On commercial units, I use an angle grinder, either air or electric. Usually I sharpen em in situ. On the smaller chit, I always use my air die grinder with a 3/4dia. carbide cylindrical burr. Absolutely will not burn the edge! If they are really bad I might use my angle grinder first, but always finish with the carbide cutter.
If I remove em to sharpen, I always balance em' with a magnetic unit, the same one that advertises on here.