Last night I tried to make another "ball" pick and determined that some hacksaw blades are hardened along the tooth edge and may be hard to file. My files--even sharp fine ones (Oberg and Grobet)--just skipped over the metal and wouldn't cut the hardened steel.
The blade had to be ground down (and dunked often to keep cool). Only after about 1/16" of material was removed along the egde could the metal then be filed.
Just another reason that a Dremel or grinder will become necessary for shaping.