AS B139-1955

Relates to three mechanical types of bevel protractors, in which the measurement is read direct from the scale either with or without the assistance of a vernier, and for one optical type in which the measurement is read on an internal circular scale by means of an optical magnifying system integral with the instrument. Requirements for accuracy in respect of the body, stock and blade, and of the accurate angle attachment, where fitted, are given and graduations for both types of protractors are specified. Illustrations of the nomenclature adopted and clauses relating to material, workmanship, and marking are included.

This standard relates to the following types of bevel protractors:-a. Mechanical.Type A. With vernier graduated to read to five minutes of arc, and with slow motion device, and acute angle attachment.Type B. With vernier graduated to read to five minutes of arc, but without slow motion device or acute angle attachment.Type C. With scale graduated in degrees, without vernier or slow motion device or acute angle attachment.b. Optical. With an internal circular scale which is graduated in divisions of 10 minutes of arc* and read against a fixed index line or vernier by means of an optical magnifying system integral with the instrument, enabling readings to be taken, by estimation or otherwise, to approximately two minutes of arc.*NOTE. Present known instruments are graduated to 10 minutes of arc but other suitable values of graduation are permissible. The fitting of an acute angle attachment is optional.

Reconfirmed 1968, 1984, 1987