I was reminded today that Grizzly has guitar nut files from .010" to.031" for $12.95 each. They are good quality,vertical sided files,but they will cut a round bottom groove that you can square up with a needle file. They sell others that have tapered sides,but this is a series of 12 files I refer to with straight sides. I would not buy their needle file sets as they are the usual Chinese stuff,but these nut files are good