED / FOUO 52
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