Shapers and slotters can preform work that would otherwise be quite difficult, such as internal features, square and rectangular holes, keyways, splines, etc. Recently, I needed to make a special hex socket; the slotter (vertical shaper) is a natural for that job, and yes, shaping segments of circles. My slotter is a 6" Pratt & Whitney, it has a 20" rotary table built in, with automatic rotary feed, plus shop bolt indexing and indexing in degrees by worm feed, and I have index plates for accurate indexing like on a dividing head. The P&W is intended primarily for tool and die work, so the ram is inclinable for die clearance. I also have a B&S slotting head for my B&S #2 universal mill, it can be sometimes better than the P&W for some work, as the slide is inclinable sideways 360 deg. Also have the Gould & Eberhardt 20-24" industrial universal shaper, especially ,,nice for such things as tapered gibs, or most any long skinny part within its range.