Geez, Kari, why go to the trouble of carving scrolls on the arms of your frame saw? It's just a utilitarian saw!
Here's my answer:
1) The Roubo illustration I'm using for reference shows scrolly arms and I think it looks cool.
2) I've always been fascinated with the volutes on violins and have wondered how they are made.
3) I like to carve.
4) I thought you'd find it interesting.
So, here is the method I came up with to carve these scroll shapes.
First, I found some violin images to use for reference. The scrolls I'm making aren't as pronounced, but they do have a similar shape. It seems to me that carving them is akin to raising a panel: you have to lower the background in order for the volute to emerge from the wood.
Next, I carved a slight downward curve toward the end of the arm and stopped at the point where the outside curve of the scroll starts. Then I defined the shape of the curves within the scroll by making vertical stop cuts with various-sweep gouges.
After that, I used gouges to cut toward the stop cuts. As I moved toward the center of the scroll, the cuts were more shallow which created a ramp that slopes upward toward the middle of the design.
Once the basic ramp shape was established, I rounded over the sharp edges with chisels and gouges. The violin images I used showed ramps that were both rising and scooped out from the outside edges toward center. If you're going to make a bunch of these, you might want to invest in some gouges that will scoop out this area in a couple passes. I don't have many gouges, so I used what I had.
The height and scoopiness of the scrolls I'm making are much less elaborate and defined compared to violins' volutes. And they're way less smooth and refined. But, I'm leaving them the way they are.
It is just a frame saw, after all.