 Hello and welcome to the video lecture on Angular measuring instruments part 1. At the end of this video lecture, students will be able to explain about various angle measuring instruments. The following topics will be discussed in this particular video lecture. Introduction to Angular measuring instruments, classification of Angular measuring instruments, vernier bubble protector, optical bubble protector, introduction to Angular measuring instruments. Precise measurement of angles is one of the important requirements in workshops and toolrooms. We need to measure angles of interchangeable parts, gears, jigs, fixtures, etc. So we can define the Angular measuring instruments as the devices or they are the devices that are used to measure the angle of the given specimens. Example it may be a vernier bubble protector, sine bar, sine center, angle gauges, clinometer, protector, so these are the various examples, classification of Angular measuring instruments. Angular measuring instruments are classified based on various standards. Line standard Angular measuring devices. Line standard gives direct angular measurement from the engraved skills in the instruments. They are not very precise, hence they are not used when high precision is required. However, they can be used in initial estimation of the angles in measurement. Example protractor bubble protectors, phase standard Angular measuring devices. These are the devices which includes angle gauges and sine bars. The measurements are done with respect to two phases of the measuring instruments. Precision obtained in such instruments is more than the precision obtained in the standard Angular measuring devices. Measurement of inclines. Precision of a surface generally represents its deviation from the horizontal or vertical planes. Gravitational principle can be used in construction of measurements of such inclinations. Spirit levels and clinometer are the instruments of this particular category. Now, in this particular video lecture we will be discussing about the Protractor, Vernier bubble protector and Optical bubble protector. Protractor. A simple protector is the basic device for measuring angles. At best it can provide least count of 1 degree for smaller protectors and half degree for large protectors. Vernier bubble protector. That is the simplest angle measuring instrument. It consists of main body, then the base plate stock, then the adjustable blade, circular plate containing Vernier scale, acute angle attachment. The body of the Vernier bubble protector is designed in such a way that its back is flat and there are no projection beyond its back. The flatness of the body is tested by checking the squareness of the blade with respect to base plate when the blade is set at 90 degree. The base plate is attached to the main body and an adjustable blade is attached to a circular plate containing Vernier scale. The main scale graduated in degrees is provided on the main body. The adjustable blade is capable of rotating freely about the center of the main scale and grabbed on the body of the instrument can be locked in any position. And acute angle attachment is provided at the top for measuring acute angles. The base of the blade plate is made flat so that it could be laid flat upon the work and any type of angle can be measured. The blade can be moved along throughout its length. The blade can be moved along throughout its length and can also be reversed. Its ends are beveled at angles 45 degrees and 60 degrees. The acute angle attachment can be readily fitted into the body and clamped in any position. The bevel protectors are tested for flatness, squareness, parallelism, straightness, etc. The main scale is graduated in degrees of arc. The Vernier scale has 12 divisions each side of the center 0 as you can see 12 divisions are there. These are marked 0 to 60 minutes of arc so that each division equals 1 by 12th of 60 that is which is equal to 5 minutes of arc. Users of bevel protector Checking inside beveled face of a ground surface. I am fixing the blade in this way and this is your stock base stock whatever you can see and this I am measuring the angle. In the second case bevel protector for checking v block so here I am placing the blade again this is your base plate stock and I am measuring the angle. The third one I am using the acute angle attachment this is the acute angle attachment this is the blade I am fixing I am placing the work base about this acute angle attachment and I am measuring what is the reading obtained. Now pause this video for a few seconds and try to write answer to the following question straight true or false each division on the Vernier scale equals 5 minutes of arc. Probably you have wrote answer to the following question. Now next we will be discussing about optical bevel protector. Optical bevel protector is recent development of the Vernier bevel protector by using this instrument it is possible to take reading up to approximately 2 minutes of an arc. The internal circular scale is graduated in divisions of 10 minutes of arc. Readings are taken against a fixed index line or Vernier by means of optical magnifying system which is integral with the instrument. The scale is graduated as a full circle marked 090 090 the zero position corresponds to the condition when the blade is parallel to the stock. Provision is also made for adjusting the focus of the system to accommodate normal variation in eyesight. The scale and Vernier are so arranged that they are always in focus in the optical system. The scale and Vernier are so arranged that they are always in focus in the optical system. These are the following references. Textbook metrology by Yamahajan, IC Gupta Engineering Metrology, Dhanpatrai Publications, RK Gen Engineering Metrology, Khanna Publishers. Thank you.