ASME B89.4.22-2004 pdf – Methods for Performance Evaluation of Articulated Arm Coordinate Measuring Machines.
For discussion purposes, we will consider two categories: those strategies that employ an artifact that represents a typical workpiece (the artifact may be an actual workpiece from the production line) and those strategies that employ an artifact specifically designed for AACMM testing. For all strategies, it is recommended that ten consecutive interim testing runs be conducted immediately after the AACMM is calibrated. The mean of these ten measurements can be used to establish a baseline value for the interim artifact, and the range of values indicates the typical variation that may be expected under these conditions. Additional factors, such as thermal conditions or different operators, may further expand the range of interim testing results. If, upon recalibration of an AACMM, the new interim baseline measurements differ significantly from the previous baseline, then the interim artifact or the AACMM calibration (or both) may be suspect and further investigation is Tarranted
Some AACMMs are dedicated to measurements of a single type of workpiece or a family of similar work- pieces. In this situation an actual workpiece may be used as the interim testing artifact. This type of artifact will be sensitive to errors that are important to actual workpiece measurements. An additional benefit is that the user is familiar with the required workpiece measurements and consequently may have an AACMM program available which can be used for the interim testing. The selected workpiece and the measured features on that workpiece should span the largest volume of the AACMM work zone that is encountered during actual workpiece measurements, to insure that the relevant volume of the AACMM is tested. For users measuring many small workpieces located all over the AACMM work zone, it is suggested that the small interim test artifact be measured at several different locations to insure that an adequate region of the work zone is tested. It is not necessary to measure every feature on the test workpiece; rather, a representative group should be selected (both for feature type and location) for the interim testing procedure. The tolerance of these selected features should be set comparable to those of the tightest tolerances found in the actual production workpieces. In general, the interim artifact should be treated, fixtured, and measured in a manner similar to that of the actual workpiece to reflect the actual measurement situation. Although the use of a test workpiece as an interim artifact has merit, it is important to note that the testing results are valid only for workpieces of a similar design and may not indicate the errors present when measuring a workpiece significantly different from the test artifact.
An artifact specifically designed for interim testing should be sensitive to common AACMM errors. On artifacts that produce several lengths upon measurement (e.g., ball plates), the longest length present will provide the greatest sensitivity to errors. Alternatively, the use of calibrated ball or hole plates may allow more than one such body diagonal to be measured in each orientation.
An artifact specifically designed for interim testing should be sensitive to common AACMM errors. The AACMM probe should be checked to ensure it is in good working order. This may involve an explicit probe test that checks the directional sensitivity of the probe, i.e., probe lobing, or may be incorporated into part of the general AACMM test, such as measuring a long gage block that is oriented in several different directions. Similarly, to test the probe calibration, which involves the accuracy of the AACMM probe calibration artifact (typically a sphere with a calibrated diameter), a true bidirectional measurement of a known length is required.
In this example, a user with an AACMM employing a hard probe has selected an interim testing artifact, consisting of a 25 mm diameter sphere connected at one end of a rod, and a conical socket at the other end. The rod length is roughly 75% of the AACMM length. The sphere is calibrated for form and diameter. Similarly, the length of the rod from the sphere center to the hard probe tip center (when placed in the conical socket) is also calibrated. The sphere is mounted in a three-point kinematic magnetic mount that allows the rod to rotate about the sphere center, sweeping out an arc of constant radius.