I made a replacement shaft for an electric model airplane motor. My wife bent the old one as part of the learning process (she's learning control line. You cannot fly ten dumb thumbs high with a control line plane -- you're always one or two mistakes away from disaster).
So, during the better part of an hour's worth of work, I cut a 3mm rod to length, made a 20-mil wide groove cutter, cut a groove, pressed out the old shaft, and pressed in the new one.
Making the cutter on my bench grinder took most of the time.
The best part is -- I saved $8.50! $7.40 for the motor, and $1.10 for a replacement shaft! For just an hour's work!