Improved direct comparison calibration of small angle blocks

In the field of dimensional metrology, direct comparison method is used invariantly for the calibration of angle artefacts e.g. angle blocks, polygons, rotary tables. A precision indexing table and an autocollimator are sufficient to perform such calibrations. These instruments are periodically compared against primary reference standard to determine their corrections. These corrections are used in direct comparison calibration. Using this method, we can achieve angle block measurement uncertainty approximately ±0.4″.At NPL-India, we have devised an improved method of direct comparison calibration of angle blocks. We measured the angle block in four different orientations. The measurements obtained in these four orientations are mathematically modeled in terms of angular position deviations of precision indexing table, pyramidal error and actual angle block size. These simultaneous equations are solved to segregate effective angular position deviations of indexing table and effective pyramidal errors from the angle block measurements. There from, the angle block measurement uncertainty is improved up to ±0.07″.

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