CMMs make contact

Manufacturing Engineering, Sep 2001 by Destefani, James D

Contact scanning can provide much higher measurement certainty than point-to-point inspection

Coordinate Measuring Machines (CMMs) with touch-trigger probes have been common and effective measurement and inspection tools for many years. Touch-- trigger probes, of course, function by contacting an individual point on the workpiece, then moving to measure the next point.

But CMMs that use contact scanning probes provide much more information about parts than touch-trigger probing, and the latest generation of scanning CMMs does so much more affordably than the original machines, which were essentially laboratory models. Current scanning CMMs can read hundreds or even thousands of data points in the time it takes touch-trigger systems to register just a handful of probings, and can do so in shop environments.

The ability to provide drastically increased data density-and thus much improved measurement certainty-is the key to scanning CMMs' improved accuracy in checking plastic injection molds, stamping dies, and other parts with complex contours, says John Westhaus, group manager, CMMs, Mitutoyo America Corp. (Aurora, IL).

"Contact scanning of contoured surfaces on a CMM can provide higher data density that yields higher measurement certainty than touch-trigger scanning, and achieves these results in a much shorter inspection cycle time than the traditional point-to-point touch-probe method," he says.

According to Richard Knebel, director of software development, Carl Zeiss IMT Corp. (Minneapolis), this ability to better describe part form is essential for improved measurements. "CMMs function by sampling a workpiece and using mathematics to relate the sampled points to basic geometric elements such as circles, lines, planes, and cylinders," he says. "When the form of the workpiece being inspected is perfect, the number and location of the sampled points becomes insignificant in computing the result.

"But, in everyday use, CMMs must inspect imperfectly formed features. This is more difficult than inspecting artifacts with perfect form, because on imperfectly formed features the result can vary based on where the CMM samples the feature," he concludes.

Part form is not the only type of measurement that can benefit from high data density, Knebel says. He points out that form deviation is present in all features we measure. When form is not controlled separately through the use of a modifier such as circularity (roundness) of a bore, the limits of size control the allowable form deviation. Therefore, when evaluating location and size, form must be understood to provide a truly accurate result. Or, as Knebel puts it, "the quality of size, location, and form information is directly related to the number of samples and location of each sampled point."

Westhaus provides an example of the value of data density to measurement certainty. "Say we start with a part with a 15" (380-mm) OD that mates with another part, so we're looking for a functional fit. We measured the OD using the extremes in terms of data density to show the changes in measurement results."

First, Mitutoyo measured four points-the minimum number required to provide even a rudimentary description of circularity-on the part OD using traditional point-topoint probing. This inspection indicated form error of 0.0003" (0.008 mm). "Next, we took 36 points around the circumference, again using point-to-point inspection," Westhaus explains. "Cycle time was 36 sec, maximum OD was found to be 0.001" (0.025 mm) smaller than the four-point analysis, and form error was calculated as 0.0008" (0.02 mm)."

Finally, technicians used continuous-contact scanning to check the OD. The process gathered nine data points per inch in 26 sec-a total of 423 data points. "We found that the maximum outer diameter was now known to be almost 0.003" (0.08 mm) larger than the 36-point test. And the form error had grown to 0.00126" (0.032 mm), much closer to the 'true' diameter and form values of the part," Westhaus says.

"This type of data density is very valuable when a functional fit diameter needs to be determined for mating part analysis or for `good part/ bad part' determination. The more data, the more accurate the measurement, and therefore the more certainty that a marginal part is either in or out of spec, and the more certainty that the result would be reproduced if you were to measure it again," he concludes.

According to Knebel, a minimum of 300 data points are required just to approach the correct result for form such as circularity of a diameter in the 1-3" (25-76 mm) range. "Only at or above this level of data density will you begin to approach 90% or more of the correct form value," he says. From information such as this we begin to understand how much data is required for size and location features as well, because if you don't know the true form, you can't know the functional size and location."

He tells Manufacturing Engineering about a Zeiss in-house comparison of an existing touch-- trigger probe program and a scanning program to inspect 45 characteristics on a cast-iron pump housing. "Eighteen of the characteristics-40% of the features inspected-differed by more than 10% of the tolerance," he says. "Scanning also uncovered eight characteristics that exceeded tolerance limits but were found to be in tolerance using touch-probe inspection. Keep in mind that this is not a problem of one machine being more accurate than another; all results were from the same CMM," he adds.