In lieu of a blog, I'll report here on the progress in making the half-round channel I need to represent the round variety of HO-scale rain gutters --- square gutters are pretty much already done with C-channel in both styrene and brass from various suppliers.
My first try was to use the Dremel in a "router table" stand with a .040" diameter round cutter to create a semi-circular groove in a solid nylon block. I then used .003" thick shim brass stock and a .045" diameter brass rod to press the sheet into the groove and form a somewhat rounded crease. Removing the brass from this "die", I then manually wrapped it half-way around the rod and formed a U-shaped piece. The curvature at the bottom is perfect, but the sides are way too long. The best I could do on the first try was to cut the sides down with precision scissors but getting a clean cut all the way along the 4" length of this sample was very difficult.
Conclusion #1: I need a deeper groove. A simple semi-circular trench isn't sufficient and I need more of a U-shaped groove than a simple half-round. If I can do this, I may be able to clamp the brass under the rod in the channel and then bend the sides over to form a good crease for cutting. I'll try to do this today if I can find the appropriate cutter burr.
Norm