It is about 150 or 300 mm in length, 3 mm wide. It is 2 mm in thickness. Its ends are bevelled at angles of 45 degrees and 60 degrees within accuracy of 5 minutes of arc.

There are four types of mechanical bevel protractors i.e. A, B, C, and D. The vernier can read to 5 minutes of arc in A and B type of mechanical bevel protractor whereas in case of type C, scale can be read in degrees.

A unidirectional approach is used in accurate worm and gear type table for setting to take care of unavoidable presence of backlash. The accurate worm and gear has an advantage for inspection and generation of angles.

It is possible to measure length to accuracy of 1 part in 10⁶ with laser interferometer. With two retro-reflectors that are placed at a fixed distance, and a length measuring laser interferometer angle change can be measured to an accuracy of 0.1 second.

If all exterior angles of upper polygon are compared with angle between any two faces of lower polygon n by comparison method of calibration of polygons, accuracy of ± 3 seconds can be obtained.

The permissible tolerance of flatness of upper and lower surfaces of sine bars is 0.001 mm and tolerance for parallelism of upper and lower surfaces with respect to datum surface is also 0.001 mm.

Firstly two trial lines namely ?1? and ?2? are marked approximately at 180 degree and two microscopes A and B are set against se marks. This practice is to divide a circle into two equal halves.

Smaller increments of angle are achieved by sine curve which is generated by electronic or optical pulsing in averaging type optical rotary table. The circular scale could be read by electrical or by Moire fringe method.

As per IS : 6987-1973, re are three types of precision polygons that is ?type-1?, ?type-2? and ?type-3?. The dimension of polygon between top surface and supporting surface is 75 mm for type -1,150 mm for type-2 and 300 mm for type-3.

Lack of true spindle rotation in accurate worm and gear rotary table results in both angular error and centre distance error. Rotary table has advantage of trouble free operation and good repeatability.