QMT Features: October 2011
Question the Status Quo
Automation lulls engineers into blissful Ignorance about the measurement process.  By Talion Edwards, The Boeing Company and 2011 CMSC Chair, Coordinate Metrology Society


I would like to start this article by making a confession.  I am an impostor in the metrology community.  My personal background is from the often maligned design engineering community - those well meaning folks who don’t quite grasp the complexities of the manufacturing process and the measurement systems that support them. 

Measurement professionals understand the cost and scheduling impact of arbitrary decisions made in the design process. Designers put 0.10mm tolerances on parts because it is as easy as a mouse click in CAD.  Ahhh, the blissful ignorance provided by my CAD terminal.  Any engineer should understand most materials expand when heated, and contract when cooled.  If we polled design engineers about what temperature their CAD models were at the time they applied the dimensions, I expect there would be a lot of blank stares.  Now that I have confessed, I would like to propose my ignorance provides a unique perspective on some aspects of the measurement business.

For example, can someone please explain to me the logic of applying linear temperature compensation to large volume coordinate metrology of non-homogeneous assemblies?  I am talking about airplane wings, car bodies, particle accelerator experiments, windmill blades and all other large assemblies made out of various materials fastened together carrying the inherent manufacturing tolerances of their components.  I have been to four industry conferences on two continents and discussed the issue with experts from three National Measurement Institutes and still can’t make any sense of it.  

I clearly understand the principle of applying compensation to measurement data to account for the affects of varying temperatures of the work piece and measurement system.  In principle, most materials behave predictably when the temperature of the component is subject to a temperature change.  A scale bar made from a homogeneous material will predictably expand with increasing temperate at a ratio defined by the coefficient of thermal expansion for the material.  Since a scale bar’s function is to define a traceable link to a reference standard, applying temperature compensation of the scale bar is both logical and necessary (Make note. Designers can be trained!).  

Given the same concept, consider a collection of parts made mostly from aluminum, with some carbon fibre composites, titanium and steel bits. Assemble them using two handfuls of fasteners and some adhesive.  Now we walk into a building that is at 30oC and collect some surface points from the assembled structure using our impressive Computer-aided measurement tool kit.  The measured data is compared to the design engineer’s intent expressed via GD&T on a drawing.  
Next we would reference ASME Y14.5-1994 for the golden rule in measurement (while my European counterparts would look for guidance in ISO 1:1975). “Unless otherwise specified, all dimensions are applicable at 68oF (20oC ).  Compensation may be made for measurements made at other temperatures”. Some will argue that before this comparison can take place, temperature compensation must be applied to the measurement data.  Our measurement software can easily make this compensation.  We can apply a compensation for aluminum of 0.0000232 because it is a simple mouse click in our measurement software. Does anybody believe 3D coordinate data collected from a freeform surface on a multi-material structure expands and contracts linearly with a temperature change?

If you didn’t catch the irony in that last statement, let me be more blunt.  As measurement professionals, we can be lulled into blissful ignorance by the automation provided by our current measurement tools.  On the bright side, there is mitigation against these issues.  The more experienced metrologists among us were brought up on first principle measurement tools.  They have the background necessary to question the answer provided by the automated tool and make sense of disparate data.  Unfortunately, there are many of us (myself included) who started in measurement with the software giving us all the answers… and no first principle knowledge to fall back on when the answers did not make sense.  As a community, we must find ways to transfer knowledge from our industry experts to next-generation practitioners to help fill in this glaring gap.  We must also maintain the connection between the scientists working at the National Measurement Institutes and the practical knowledge provided by measurement practitioners working in industry.

There are dedicated groups committed to supporting the growing 3D measurement workforce.  The Large Volume Metrology Conference holds a yearly conference in the UK* to support this community.  The Coordinate Metrology Society (CMS) in the Unites States continues to make progress towards a centralized certification for industrial coordinate metrology practitioners. The Certification Workshop held in July announced The American Society of Photogrammetry and Remote Sensing (ASPRS) as their partner in this endeavour. The journey begins toward an industry-recognized personnel certification in portable 3D metrology that includes laser tracker, area scanners, laser radar, hand-held scanners and data post processing.

The CMS has also been focused on activities that highlight the operator’s impact on the measurement process. The 2011 measurement study conducted this past July was based on large-volume measurement principles and focused on the user’s ability to influence measurement results.  The exercise was executed by CMS volunteers from the U.S., Canada, and Europe, and directed by the National Physical Laboratory.  The resulting data is currently being analyzed, and a final, comprehensive report will be issued in October for the membership.
Collectively, these efforts are a good start for sharing knowledge with the metrology community. By instituting a “question the status quo” culture (that we like to see from design engineers) in the measurement industry, this mindset can have a significant impact on our ability to make efficient use of automation provided by CAMS without losing the ability to think our way out of a problem. l

*Talion Edwards of The Boeing Company served as the 2011 CMSC Chair, Coordinate Metrology Society and is the keynote speaker at the
LVMC ‘11 conference and exhibition, 15 - 16th November at the  Concorde Conference Centre,  Manchester,UK
www.lvmc.eu
email: Talion.edwards@boeing.com
  
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