I have tried to use a Dremel tool to engrave sundials in brass, but even after placing the tool into the router base, it was not possible to achieve any degree of control manually. The results were not at all satisfactory, downright embarrassing actually. I have also used ferric chloride to etch a dial in brass, and while it does work, it takes a great deal of patience and/or skill to produce a satisfactory result. A clean mask is critical to success. In addition, I did not etch the design deep enough, and now that the brass is weathering, the dial is barely readable. I still believe it is possible to produce results this way without spending much money, but a great deal of time, care and patience is required.
About five years ago, Roland Digital Group introduced a computer controlled engraver that interfaces to the PC (serial or parallel port), and shares the instruction set of an HP plotter. This machine will engrave a variety of material, including brass, and has a table accuracy of .0004 inches. The straightforward interface has made it very simple to utilize CAD software on a PC, and directly "plot" the output into brass, or engraving plastic as my needs require. Although the price of the machine was an obstacle (it took me quite a while to convince my wife I should get it) the results have been very satisfying. The table size is 5.5" by 8", so I limit my dials to this range. This is not a problem because the design I developed incorporates a wire gnomon going from the dial face to a post "north" of the scales, and even on fairly small dials the readings are very accurate. All this started for me by plotting paper dials that the sun would destroy in a matter of weeks - even with UV resistant polyurethane. The only material I didn't try was stone, and as Bob Terwillager mentioned in another e-mail, sandblasting would probably be best. I did consider making a wooden mold for concrete, but have been side-tracked by the engraving. I hope this will save you some time in your pursuit of a n accurate dial. Steve Harrington
