[QuinnSjoblom]

An interesting dilemma. Speed, accuracy, cheap, pick any two. I'm going through somewhat the same thought process on my lathe which right now does not have spindle indexing capability, but does have live tooling. 7.5 KW (10HP) spindle, just to make it more interesting.

In your case, why not put the spindle motor of your choice on the spindle, any cheap motor would do for turning. Then a separate small servo motor with a timing belt drive, driving a large sprocket on the spindle. But the sprocket freewheels until you lock it to the spindle. The sprocket lock mechanism could be as simple as screwing in a bolt, pushing in a pin, or for full automation could use an automotive air conditioner clutch (those will transmit an immense amount of torque, around 10 HP worth). For additional holding power, you can always add a disk brake, motorcycle or go-cart parts.

For servos, take a look at DMM Tech products also.

Not a bad idea. Probably even cheaper. To get a servo that will provide the needed power for turning, it's 600 bucks minimum. If I separate it into a servo and a bldc motor, the servo doesn't need nearly as much power for the indexing as long as it's geared down enough, more like 300 dollar servo. One hang up I have on the heavily geared down servo though, 10 to 1 with pulleys is gonna be a huge pulley and a tiny pulley, i dont picture the tiny pulley holding onto the belt with total rigidity during indexing. I can imagine there will be a tad bit of give there. Maybe not as bad as I think. I guess there's still the option of putting the servo on a 50 to 1 harmonic, then link that to the spindle with 2 decent size pulleys. Then i would have all the resolution i could ask for and still probably 100rpm after 50 to 1 gearing which is plenty snappy for a 4th.
Actually that gives me a good idea. Only 2 pulleys, one on the spindle, one next to it. The pulley to the side is supported by it's own bearings with a shaft protruding out both sides. One end of that shaft will have a link to the harmonic reduced servo, other end linked to bldc motor. Some kind of detachable coupling for both ends or clutch system. I'll take a look at the a.c. compressor clutch mechanism. I can also picture some kind of 2 in 1 actuated system. Dowel pins passing through that offset pulley. When pushed one way or the other, they will disengage one and engage the other in one single mechanism. Hmm.i think I can start playing around on cad now, see how this will lay out. Thanks for the dual motor idea, might make the most sense.
So along the lines of picking a bldc, what would you recommend? Does 2hp come in a pretty small package? How do you control it with mach3? Guess I can probably figure that out if I look. Never used them before