Im wanting to build a horizontal spindle for my mill that will double as both a lathe and a 4th axis, mostly for smaller diameter (roughly up to 2 inch max diameter) aluminum parts, turning with tools mounted to main milling head and index milling with main spindle. At this point im leaning towards a foundation of a premade dunham 5c headstock/spindle. Seems to be pretty good quality, very low runout, 5c taper is ground after mounted, 1700 bucks. The challenge is choosing a proper servo to drive this thing and come up with a solution that will give enough rpm as well as enough holding torque and resolution for 4th axis operations. These are the goals:

1-At least 2500rpm for turning, more is better, 4k would be great

2-At least 1hp for turning. Smaller aluminum parts so don't need crazy power

3-A minimum resolution of .01 degree step for 4th axis operation. 90 percent of what I'm doing doesn't require this kind of resolution, but one exception to the small aluminum parts is hobbing large helical delrin gears, about 4inch diameter so .01 degree step will be about 3 tenths on the edge of a gear. Higher resolution would be better, but I figure this is a minimum.

4-enough holding torque for indexing a mini horizontal tombstone with tail stock support, smaller tools up to 1/4inch machining out at a max radius of 2.5 inches, only light machining, rarely more than 1 cubic inch mrr, no drilling larger than 3mm.

5-budget of 3500 max including the 1700 dollar headstock /spindle

At first i was hoping to find a powerful enough servo to do all this without changing gearing, but after some research it doesn't seem too realistic. A big 1200 dollar clearpath servo will give far more than enough power, 2700rpm, and probably just enough holding torque (4800oz/in) with direct drive, but not quite the resolution i need. This would give .03 degree resolution which would be about a thou step at the edge of a 4inch gear, don't think that's gonna cut it for precision hobbing. So the main challenge in my mind is figuring out how to give this thing 2 gearing options that can be swapped fairly quickly, better yet an automated changed over. If I choose a more realistic size nema 34 dc servo, it wants to run somewhere between 1 to 1 and 2 to 1 ratio for turning, and between 5 to 1 and 10 to 1 for indexing. 10 to 1 or more would be preferred. Pulleys and belt is fine for the turning, but a 10 to 1 ratio with a belt for indexing isn't really optimal. Not sure what kind of rigidity and backlash I can count on with that. Then I was thinking maybe somehow use harmonic drive for the 4th axis reduction and belt for turning, but I would still need to place the harmonic drive next to the spindle and drive it with a 1 to 1 belt since it's a 5c spindle and harmonic drive can't couple directly to the back of it.

So, any ideas of the best way to tackle this? Would be great if there was a servo capable of what I want with only one gear ratio and within the budget, but I don't think that's realistic, so the main puzzle is coming up with how to give this thing 2 quickly swappable gearing options (automated would be great) to meet the goals.