QMT Features: June 2009
The solution is out there
Laser scanning solves a common metrology problem of inconsistent measurement of a long radius with a small pan for an international component manufacturer.

ITW Deltar International Components Group is a global corporation based in Germany that manufactures components for many industries: construction, automotive, food and retail services, consumer goods, and general industrial.  With locations in over 48 countries, the company’s diverse products earn billions in revenues each year.

The company was having difficulties in inspecting a long radius with a very small span on a master buck fixture part used in door handles. Results were not repeatable and the error was up to ± 1mm when contact-measured on various high accuracy CMMs. As a result, company was unable to proceed with production of the part or submit samples to its customers.  Since the value was different each time the radius was measured on the CMM, the customer was not sure of the true physical dimensions of the part. 

Performing accurate inspections on a CMM requires optimal environmental conditions:  consistent temperature, the overhang of the measuring probe, and the CMM’s overall condition all contribute to the uncertainty in measurement which is ± 1 micron, though cumulatively it is much more.

Any CMM software constructs or calculates the value of radius by picking and projecting a minimum of three points on to a constructed or measured plane. A measured or constructed plane is never practically perpendicular to the axis of the circle or arc, which adds another uncertainty to the measurement process. All these factors play a very important role in this contact type of measurement procedure.
The design intent for this fixture part needed to be closely executed in order for it to function correctly.  Its instructions state: “Under no circumstances is the material to be more or less than the specified tolerances. Maintaining the specified tolerance will prevent interference, loose fit, incorrect operation or loss of efficiency.”

GKS Inspection Services—India had been performing 3D scanning work for the company for several years, so the ITW engineer knew that high-accuracy laser scanning would generate definitive results in ascertaining the radius’s length.  GKS’s expert metrologists have years of experience in measuring these types of radii and creating consistent, very accurate results with non-contact laser scanning.  In fact, this issue is rather common with these kinds of parts or features that have a long radius/diameter and the span of the radius is very small.  

The metrologists at GKS explained to the customer that after scanning the part, they needed to perform a deviation check so it could be determined how much the deviation in the radius value was compared to the CAD value.  They set up the master buck fixture part on Laser Design’s Surveyor DS-3040 laser scanning system with a high-accuracy RPS-450 laser probe.  The scan was completed with a single set up in 30 minutes on the high-speed DS laser scanning system. 

Scanning small free-form shapes and irregular surfaces is a prime application for non-contact laser scanning; there is no operator error introduced from differences in technique or the pressure of contact measurement. Because the laser scanning system projects a line of laser light onto surfaces while cameras continuously triangulate the changing distance and profile of the laser line as it sweeps along, the problems of missing data on an irregularly shaped surface are eliminated.  The laser line moves back and forth over the area until the complete surface is captured. The system measures small details and complex free-form geometry so that the object can be exactly replicated digitally. The laser scanner measures articles quickly, picking up to 14,400 coordinate points per second—a thousand times faster than a traditional CMM—and generates huge numbers of data points without the need for templates or special fixtures.

After gathering the large volume of point cloud data, GKS metrologists processed the scan files and generated an inspection report in Geomagic Qualify in less than 2 hours. 

The measurement data in this case showed the measurable span of the arc to be 147.260mm. The two end points of the arc lie symmetrically about an axis and on line with “Y” = 0.000. That means that if the CMM is calculating the arc values for a radius of 660.50mm, the three ideal points that would define this form perfectly would be:   a) -73.630, 0.000   b) 0.000, 4.116824   c) 73.630, 0.000. However, due to the inaccuracy introduced from CMM measuring, point B is shifted by +1 micron, which produces these values:  a) -73.630, 0.000   b) 0.000, 4.117824   c) 73.630, 0.000.  Therefore, the calculated result is 660.3406.

That means that the error of 1 micron is present at all the three points, so that the result deviates in mm’s.  In order to obtain the best results possible, this type of feature’s “form” or profile should be checked with the help of special inspection software like Geomagic Qualify and Cam2.

The Geomagic Qualify software used at GKS checks the form of the component’s surface or edge and gives a whiskers result which shows if the “material” is within the specified tolerance zone.  The whiskers result would immediately highlight in red if the material is out of specified tolerance zone (± 0.10mm in this case).  The results obtained three times with the 3D laser scan and inspection software produced redundant results within ± 3 microns.  This deviation was accepted by GKS’s customers and their clients because they found absolutely no fitment problems, nor loss in efficiency and suitability at the final stages of production.

email: measure@gks.com 
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