I use a harbor freight el cheapo 4.5 hand angle grinder[actually i like it better than my makita].Try not to get them too hot ,put them in a vise,try to keep original angles,also try to keep each end exactly the same size for balance. remove wire edge and do final touches with a flat file,
T hey work as good as new,unfortunetly every body and their brother wants me to do there blades,more so with the ones with the spiral shaped mulching blades
ALAN