Lesson plan
Precision polygon
Precision polygons are basic standards for angle measurement, which are used and calibrated by the national standards laboratories in particular. Multifaced polygons 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.…
Article summary
Precision polygons are basic standards for angle measurement, which are used and calibrated by the national standards laboratories in particular. Multifaced polygons 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.…
Key claims
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Discussion questions
- Summarise the article in your own words. Which single claim would you verify first, and why?
- Which claim in this article is most likely to be revised as the sources update? How would you check its current status?
- If two sources disagreed about one of these facts, what would you need in order to trust one over the other?
Bibliography
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Key claims and bibliography are generated from the same tracked claims and sources as the article page.