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Chapter: Mechanical : Metrology and Measurements : Linear and Angular Measurements

Bevel protractors

The different types of bevel protractors used are: 1) Vernier bevel protractor 2) Universal protractor 3) Optical protractor



Bevel protractors are nothing but angular measuring instruments.


Types of bevel protractors:


The different types of bevel protractors used are:




1) Vernier bevel protractor


2) Universal protractor


3) Optical protractor








Working principle


A vernier bevel protractor is attached with acute angle attachment. The body is designed its back is flat and no projections beyond its back. The base plate is attached to the main body and an adjustable blade is


attached to the circular plate containing Vernier scale. The main scale is graduated in degrees from 0° to 90° in both the directions. The adjustable can be made to rotate freely about the center of the main scale and it can be locked at any position. For measuring acute angle, a special attachment is provided. The base plate is made fiat for measuring angles and can be moved throughout its length. The ends of the blade are beveled at angles of 45° and 60°. The main scale is graduated as one main scale division is 1° and Vernier is graduated into 12 divisions on each side of zero. Therefore the least count is calculated as

Thus, the bevel protractor can be used to measure to an accuracy of 5 minutes.

Applications of bevel protractor


The bevel protractor can be used in the following applications.








Auto-collimator is an optical instrument used for the measurement of small angular differences, changes or deflection, plane surface inspection etc. For small angular measurements, autocollimator provides a very sensitive and accurate approach. An auto-collimator is essentially an infinity telescope and a collimator combined into one instrument.


Basic principle


If a light source is placed in the flows of a collimating lens, it is projected as a parallel beam of light. If this beam is made to strike a plane reflector, kept normal to the optical axis, it is reflected back along its own path and is brought to the same focus. The reflector is tilted through a small angle ‘0’. Then beam the is deflected parallel twice the angle and is brought to focus in the same plane as the light source.

The distance of focus from the object is given by





There are three main parts in auto-collimator.


1. Micrometer microscope.


2. Lighting unit and


3. Collimating lens.


Figure shows a line diagram of a modern auto-collimator. A target graticule is positioned perpendicular to the optical axis. When the target graticule is illuminated by a lamp, rays of light diverging from the intersection point reach the objective lens via beam splitter. From objective, the light rays are projected as a parallel rays to the reflector.



A flat reflector placed in front of the objective and exactly normal to the optical axis reflects the parallel rays of light back along their original paths. They are then brought to the target graticule and exactly coincide with its intersection. A portion of the returned light passes through the beam splitter and is visible through the eyepiece. If the reflector is tilted through a small angle, the reflected beam will be changed its path at twice the angle. It can also be brought to target graticule but linearly displaced from the actual target by the amount 2θx f. linear displacement of the graticule image in the plane tilted angle of eyepiece is directly proportional to the reflector. This can be measured by optical micrometer. The photoelectric auto- collimator is particularly suitable for calibrating polygons, for checking angular indexing and for checking small linear displacements.





Auto-collimators are used for


1)    Measuring the difference in height of length standards.


2)    Checking the flatness and straightness of surfaces.


3)    Checking square ness of two surfaces.


4)    Precise angular indexing in conjunction with polygons.


5)    Checking alignment or parallelism.


6)    Comparative measurement using master angles.


7)    Measurement of small linear dimensions.


8)    For machine tool adjustment testing.


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Mechanical : Metrology and Measurements : Linear and Angular Measurements : Bevel protractors |

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