Linearity represents the change in accuracy through the expected operating range of a measurement device. For example, does the bathroom scale perform the same when weighing a pet of 10 pounds to a man of pounds? The scale has an operating range of 0 lbs to lbs but the scales accuracy may change at various levels of measurement.
Sources of linearity error may come from age, wear, calibration error, or there may be known linearity error. If there is known error then there may be a calculation to account for it and various ranges of measurement. Stability also referred to as "drift" of a measurement system can be analyzed using control charts.
Ensuring the measurements taken by appraiser s for the process indicate stability and consistency over time. Each appraiser should measure the same way every time over a long period of time and each appraiser should measure the same way as all the others. Stability is the total variation spread and shape of the data distribution of the measurements using the same parts, measured using the same gauge over a long period of time.
SPC Charts use a variety of tests to determine stability. Many software programs will have these as options when analyzing data and will even indicate the point s and test that each failed. Some of the corrective measures once again include Standard Operating Procedures and re-calibration. Sometimes the gauge will have wear from use over a long period of time and this can not be repaired or re-calibrated.
Other times, there may be a build up of dirt, dust, or contamination. Special cause variation can also occur within the process control limits and these must be given corrective action before proceeding to validate the measurement system. The I Chart below shows stability in this measurement system example, assuming this is a "longer" period of time that represents actual conditions as close as possible. And each process output is measured times by each operator.
Depending on the cost and time involved you can add more appraisers and measurements and replications. When performing the replicated appraisals it is critical that the measurement are randomized so that no patterns or predictability can be entered in by the appraiser.
For example, an appraiser may remember the 7th part that was measured was borderline and made a decision to give it one measurement. The appraiser may have spent a lot of time on that part in the initial assessment and if the 2nd round of measurements are not randomized, that person will remember the measurement appraisal they concluded during the first round.
In summary, the message is to move the parts around each repetitive set of measurements. However, the parts must be identified so the person entering the data into the statistical software enters the reading under the correct part. The following four areas will be assessed. A statistical software program will generate these values once the data is entered. Process capability can not be determined with reliability if the measurements the data are suspect.
Examining the visual aids below shows commonly used judgement criteria for each category. In this case we are using 3 appraisers measuring 6 different parts. This study shows the measurement error as a percent of tolerance in short period of time. It includes both repeatability and reproducibility, can not be separated.
This value is a constant that is found by looking at the column with 3 appraisers and going across with the row with 6 parts. In this example the d 2 value is 1. The range of the three measurements for each part is shown on the right.
To convert this gage error of 2. The process tolerance is the difference in the specification limits. With the tolerance being 25, then:. Reproducibility is the ability of one appraiser to get the same result and another appraiser or the ability of all appraisers to get the same results AMONG each other. Pictures of good, bad, in the middle, and colors, will help each appraiser standardize their response, improving the reproducibility. When using humanly subjective "touch" devices such as micrometers and calipers it is important that all appraisers "squeeze" the same amount.
When using an optical comparator to get a higher resolute data point there is subjectivity where to place the template or the starting and end point s on the shadow. Pictures of acceptable and non-acceptable will help reduce this variation. In destructive testing such as tensile testing these reading will not be possible and some statistical software programs have options to select for destructive testing.
The goal is to have an appraiser repeat unit readings at least three times. Avoid writing down measurements and then typing them into a statistical program. Precision is the ability to have the same repetitive result or appraisal in this case. Template - TRIZ Contradiction Matrix This interactive template allows users to select a pair of improving and worsening factors and returns up to 4 innovative principles that have been successful at solving the technical contradiction.
Special thanks to Mark Jones of Catalyst Consulting Ltd for his input and expertise in building this template. The template provides a process-oriented methodology for identifying the causes of failure in a design and evaluating the risks associated with each cause using the new Action Priority scoring.
The template provides a process-oriented methodology for identifying the causes of failure in a process and evaluating the risks associated with each cause using the new Action Priority scoring. Template - Kano Model This template categorizes customer requirements into Kano Categories based on customer survey responses and plots them on a Kano graph. It also categorizes customer requirements into Kano Categories based on customer survey responses and plots them on a Kano graph.
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Template - Harvey Balls Use to visually communicate or compare qualitative information. There is a solution called Soft Bypass, which allows yo.
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