A sine bar is a tool used to measure angles of a block. It consists of a hardened steel body with two precision ground cylinders fixed at the ends. The distance between the centres of the cylinders is precisely controlled, and the top of the bar is parallel to a line through the centres of the two rollers as shown in Figure 18.20. The dimension between the two rollers is chosen to be a whole number and treated as the hypotenuse of a triangle. Generally, the centre distance between two cylindrical rollers is 10 in. or 100 mm sine bar (however, 5 in. sine bar is also used).
Figure 18.20 Sine Bar
Figure 18.21 Sine Bar Measuring the Inclination Angle of a Workpiece
When a sine bar is placed on a flat surface, the top edge will be parallel to that surface. If one roller is raised by a certain distance using gauge blocks, then the top edge of the bar will be tilted by the same amount forming an angle that may be calculated by the application of the sine rule. Angles are measured using a sine bar with the help of gauge blocks and a dial gauge or a spirit level. For example, to measure the angle of a wedge, the sine bar is placed on a horizontal surface plate. The wedge is clamped over the sine bar with inclined surface on the top. At this position, the top surface of the wedge is inclined with respect to surface plate. Using slip gauges, the top surface of wedge is made horizontal. The sine of the angle of inclination of the wedge is the ratio of the height of the slip gauges used and the distance between the centres of the cylinders as shown in Figure 18.21.
where θ is an inclination angle, h is height of slip gauges, and l is distance between rollers of sine bar.
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