Accurate angle measurement

[mklotz]

In a Tool Talk thread, I talked about the need to measure the angle of a planer gauge accurately...

http://www.homemadetools.net/forum/h...7175#post85187

While this has more to do with how to use the tools you have rather than the construction of a new tool, like my taper measurement method, I feel it's a tool in the conceptual sense of the word, hence its presence in the tool forum.

The basic idea of this method is to use a sine bar to "cancel out" the angle to be measured and, having done this, the stack height under the sinebar becomes an indirect, accurate measure of the angle in question.

As the photo shows, the object is placed on a sine bar in such a way that raising the end of the sine bar can make one side of the object horizontal.

How do we determine when it's horizontal? A DTI is swept across the top of the object; when it shows no change in value from one end of the object to the other the top of the object is parallel to the surface plate on which the work is done. A surface plate is essential; for this scheme to be accurate the surface must be flat and featureless.

The DTI is supported on one of my height gauges. The height gauge plays no part in the measurement; it's simply a convenient way to adjustably support the DTI and slide it over the surface plate.

Purists may note that I used an adjustable parallel rather than Jo blocks to create the stack for the sine bar. It's easier and the effect on accuracy is negligible.

So, what were the results?

After the DTI showed no slope of the gauge, the adjustable parallel miked to be 1.136" high. With the 2.5" sine bar I was using this translates into an angle of...

ang = arcsin (1.136/2.5) = 27.03 deg

That's only 0.01 deg = 0.6 arcmin difference from the value I obtained in the referenced thread using a no sinebar method of measurement.

You'll only rarely need to measure angles with this degree of accuracy but, should you need to, now you know how to do it.