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How to Build a Sine Bar

Sine bars are tools used with gauge blocks to accurately measure angles to within thirty seconds of a degree. A sine bar consists of two equal cylinders that are an exact distance apart, which is usually a round number, such as 5 or 10 inches, and a plane on top of the cylinders that is parallel with the axes of the cylinders. Because they are designed to give precise measurements, heat treatments and grinding are used in the production of commercial sine bars to make sure the tools are stable over time and are uniform in size.

Things You'll Need

Masonite, 1/4-inch thick
2 dowels
Glue
Drill
Jig saw

Instructions

- 1
  Cut a board from a piece of Masonite that is 13 inches long. This board should be cut so that there are two equally spaced pieces that extend 2 to 3 inches lower than the rest of the board, where the cylinders or wheels will be attached. These strips should be cut to allow the wheels to be attached at a distance of exactly 10 inches. The sine bar can be made smaller, but should still be cut to be an exact distance. Any inaccuracies caused by a board that is not precisely uniform will lead to greater measuring inaccuracies in a board of a shorter length.
- 2
  Cut two circular discs from the Masonite with a diameter of three inches to serve as the sine bar's wheels or cylinders. Measure to the exact center of the disc and drill a hole of .25 inches. While the size of the disc can be modified, it is important that the discs be as close to the same exact size as possible. You should also take care to make sure that the drilled hole is at the exact center of the disc.
- 3
  Drill a matching hole in each of the longer strips that come off of the board. These holes should be exactly 10 inches apart -- or whatever distance you have chosen for the length of your board. Use a dowel and glue to attach the discs to the board. Sand down the dowel so that it is flush with the board and disc.
- 4
  Use your sine bar to measure the angle of a particular object by using gauge blocks or another item of uniform height. The height is equal to the sine of the angle multiplied by the distance between the two discs. Since you already know the distance between the discs and should know or can measure the height, you can determine the size of the angle.

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