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.

Measurement Uncertainties

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.

Contact

Customer Service tel: +44 20 8943 8631
E-mail: dimensional_enquiries@npl.co.uk