Providing accurate angular measurements
We offer calibration services for a number of types of angle standards, including autocollimators, polygons, angle gauges and index tables. The basic standards for angle measurement used by NPL depend either on the accurate division of a circle or on the generation of a known angle by means of a precision sine-bar. Several methods are available for dividing a circle but the one employed by NPL for undertaking measurements for the precision engineering industry is based on the accurate meshing of two similar sets of uniformly spaced vee-serrations formed in the top (rotatable) and base (fixed) members of an indexing table.
Calibration of combination angle gauges
NPL provides a UKAS accredited service for the calibration of combination angle gauges. Angle gauges are examined for compliance with NPL Specifications of Accuracy MOY/SCMI/18-45. The angles of the gauges are determined by means of a precision indexing device and the NPL small angle generator. Measurements can also made of the flatness, squareness and parallelism of the faces.
The uncertainty of measurement for the determination of angles is largely dependent on the quality of flatness of the measuring faces. The requirement for the deviation from flatness of the measuring faces of angle gauges made to comply with the relevant NPL Specification is 0.000 2 mm. For such gauges the best measurement capability expressed as an expanded uncertainty (k=2) is ± 0.3 second of arc.
Calibration of autocollimators
Calibration of an autocollimator is made using the NPL-designed small angle generator. In the case of visual- and photoelectric-setting type autocollimators, small angles are generated to check the periodic and progressive errors of the micrometer screw which enables the displacement of the reflected image of the target cross-lines to be measured.
The best measurement capability expressed as an expanded uncertainty (k=2) is given by visual and photoelectric: Up to 10 minutes of arc, 0.06 seconds of arc.
Calibration of index tables
Generation of angle by indexing tables is achieved by the meshing of two similar sets of serrations. Calibration of such tables submitted for test is effected by mounting the table under test on top of one of the NPL indexing tables and using a mirror-autocollimator system to compare angles generated by the table under test with similar angles generated by the NPL table. For the purpose of assessing the accuracy of performance of the serrated type of table it is considered sufficient to intercompare each successive 30-degree interval with the NPL table, thus providing 144 comparative measurements. The small angular differences between the two tables are measured by a photoelectric autocollimator capable of a discrimination of 0.01 second of arc. This service is UKAS accredited.
A shortened test may be applied to indexing tables which have a reproducibility of setting significantly poorer than the NPL tables, that is, greater than 0.05 second of arc. For such tables three sets of measurements of twelve consecutive 30-degree rotations of the table under test are compared with the NPL table. Between each set of measurements the test table is moved through 120 degrees relative to the NPL table.
The uncertainty of measurement is largely dependent on the quality of the two sets of serrations. The criterion for assessing this quality is to check the reproducibility of angular positions of the upper table relative to the base. Indexing tables similar to the NPL tables will normally repeat angular positions to between 0.02 to 0.05 second of arc. The best measurement capability expressed as an expanded uncertainty (k=2) for the calibration of these tables, based on 144 comparative measurements, is ± 0.04 second of arc.
Indexing tables having a slightly lower precision of angular setting, say between 0.05 and 0.2 second of arc, are calibrated by making 36 comparative measurements and the best measurement capability expressed as an expanded uncertainty (k=2) of the calibrated values will be between ± 0.25 and ± 0.5 of arc.
Calibration of polygons
NPL offers a UKAS accredited service for the calibration of precision polygons. Regular polygons are examined for compliance with the NPL Specifications of Accuracy MOY/SCMI/83 (glass polygons) and MOY/SCMI/87 (steel polygons). Only Reference and Calibration grades of steel polygons are accepted for calibration. The measured errors of the exterior angles of the reflecting faces are determined by means of a precision angle indexing table, which is maintained and calibrated by NPL from first principles. Flatness and squareness of the measuring faces is also determined.
The uncertainty of measurement is largely dependent on the quality of flatness of the reflecting faces. The faces should be flat to 0.05 µm in order to achieve a best measurement capability expressed as an expanded uncertainty (k=2) of ± 0.11 second of arc or better. This uncertainty is achieved by comparing each nominal angle with every corresponding angle of an NPL master indexing table.
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