What is a Sine Bar?

A sine bar is a tool used to measure angles in wood and metalworking. It can be used to find a desired angle or to measure an existing angle within an object or project. The sine bar typically comes in standard sizes, with a 5-inch (12.7-cm) sine bar among the most common in the U.S. In other parts of the world, a 100-mm (3.9-inch) bar tends to be the most common size, though other sizes are also available.

Manufacturers produce sine bars to very specific dimensions in order to ensure precise measuring capabilities. Each bar consists of steel, stainless steel, or aluminum, and may include one or more magnetic surfaces. The bar includes a cylinder at each end, with each cylinder equidistant from the center of the bar. These cylinders must measure exactly the same size to maintain a high level of precision.

To use a sine bar, workers must place the bottom of one cylinder on top of a flat surface. The top of the bar should rest along the angle being measured. To calculate this angle, users must divide the height of the raised end of the sine bar by the length of the bar. This figure gives the user the sine of the angle in question. By using a sine calculator or mathematics chart, users can then calculate the size of the angle in degrees.

Many sine bars also come with charts that tell the users how high to raise one side of the bar to achieve a specific angle. users stack small steel blocks under one end of the sine bar to raise the bar to this height. These blocks, known as precision blocks, or gage blocks, come in precise units to achieve the desired measurement.

A sine bar can help wood or metalworkers set a vise angle or clamp an object in place at the desired position. It can also be used to correctly position tools on a lathe or milling machine. The sine tool can even be clamped in place along with an object to ensure precision as the work takes place. Finally, a sine bar may be used to ensure objects conform to the correct measurements and angles as part of the quality control process.

In some applications, a standard sine bar may not be large enough to measure oversized objects. On these products, workers use a similar tool known as a sine plate. In more complex products, users rely on a compound sine plate or table. This tool allows workers to measure compound angles over large surfaces.