G'day again, Roderick. Two-arm and three-arm protractors may be had from marine instrument dealers, as well those who supply tool and die makers. Bought new, they are quite expensive, and while you might find used ones in some seaport-city hock shop, chances of doing so are not good.
Tool makers use a surface plate, sine bar, and height gauge technique for precision angle work. This is, of course, a trig. method. With any good 18" metal scale, a hand magnifier or an eye loupe of modest power(5x or so,) and digital calculator with trig. functions, you can work to +/- about a minute of arc or better if need be. This is quite good enough for almost any dialing purpose. Usually, the layout can be done directly on the dial plate's face. (There is no advantage to hinging two scales for this application.) My article "Doing It With Style" in Vol.2, No.4, December 1995, NASS Compendium, pp(8-13), covers some of the trig. techniques as applied on a larger scale to orientation of the dial and its style. The treatment includes a quick run through 'law of cosines' application, and a refresher 'box' in plane trigonometry. The dial face layout is actually in most ways simpler than the orientation problem. (There are a couple of errata in the Compendium piece, but they shouldn't bear on your question.) I really believe that the trig. approach is the way to go. If you have particular problems about this, please direct them to me. When (if) you approach the implementation stage, we can go to single recipient e-mail on any points that would clutter the Sundial List with details. Cheers, Bill.
