Precision polygons are basic standards for

angle In Euclidean geometry, an angle is the figure formed by two rays, called the '' sides'' of the angle, sharing a common endpoint, called the '' vertex'' of the angle. Angles formed by two rays lie in the plane that contains the rays. Angles a ...

measurement, which are used and

calibrated In measurement technology and metrology, calibration is the comparison of measurement values delivered by a device under test with those of a calibration standard of known accuracy. Such a standard could be another measurement device of known ...

by the national standards

laboratories A laboratory (; ; colloquially lab) is a facility that provides controlled conditions in which scientific or technological research, experiments, and measurement may be performed. Laboratory services are provided in a variety of settings: physici ...

in particular. Multifaced

polygon In geometry, a polygon () is a plane figure that is described by a finite number of straight line segments connected to form a closed ''polygonal chain'' (or ''polygonal circuit''). The bounded plane region, the bounding circuit, or the two to ...

s are used for calibration of rotary tables and dividing heads. Any number of faces can be produced. The minimum size of the reflecting face should be approx. 13 mm x 13 mm. Thus maximum number of faces can be 72, corresponding to an included angle of 5°. Steel polygons are made of high grade steel, hardened and subjected to a suitable heat treatment for stabilisation and the sharp corners are chamfered or rounded off suitably. The reflecting faces are lapped flat to within 0.0001 mm. According to IS: 6987—1973, these are divided into three types, 1, 2 and 3, with working faces between 5—20, 21—40 and 41—72. The top and supporting surface are lightly lapped and legibly marked. In order for the result of the calibration to be recorded and subsequently applied, the face of the polygon is identified. Each face is identified by engraving on the top surface, the nominal angles with the face and a datum face. The faces may be identified with a series of numbers starting from 1 if nominal angle is not a whole number of degrees. Around 4 holes (plain for type 1 and threaded for types 2 and 3) are provided for clamping purposes. The dimension of polygon between the top and supporting surfaces is 75,150 and 300 mm for types 1, 2 and 3 respectively. The top and supporting surfaces are made flat and parallel to accuracies of 0.0025,0.005 and 0.007 mm, 0.0035,0.005 and 0.04 mm for types 1, 2 and 3 respectively and departure from flatness is permissible only in concave direction. All faces are made square to the bottom supporting surface to an accuracy of 2.5 \xm over the thickness of the polygon. The error in angle between any working surface and the datum face and between any two adjacent working faces shall not be more than 10 seconds of arc.

References

Dimensional instruments {{measurement-stub