Metrology Monday: Sine Bar

What is a Sine Bar For?

I consider this sine bar to be the most exotic measurement tool I have. I bought it mostly for fun. A sine bar consists of two cylinders of the same diameter fixed with their centers a known distance apart. The top of the connecting bar is parallel to the line between the two centers. All this done with as much precision as you are willing to pay for.

What the bar is used for is to “construct” precise angles between the bar surface and the surface the bar is resting on, typically a surface plate. Given the desired angle, one does the appropriate calculations (hence the sine name) to get vertical displacement for one end and builds that height out of gauge blocks.

Here as an arbitrary example, is a setup to get an angle of 32 degrees, 16 minutes and 27 seconds. Reference to an online calculator produced a displacement of 2.670″.

I called this tool exotic because it is difficult to conceive of a circumstance where I would actually need to measure to set up an angle this precise. I can use it as part of a machining setup or to check something like my engineer’s protractor for accuracy. I expect that an experienced machinist has more uses for one so who knows.

Alternatives

Budget sine bars cost less than $50CDN new so it wouldn’t break the bank if you decide you needed one. There are lots of protractor variants that are probably more practical for most jobs.

Options

The “sine tool” family includes plates and vises. The plate is just a wide bar. The vise is a vise but includes some sort of angle locking mechanism and the two cylinders to allow precise angle setting. Like a lot of precision tools, spend as much money as you want for increasing precision well in excess of the average hobbyist’s needs.

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