Adding second shaft and encoder to small surplus motor

In this case the existing shaft was 5/32". A 1/4" shaft was pressed over the existing shaft then supported by ball bearings. Motor cost
$25.00, encoder cost $40.00. Since then, US Digital has introduced an even lower cost encoder for $20.00. Link to pictures below.
http://www.pbase.com/eldata/motor_mod
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Neat. I've done several conversions where there was no tail-end shaft at all:
http://www.truetex.com/servomod.htm
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I found that link in a post of yours while Googling before undertaking the mods. It gave me the confidence to press on with mine.
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...

<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
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If there is a live center dimple then you can proceed as in that article. However, suggest you match the diameter of the shaft to be pressed with the correct size drill bit before you start the drilling. Wire gauge sizes are useful for that purpose. A dry run in the lathe to confirm correct size hole for pressing would also be prudent. I used a precision shaft with matching drill bit and precision reamer since the extension shaft also became the motor shaft.
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