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Lab Session: 2
To measure the internal and external taper of various shapes/polygons using bevel protractor.
Objective:
Main objective of this experiment is to develop an understanding of bevel protractor and their use. How bevel protractors can be helpful in measuring internal and external angles of different shapes.
Problem statement:
Precise measurement of angles is an important factor in workshops and tool rooms. The parts produced are useless if they are not according to the dimensions provided by the consumer. Proper care of tools and instruments is required for accurate measurements. It is expected that, after concluding the experiment, you will put down critical and relevant comments to increase value to the learning experience.
Apparatus:
Bevel Protractor
Shapes of different sizes
Theory:
In Metrology, the angular measurements are frequently necessary for manufacturing industries. Vernier Bevel protractor is one of the angular measuring instruments used for close tolerances
Bevel Protractors:
Bevel Protractors are the Angular measuring devices. Vernier Bevel protractor is the simplest angular measuring device which is having a Vernier Scale (Similar principle as the Vernier Caliper) along with the acute angle attachment.
Figure 2.1: Bevel Protractor
The bevel protractor is used to establish and test angles to very close tolerances. It reads to 5
arcminutes (5′ or
1/12°) and can measure angles from 0° to 360°.
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Construction:
The bevel protractor consists of a beam, a graduated dial and a blade which is connected to a swivel plate (with Vernier scale) by thumb nut and clamp. When the edges of the beam and blade are parallel, a small mark on the swivel plate coincides with the zero line on the graduated dial. To measure an angle between the beam and the blade of 90° or less, the reading may be obtained direct from the graduation number on the dial indicated by the mark on the swivel plate. To measure an angle of over 90°, subtract the number of degrees as indicated on the dial from 180°, as the dial is graduated from opposite zero marks to 90° each way. Since the spaces, both on the main scale and the Vernier scale, are numbered both to the right and to the left from zero, any angle can be measured. The readings can be taken either to the right or to the left, according to the direction in which the zero on the main scale is moved.
Figure 2.2: Parts Name
How to read bevel protractor?
The bevel protractor Vernier scale indicates every five minutes or 1/20 of a degree. Each space on the Vernier scale is 5 minutes less than two spaces on the main scale. Twenty-four spaces on the Vernier scale equal in extreme length twenty-three double degrees. Thus, the difference between the space occupied by two degrees on a main scale and the space of the Vernier scale is equal to one twenty-fourth of two degrees or one twelfth of one degree (or five minutes).
Figure 2.3: Variety of uses of the Bevel Protractor
Read off directly from the main scale the number of whole degrees between 0 on this scale and the 0 of the Vernier scale. Then count, in the same direction, the number of spaces from the
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zero on the Vernier scale to a line that coincides with a line on the main scale; multiply this number by 5 and the product will be the number of minutes to be added to the whole number of degrees
Procedure:
Note down the lease count of Bevel protractor.
Keep the workpiece on a surface plate.
Adjust workpiece in protractor (place one edge or workpiece on the beam and rotate the blade on the side of a workpiece whose angle is to be measured)
Tighten the screw of the blade
Note the reading
Repeat the same steps to get other readings for different sides/shapes
Observation:
Least count of Bevel Protractor= 5 minutes
References:
https://www.craftsmanspace.com/knowledge/vernier-bevel-protractor.html
https://extrudesign.com/vernier-bevel-protractor/
Sr No External angles (Degree, Minutes) Internal angles Degree, Minutes)
1
239°15’
121°15’
2 239.5°15
120.5°15’
3
239.5°15’
120.5°15’
4
239.5°30’
120.5°30’
5
240°30’
120°30’
6
242°45’
118°45’
Sr No External angles (Degree, Minutes) Internal angles Degree, Minutes)
1
118°50’
342°50’
2
111°40’
349°40’
3
90°20’
270°20’
4
42°10’
318°10’

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