Note also that John uses nylon spacers to mount the motors. If there is any very minor misalignment at the motor shaft, the nylon tends to absorb it a little.
I have had one shaft break due to misalignment. I was surprised to find that they are two piece shafts with an electric weld joint that is apparently intended to break as a safety feature. The coupler was a DumpsterCNC delrin part. Obviously, it was a very bad misalignment situation that was self induced from lack of experience with these things.
CarveOne
http://www.carveonecncwoodcraft.com