When I was a kid I used to turn the crank on the grinder while Dad sharpened the sickle blades. It took years before we got a grinder with a motor to sharpen sickle blades. The stone was "V" shaped so you could sharpen two sides at once. Haven't seen a "V" shaped stone in years, but they are probably still available.
I haven't tried it, but I think you might be able to sharpen blades with a 3" or 4" angle grinder.