Measuring and Marking Metals for Home Machinists by Law Ivan;

Author:Law, Ivan;
Language: eng
Format: epub
Publisher: Fox Chapel Publishing

THE SINE BAR

The tool which engineers use to determine angles, using the above method, is called the sine bar. It is an accurate parallel steel bar with two identical rollers attached at a definite center distance. This distance can vary depending on the size of the workpiece involved but usually the center distance is either 10in. or 5in. Fig.53 shows a sketch of a typical sine bar. It was the author’s intention when starting out to compile this book, to keep mathematics out of it as far as possible but unfortunately should any reader wish to use the sine bar, or the principle involved, then he must resort to the use of simple trigonometrical tables, or at least to the table listing the sines of angles.

If the reader is not familiar with trigonometry it is still possible to use the sine bar and sine tables to obtain an angle without having to make a study of triangles or their trigonometrical function. Refer to fig.54 – here we see a sine bar with one end resting on a flat surface such as a surface plate ad the other end resting on a packing. The sine bar is now at an angle to the surface plate. Since the length of the sine bar remains constant it follows that if we alter the size of the packing it will also alter the angle. In other words, the size of the packing will determine the angle, or, to put it the other way round, the angle required will determine the size of the packing needed. Now, if we divide the size of the packing by the length of the sine bar we get a figure – always less than one – and this figure is called the sine of the angle. It is a simple as that!

Many workshop manuals, and certainly all books on trigonometric functions, contain a table of sines, and this table lists side-by-side the figure obtained by dividing the height of the packing by the length of the sine bar, with the angle which corresponds to this figure. If, therefore, we know the angle but do not know the sine, look down the list of angles until the required figure is revealed and simply read off the sine. If the sine is known but the angle is not, look down the sine column and read off the angle. For example, supposing we require to know the size of the packing required to give an angle of 20°. The table of sines for 20° gives a figure of .342, now this is the figure we get when the size of the packing is divided by the length of the sine bar, so if we multiply the length of the sine bar – which is 5in. – by the .342, we shall obtain the size of the packing, i.e. .342 x 5in. = 1.710in. It is now apparent why 5in. was chosen for the sine bar length, it is very easy to multiply

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