Hello,
I've got a 1969 Series I J-head machine that's got ballscrews and used to be CNC. The old servos were not part of the deal (nor the refrigerator sized NC controller!) So right now it's a manual machine while I learn how milling works, but I want to get it back to CNC.
I understand enough about steppers that there's no problem piecing together a system, as it's pretty straightforward. Would steppers rated 900-1200 oz/in be good enough to drive the X/Y on 9x42 table with .200" lead ballscrews? I've looked at the motor torque curves and figured out the max power zone for various feed rates (ie, 900 oz-in model has lower inductance and does almost as well as 1200oz-in model, except at low speeds,) but I don't know how much torque is required.
But if I can afford it, I'd rather go with servo motors.
I see places like Keling have inexpensive DC brush servos (which seem easy to setup,) but I'm not sure if their largest DC servo is going to be powerful enough. It says 1125 peak oz/in @ 40Amps, but the Gecko drives they sell only do 20 Amps, so presumably the peak torque would be cut in half. Constant torque is listed at 226 oz/in, not sure if that's enough. I suppose servos would require belt gear-down to get the servos going faster into their high-power zone.
Then there are brushless/AC servo motors. I see these on E-Bay all the time, but the drives either require 3-phase 220, or have a serial interface rather than a simple step/direction interface that I can patch right into my parallel port. Another big turnoff is the AC servos I see on e-bay also have some connectors which are probably expensive and/or hard to find.
The drives usually have some funny connectors with some weird pinout with all these pin functions that are not clear to me. Some have encoder inputs with a whole lot of pins - are those for absolute/grey-code encoders? Probably I'd just need to dig into the tech manuals for each drive and figure it out - I've read through some, but they seem to assume that you know the basics, ie, they show just a pinout, not a thorough explanation of each signal. What do most of these drives need?
(BTW - I'm patient enough to wait for affordable parts to show up on e-bay, I'm willing to get an o-scope for PID tuning if needed, and given enough information I'm technically savvy enough to eventually figure out if a drive is well suited for a particular motor.)
I don't need insane feed rates, but I don't want to buy a whole lot of stuff and spend the time/effort hooking it up just to be disappointed with an underpowered setup.
I'd rather have servos that go all-out, instead of steppers which have to be conservatively held back so they have a huge safety margin on their non-linear torque curves, vibrate a lot, and potentially lose position.
So in each category (stepper, DC servo, AC servo,) what would be the recommended motor sizes, or power/torque ratings for a 9x42 bridgeport with ballscrews to get decent enough power/speeds to not be considered grossly underpowered?
-Matt