A sinebar is the preferred way to accurately set or measure an angle. On the other hand, a protractor is a very handy tool in a number of applications - not only metalworking but woodworking as well. Wouldn't it be nice if we could combine the accuracy of the sinebar approach with the convenience of the protractor?
As an experiment, I tried doing this and it worked out well for my purposes. Some of you may want to explore the idea further and the included program will be helpful if you do.
I hinged two bars together with an offset, half-lap hinge such that when fully "closed" the edges of the two bars were parallel and in contact along their length. With this sort of hinge, the measuring surfaces of the two bars are always in the same plane - unlike the case where two bars are hinged together by simply drilling a hole through both and dropping in a hinge pin.
I made a high quality hinge consisting of reamed holes in the bars with a close fitting pin that had a protruding threaded section. A lock nut on the threaded part clamps the two (half-lapped) bars together when tightened. The hinge pin is also centerdrilled so that locating the hinge axis with a center finder is done easily.
At a fixed radius from the hinge axis, I drilled the bars and mounted two precision pins - denoted by the '.'s in the crude ascii diagram above. Although it's not clear from this crude diagram the distance from the hinge axis to the center of *each* pin is the *same* (known) radius. The included protrac.jpg diagram will make this clearer. [Replaced with image below—jhmiii]
Now, the idea is that, by measuring the distance between the two pins (say with a vernier caliper) we can set or calculate the angle formed between the two bars. The process is analogous to hinging a sinebar in the middle and setting the included angle by adjusting the distance between the two sinebar rolls.
On my experimental model the pin diameter was small enough (i.e., less than the bar width) that the pins did not touch when the protractor is fully closed. At a zero angle, there is a finite gap between the two pins. This gap must be accounted for in the calculations.
If I've left out some dimension you need to know, contact me and I'll add it. mklotz@alum.mit.edu.
The defaults are for the protractor made in Protractor & sine bar.