 Hello, and welcome to the video lecture on Angular measuring instruments 4. At the end of this video lecture, students will be able to explain about various Angular measuring instruments. The following topics will be discussed in this video lecture. Recap of third session, then we will be discussing about the sign center, sign bar. It is 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 rulers. The sign 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 axes of these rollers are parallel to each other and also to the upper surface of the bar. The normal distance between the axes of the rollers is exactly 100mm, 200mm or 300mm. When the rollers are brought in contact with a 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. Sign bars are graded as A grade or B grade sign bar. A grade sign bar are made with an accuracy of 0.01 mm per meter of length and B grade sign bar with an accuracy of 0.02 mm per meter of length. Types of sign bar. The sign 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 sign bar. Figure B shows a sign bar with hollow rollers in which outside diameter is equal to width of the sign bar. It is useful in instances where the width of the bar enters into calculation of work height. Figure C shows a sign bar with pins on both sides. This is used where the ordinary sign bar cannot be used on the top surface due to interruption. Figure D shows a sign bar which is generally preferred as the distance between the rollers can be adjusted exactly. Principle of sign bar. The principle of operation of a sign bar is based on the loss 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 the combination of slip gauges and L the distance between the roller centers then sin theta equal to H by L or theta equal to sin inverse of H by L. Thus the angle to be measured or to be set is determined by indirect method as a function of sin. For this reason the device is called a sign bar. Accuracy requirements of a sign bar. If a sign bar is to be accurate then falling conditions relating to its constructional 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. Users of sign 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 can be treated as horizontal. One roller of the sign 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 sin it to be set at an angle theta. Then sin theta 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 sign bar is inclined at an angle theta to the surface plate. For better results both the rollers could also be placed on slip gauges of height H1 and H2 respectively. When sin theta equal to H1 minus H2 by L theta will be equal to sin inverse of H1 minus H2 divided by L. Checking for measuring unknown angles. One 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. Sign bar is then set 2 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 sign bar. The dial gauge is then set at one end of the work piece and mowed 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 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 the component is then calculated by the relation theta equal to sin inverse of H by L when the component is of large size or heavy. In such cases the component is placed over a surface plate. The sign bar is placed over the component, the height over the rulers 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 probe of dial test gauge showing same rating for the topmost position of rulers of the sign bar. If H is the difference in the heights and L the distance between the ruler centers of the sign bar then theta will be equal to sin inverse of H by L. Limitations of the sign bar. It's reliable for angles less than 15 degree and becomes increasingly inaccurate as the angle increases. It is impractical to use sign bars for angles above 45 degree. It is physical clumsy to hold in position. Size of parts which is to be inspected by sign bar is limited. Sign center. Due to difficulty of mounting conical work easily on a conventional sign bar sign centers are used. Two blocks as shown in figure are mounted on the top of the sign bar. The blocks accommodate centers and can be clamped at any position on the sign bar. The centers can also be adjusted depending on the length of the conical workpiece to be held between centers. Sign centers are extremely useful for testing of conical work. Use the centers and ensure correct alignment of the workpiece. The equipment consists of a self-contained sign bar hinged at one end and mounted on its datum surface. The table is quite rigid one and the weight of the unit and workpiece is given fuller and safer support. The table may be safely swing to any angle from 0 to 90 degree by pivoting it about its hinged end. Due to the work being held exactly between centers the angle of inclination will be half the included angle of the work. It provides a convenient method for measuring the angle of a taper plug gauge. So this will be the angle of inclination. Now pause this video for a few seconds and try to write answer to the following question. What are the applications of sign center? Probably you have wrote answer to the following question. These are the following references. A textbook metrology by M. Mahajan. Second one is about IC Gupta, Engineering Metrology, Dhanpatrai Publications. Third one is about R. K. Chen, Engineering Metrology, Khanna Publishers. Thank you.