[footpetaljones]

I am considering upgrading the motor on my G0704 and down the road I might look to convert the spindle to use BT30 tool holders. I know that an encoder can be added to enable rigid tapping, but my understanding is that most control software slaves the Z axis movement to the motor RPM when doing so. I am wondering if a similar encoder system would allow spindle orientation capabilities when using a BLDC or 3 phase induction motor, or if that would not be possible.

Thank you

[Jim Dawson]

Spindle orientation might be possible with a 3 phase induction motor. When I get time, that is one of my projects on my lathe. It already has a spindle encoder, is being driven by a sensorless vector VFD, and controlled by a motion controller. The motor is the same but the original drive was a Fanuc, and would it index/orient to 1°, but I think the drive had tighter control than the VFD. We'll see how it goes.

The really easy way to make the spindle do anything you want is to convert to a servo motor. That will index to one encoder pulse. The DMM 1.8 KW servo is a popular spindle conversion. ClearPath has also come out with some larger servos.

[footpetaljones]

Going with a servo motor certainly would be the easiest solution. However, both DMM and ClearPath's motors are quite limited on speed and I mostly cut aluminum. A pulley arrangement is simple enough to add, but the Centroid Acorn software I use requires a 1:1 ratio between the encoder and spindle, meaning I'd have to add in another encoder anyway. I've been looking at ebay, but even used a proper spindle servo motor and drive combo would be somewhere between $2000-$3000.

[skrubol]

Most BLDC motors outside of the hobby market have at least hall sensors on them. You should be able to get at least 6 reference points from that (3 rising, 3 falling edges for a 2-pole motor, double that for a 4-pole, etc.)
I don't think that would be enough resolution for rigid tapping though, I think rigid needs quite a bit better resolution than single point threading on a lathe because the lathe is always spinning at a constant(ish) speed, whereas tapping has to decelerate and accelerate at the bottom of the hole. I think having an accurate speed or index helps with tension/compression taps though (allows you to get closer to the bottom without risking crashing.)
I'm not sure if it's possible to pick up an index from an induction motor, as their magnetic field doesn't rotate in sync with the rotor.

Just reread OP and I don't think what I said had much to do with it (but I'll leave it anyway.) You can definitely add an encoder to a spindle. If it's belt driven, you have some options, you may be able to clamp an encoder disk to the spindle somewhere or add a second (light weight) pulley to drive a timing belt for an encoder on a shaft (if you can't do 1:1 with the motor.) Klugey, but can work.
I see tons of spindle motors for cheap on ebay, though usually much bigger than what you'd want for a bench mill. The problem is matching up a drive. If it's an induction motor you can usually just hook it up to any old VFD and figure out how to interface the encoder or just replace the encoder with a quadrature encoder.