Looking forward to seeing some pics of your machine as the project unwinds
. The Fanuc motors are very good quality and work really well in my experience...after all they are still making money for you 26yrs on!!
The 401 alarm is usually motor related ie. a build up of carbon dust and damp in the motor or in extreme cases can be a faulty tacho or encoder but this is quite rare.
Slop in the motor bearings can cause this too if the windings are moving about too much.
Good thing to check the brushes for wear although i'm sure you already have this under control!!
If the motors are left standing for a long period the brushes can rust the commutator too..been there done that!!
Will you be using the Fanuc spindle motor and drive..? Would be quite expensive to replace ! I'm presuming its a Fanuc AC model 3 or above with an A06b-6044 drive..?
Not sure if you can rigid tap with the Fanuc motor by adding an encoder..? I think the motor uses a Tacho for speed.
Mine has a resolver to position the spindle for a tool change (M19) as well.
John.