 Hello and welcome to the video lecture on Angular measuring instruments 3. 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. Recap of second session, Sign Bar, Types of Sign Bar, Principle of Sign Bar, Uses of Sign Bar, Limitations of Sign Bar. Spirit Level A spirit level has a sealed glass tube ground on its inside surface to a convex form with large radius of curvature R. The tube is almost completely filled with ether so that only a small volume remains at the top part of the tube, which contains ether vapor in the form of a bubble. Cleanometer A cleanometer is a spirit level mounted on a rotary member. The angle of inclination of the rotary member relative to its base can be measured by a circular scale. There are various types of cleanometers, one-year cleanometer. It consists of a spirit level mounted on a rotary member carried in a housing. One phase of the housing forms the base of the instrument. There is a circular scale on the housing. The angle of inclination of the rotary member relative to the base can be measured by the circular scale. The scale may cover the whole circle or only a part of the arc. Generally used to determine the angle included between two adjacent phases of a work base. The base of the instrument is placed on one of the surfaces and rotary member is adjusted till zero reading of the bubble is obtained. The angle of rotation is then noted on the circular scale against an index. The instrument is then placed on the other surface and the reading is taken in a similar manner. If theta and phi are the reading of the instrument then the included angle alpha between the surfaces alpha will be equal to 180 degree minus theta plus phi. Now let's discuss about the next cleanometer that is your micrometer-cleanometer. In this type spirit level is attached at one end of the barrel of a micrometer. The other end of the spirit level is hinged on the base. The base is placed on the surface whose inclination is to be measured. The micrometer is adjusted till the level is horizontal. This type of cleanometer is used for measuring small angles, uses of cleanometer. They are used for checking included angles, relief angles as well as angular phases on large cutting tools and milling cutter inserts. They can also be used for setting inclinable tables on jig boring machines and angular work on grinding machines. Now let's discuss about sine bar. It's the precision instrument used along with slip gauges for the measurement of angles. It is used to measure the angles very accurately to locate the work to a given angle within very close limits. It consists of a steel bar and two rollers. The sine bar is made of high carbon, high chromium, corrosion resistant steel, suitably hardened precision ground and stabilized. The rollers are of accurate and equal diameters. They are attached to the bar at each end. The axis of these rollers are parallel to each other and also to the upper surface of the bar. The normal distance between the axis of the rollers is exactly 100 mm, 200 mm or 300 mm. When the rollers are brought in contact with the flat surface, the top of the bar is parallel to the surface. The various parts are hardened and stabilized before grinding and lapping. All the working surfaces of the bar and the cylindrical surfaces are fine to surface finish of 0.2 micrometer RA value. Sine bars are graded as A grade or B grade sine bar. A grade sine bar are made with an accuracy of 0.01 mm per meter of length and B grade sine bar with an accuracy of 0.02 mm per meter of length. Types of sine bars The sine bars are available in several designs for different applications. The simplest one is shown in figure A. In which the rollers are so arranged that their outer surfaces on one side are level with the plane top surface of the sine bar. Figure B shows a sine bar with hollow rollers in which outside diameter is equal to the width of the sine bar. It is useful in instances where the width of the bar enters into calculation of work height. Figure C shows a sine bar with pins on both sides. This is used where the ordinary sine bar cannot be used on the top surface due to interruption. Figure D shows a sine bar which is generally preferred as the distance between the rollers can be adjusted exactly. Principle of sine bar The principle of operation of a sine bar is based on the laws of trigonometry. To set a given angle one roller of the bar is placed on the surface plate and the combination of slip gauges is inserted under the second roller. If H is the height of combination of slip gauges and L the distance between the roller centers then sine theta is equal to H by L. Thus the angle to be measured or to be set is determined by indirect method as a function of sine. For this reason the device is called a sine bar. Accuracy requirements of sine bar If a sine bar is to be accurate then following conditions relating to its construction features must exist. The axis of rollers must be parallel to each other and the center distance L must be known. The top surface of the bar must have a high degree of flatness. It should be parallel to the plane connecting the axis of the rollers. The rollers must be of identical diameters and round within a close tolerance. Uses of sine bar, locating any work to a given angle. To set the given angle the surface plate is assumed to be perfectly flat so that the surface plate can be treated as horizontal. One roller of the sine bar is placed on the surface plate and a combination of slip gauges is inserted under the second roller. Let H be the height of the slip gauge, combination and sine it to be set at an angle theta. Then sine theta is equal to H by L where L is the distance between the center of the rollers. Thus knowing theta H can be found out and any work could be set at this angle as the top face of the sine bar is inclined at an angle theta to the surface plate. For better results both the rollers could be placed on slip gauges of height H1 and H2 respectively. Then sine theta will be equal to H1 minus H2 divided by L checking for measuring unknown angles. When component is of small size for measuring an angle it is necessary to fast find the angle approximately with the help of a bevel protector. Sine bar is then set up at that nominal angle on a surface plate by suitable combination of slip gauges. The component to be checked is placed over the surface of the sine bar. The dial gauge is then set at one end of the work and moved along the upper surface of the component. If there is a variation in parallelism of the upper surface of the component and the surface plate it is indicated by the dial gauge. The combination of the slip gauges is so adjusted that the upper surface of the component is truly parallel with the surface plate. The angle of component is then calculated by the relation sine theta equal to H by L when the component is of large size or heavy. In such case the component is placed over a surface plate. The sine bar is placed over the component. The height over the rollers can be measured by a 1 year height gauge using a dial test gauge mounted on the anvil of height gauge to ensure constant measuring pressure. The anvil of height gauge is adjusted with the probe of dial test gauge showing same reading for the top most position of rollers of sine bar. If H is the difference in the heights and L the distance between the roller centers of the sine bar then theta will be equal to sine inverse of H by L. Now pause this video for a few seconds and try to write answer to the following question. State true or false slip gauges are used for measurement of height. Probably you have wrote answer to the following question. Limitations of sine bar. It is reliable for angles less than 15 degree and becomes increasingly inaccurate as the angle increases. It is impractical to use sine bars for angles above 45 degree. It is physical clumsy to hold in position. Size of parts which is to be inspected by sine bar is limited. These are the following references. Thank you.