Portable CMM Accuracy Tests: the MicroScribe® X Series

Two well-known testing standards for portable CMMs guide accuracy specification reporting:

(1) The American Society of Mechanical Engineers (ASME) B89.4.22-2004 draft (“ASME B89”) and (2) the International Organization for Standards (ISO) 10360-2:2001, Geometrical Product Specifications – Acceptance and reverification tests for coordinate measuring machines – Part 2: CMMs used for measuring size (“ISO”).

While some manufacturers of portable CMMs refer to ASME B89 specifications for large diameter arms, the test standards as written become impractical due to the length of the ball bar gauges required and other factors.

To help our customers sort through the true differences among various portable CMM manufacturers’ test methods, we have designed tests that represent the MicroScribe® X portable CMM performance under typical manufacturing applications. We clearly describe our methodology, calculations, and results. Following is a summary of these tests. Please contact us for more detailed information.

Note: In the test descriptions below, the term "standard height" refers to the midpoint between the machine’s base and its first (beta) joint.

Volumetric Accuracy
Sphere Center Repeatability and Effective Diameter

Volumetric Accuracy

The MicroScribe X portable CMM Volumetric Accuracy Test is similar to the ASME B89 Volumetric Performance Test and the ISO Size Test. Volumetric accuracy is designed to determine the system’s performance in measuring length accuracy throughout its workspace.

The test uses a calibrated ball-bar approximately 700 mm (27.6 in) long. Each ball is approximately 20 mm (0.79 in) in diameter. Length is determined by performing three measures of sphere center distance when the gauge is in 20 different orientations distributed among 4 specific locations within the measurement workspace. The sphere center is calculated with a least squares algorithm applied to eight specified points.

The 20 different gauge orientations and 4 locations were chosen to provide data representative of the MicroScribe X portable CMM’s performance within its work envelope. The test includes measurements taken of the gauge between 15% and 75% of the arm radius; at several heights; in vertical, horizontal, and diagonal orientations; and in different positions around the arm. For each measurement taken, the error is calculated as the absolute difference between the calibrated length and the measured length of the ball-bar. The test results report a two sigma deviation of error over all the measurements.

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Sphere Center Repeatability and Effective Diameter

Immersion uses the same set of measurements to calculate both accuracy and repeatability using a calibrated sphere. The test and the calculations use principles from the ASME B89 standard called Effective Diameter Performance Test and the ISO standard called Probing Error Test. Both ASME B89 and ISO tests calculate the difference between the measured and the calibrated diameter or radius of a sphere. Sphere Repeatability calculates the system’s ability to provide repeatedly similar values for sphere center, and Effective Diameter calculates the system’s ability to provide accurate measurements of sphere diameter.

With the arm in approximately the same orientation throughout this test, 10 sets of 8 specific points are taken on a sphere with a diameter of approximately 20 mm (0.79 in) positioned at standard height and at 50% of the arm’s radius from the MicroScribe X system’s base. For each set of points, a least-squares calculation of sphere center is performed, and the estimated radius is determined by averaging the distance from each measurement to the estimated center. The estimated radius is multiplied by 2 to get the estimated diameter.

For the Sphere Center Repeatability Test, the deviation from the mean sphere center is calculated for each of the 10 estimated sphere centers. The two sigma deviation of the sphere center measurement is reported as the result of the Sphere Center Repeatability Test.

For the Effective Diameter Test, the difference between the calibrated diameter and the estimated sphere diameter is determined for each of the 10 sphere measurements. The absolute maximum difference between the calibrated diameter and the measured diameter is reported.

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