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Measuring For Proof of Compliance vs Process Feedback

When Dr. Greg Hetland, International Institute of Geometric Dimensioning & Tolerancing, first started out as an inspector, he thought his job was principally to determine if a manufactured part conformed to the specification requirements defined on the engineering drawing. If the part conformed he would accept/release the parts to the next operation or to be shipped, however if the part did not conform he would write a multiple copy non-conformance sheet and distribute copies to the interested party and put the parts on hold for members of the management team and technical staff to review. What he did not understand was that a significant part of his job was to provide feedback to the manufacturing engineers and machinists so they could truly understand the problem and put them in a better position to know how to fix it and/or optimize their process.

This was all assuming his measurements were always correct, which he determined based on doing a measurement repeatability and reproducibility study. Sadly, what he did not know at the time was that many of his measurements were repeatably and reproducibly less than optimal and included biases he was not aware of, which also meant he did not know how to correct for them.

How often has manufacturing received measurement data from the quality group that has been of little use to aid manufacturing in determining how to solve the manufacturing problem? The 2D example below is an example of the quick analysis (with the proper software) that can be completed to provide value to both quality and to manufacturing. In the figure, the result on the left is the optimal results quality is looking for from a proof of compliance perspective (minimum zone fit) while the result in the middle is what manufacturing is looking for from a process optimization perspective (least squares fit) to know what to fix.

Historically the values (not the graphics) is what manufacturing receives from quality and are based on the default algorithms (least squares) used in the majority of CMM software’s used in industry today. The result of this feedback would reject the part (in this case) or show a larger portion of the tolerance used than desired. The implications of this would motivate manufacturing to indicate to design engineering that they are not able to optimally meet the specification tolerance and encourage the designer to undesirably increase the allowable tolerance.

By providing the graphic in the middle it can clearly be seen by the manufacturing engineer that it is only the lower right corner of the part that shows excessive variation would could simply be caused by 1) built up in a molding die, 2) break down in a stamping die, 3) interpolation error in a milling process. It is even possible that it was not a manufacturing problem as it potentially could have been caused by the measurement process due to contamination on the part or build up on a probing sphere. All considerations worth evaluating prior to any consideration of discussing the topic with the designer.

By removing the 12 points identified in the middle graphic and re-analyzing the dataset the potential magnitude of improvement can see that could result in the core contributor being fixed, resulting in an optimized process and tremendous improvement in process capability.

Dr. Hetland now teaches GD&T Intermediate & Advanced classes providing a more in-depth knowledge on the topic along with other insight into the ASME Y14.5 standards. Classes are offered as public seminars or can be taught directly at a company whereby training can utilize company specific part drawings.

Dr. Greg Hetland has 30+ years’ experience in the Aerospace, Defense and Commercial industries with extensive expertise in the Mechanical and Precision Engineering fields as an Engineer, Manager, Consultant, Educator and Author. Dr. Hetland is a recognized leader in the dimensional tolerancing and physical metrology fields. Dr. Hetland has extensive technical society affiliations. He is recognized worldwide as chairman and member of U.S. committees as well as U.S. representative on international standards’ development in the areas of physical metrology, dimensional tolerancing and uncertainty analysis, with emphasis in the sub-micrometer regime.

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