OK... forgive my ignorance but if I understand the term broaching correctly its where the workpiece stays still and the cutter is not rotary but cuts with a forward/backward motion. So to cut a spline you'd hold the shaft in a rotary 4th axis and move a stationary cutter (sort of like a boring bar with a different head on it) in and out and down to form the spline... then rotate n degrees and repeat? I guess the cutter has three cutting edges bottom, left and right and is the shape of the final 'slot'? Sort of like gear hobbing...

SO... what makes it so hard under CNC control? Is it the forming of the cutter to get the splines accurate?