....
Neat. I've done several conversions where there was no tail-end shaft at
all:

http://www.truetex.com/servomod.htm

Clip from above link:
To add an encoder to the motor, one needs a rear shaft extension with a
diameter to match the encoder wheel, typically 1/4". While some motors have
such an extension by design, I had to add it to this surplus unit. Figure 3
shows the end of the 5/8" shaft of the motor, into which I have drilled a
1/4" hole. Based on the dimensions of a 2" Renco encoder, which requires a
0.65" to 0.74" shaft projection, the hole is drilled 0.570" deep into the
shaft. This drilling depth is chosen to yield about 0.5" of depth into the
shaft proper, since the end of the shaft is recessed slightly inside the
motor end housing. The length of the encoder shaft projection should be
0.74", and thus the drill rod length is 1.31". The Agilent encoders use a
slightly shorter extension, but can accommodate the longer one. And it is
easier to shorten the projection after it is installed, versus putting metal
back!
end clip


I'm needing an encoder on a 5 hp motor. Anyone done this trick on a large 3
phase motor? My 'PLAN A' was an extra pulley, belt, and encoder bracket on
the front. This sounds easier. OTOH, mounting a large armature from a 5 hp
motor in the lathe won't be a small job.

Karl