Servo Drive for Lathe Spindle

[barnbwt]

That Baldor is a beauty and, as usual, expensive as hell. One thing about this motor is that it is actually a permanent magnet synchronous motor, a cousin to the brushless DC motor but with sinusoidal rather than trapezoidal BEMF. You can't do much better than that motor, it is pretty much state of the art. Looking forward to seeing it in action, if you decide to go that route.

Well, I've 'gone that route' since those two units are on the way here, for 390$ used (not bad, assuming I find a decent cabling solution)

So, as my last question for those with experience, if I'm running my spindle servo via step/dir like my axis stepper motors, and I'm using the servo's encoder feedback to ensure the spindle is following steps, do I have the control/coordination needed to do threads/tapping from an off-the-shelf unit (let's say Smooth Stepper for argument), or is additional feedback to the actual motion controller necessary? From what I understand it absolutely is in the case of the speed control "spindle" output these cards have (the +/- 10VDC analog signal*) to bring additional encoder feedback to the motion controller so it can make double-sure the feed axis is keeping up with the rotary, but what about the same maneuver between two proper stepper drives, assuming no missed steps? If so, then I need to make sure to use a controller that can parse the simulated encoder output of the servo driver.

TCB

*That output may ultimately control the spindle of a live tool, I think

 

[cmantunes]

Well, I've 'gone that route' since those two units are on the way here, for 390$ used (not bad, assuming I find a decent cabling solution)

Awesome! Did you buy this on eBay?

if I'm running my spindle servo via step/dir like my axis stepper motors [...] do I have the control/coordination needed to do threads/tapping

For threads/tapping, usually you run your servo in speed, rather than position, mode. The CNC controller gets the pulses either from the encoder directly or from a "synthesized" encoder provided by the servo drive. The CNC controller then synchronizes the Z-axis movement to the angular position of the spindle. It is only when you want to do milling on the lathe that you switch the servo to position mode and the spindle becomes the C-axis. Then you control the X, Z and C axes to do a (limited) amount of milling on the lathe with live tooling. This is the way a "real" CNC turning controller works. I don't know enough about the SmoothStepper to know if it supports this kind of operation. Here's an example of what I'm talking about:

[/QUOTE]

 

[barnbwt]

I've not even gotten to the programming side; is that control method switch just a G-command, and can be changed mid-tape? I'd figured it was dependent upon how you wired things, and it was either/or. I wonder if the big machines have an entirely parallel driver that a controller selects depending on local parameters like commanded spindle speed.

TCB

 

[cmantunes]

is that control method switch just a G-command, and can be changed mid-tape?

A Fanuc controller, for example, has a pair of M-codes to switch-on/off C-axis mode and it can certainly be done mid-tape. This usually controls a digital I/O to tell the servo drive whether to do speed or position control.

 

[barnbwt]

M code was actually my second guess, lol (see, I am learning )

TCB

 

[JimDawson]

Basically you would run the C axis at some RPM by a M3 Sxxxx, then slave the Z axis to the C axis at some ratio that would move the Z axis the proper amount per revolution to generate threads. This would be commanded by the G-code with a G76, but all of the real programming to make this happen is internal in the motion controller.

 

[barnbwt]

Okay, if you can switch between drive modes on the fly like shifting gears, then things make a bit more sense. So for threading, the servo drive will be looking for an analog spindle velocity/torque signal on the 10V input line from the motion controller (both with encoder feedback), which will direct a simultaneous step/dir signal output from the controller to the stepper driver running the relevant axis at the proper ratio. For milling or engraving, the servo drive will make the desired finite spindle moves based on step/dir inputs off the motion board (both with encoder feedback) independent of the other axes.

That does seem more efficient from a data standpoint, than "go-go-go-go-go-go-go-go-go-go-go-go-go-go..." for the entire spindle operation . Is velocity control generally 'smoother' than positional control for a servo, and more like a conventional VFD variable speed control (only precise)? Or perhaps the electronics in the driver can 'see ahead' to the incoming code and interpolate between steps on the fly to smooth them out microstep-style so there isn't much functional difference, apart from the bandwidth usage? I suppose it doesn't matter a huge amount, if both modes are accessible, since I can simply choose whichever yields the best result.

I guess my thought was for something like single-cut threading with a number of passes, the step based approach made more sense due to the number of spindle velocity changes involved (and also precise positioning). I guess what you're saying is the accuracy of either servo control method is comparable due to the encoder feedback, and whether slaved or independent, the two axes involved in the thread will be synched by the same motion control board that compensates for their latency (with at least the spindle running closed-loop on its encoder). Do you think there's much to be gained by putting an encoder on the Z-axis early on? I planned to do this later, as a means to incrementally improve upon hopefully already-acceptable performance.

TCB

 

[Uguessedit]

Anyone having luck using a servo-driven C-axis spindle to turn *and* mill stuff?

I am trying to determine my best options for an upcoming CNC lathe project. As is always the case, the goal is a machine that 'does eveything' but compromises for the resources available. Threading at the very least is a necessity, since this project needs to deliver more functionality than my manual lathe to be worthwhile.

The requirements thus far:
- 110VAC/15A-1ph wall power source
- 1.0-1.5hp spindle power (so I still have enough reserve juice for the axis steppers & controller)
- 4000 spindle speed, belted to motor (fast enough for proper carbide sfm on 1"-2" aluminum parts)
- Articulating spindle (single point threading, live-tool engraving, live-tool light milling/slotting/broaching, possibly rigid tapping if motor is strong enough)
-widget-sized parts in small quantities (so no need for very long duty cycles or aggressive cutting)

Points 3 and 4 seem to suggest a servo, so I am following that rabbit hole at this time.

The questions thus far:
-If using the servo as a controllable C-axis for engraving or light milling (1/8" shank or smaller), will the servo's resistance & mass be sufficient to control tool chatter & cutting forces? These are 4N-m range.
-Is it more efficient to run a high-speed stepped-up-high-voltage servo geared down to the desired RPM, or gearing up a low-speed high-torque servo that operates at near line voltage?
-Most drivers run on Asian/industrial 220-240VAC sources; are there any downsides to using a driver that operates closer to my line voltage?
-Is there any advantage to high vs. low voltage AC brushless servos, or is it really just a function of which controller you want to use?
-Any estimate of losses for transformer or switching voltage booster systems would be helpful to determine my final current draw.
-At what region of the operation envelope do servos+controllers draw the most source current? Is it whenever they are applying max torque, or rather when they are running at high speeds under maximum voltage?
-Could I feasibly use a larger motor and limit its current through the driver software, and would I be giving up torque or speed (or both)?

Anyone having luck using a servo-driven C-axis spindle to turn *and* mill stuff?

Currently I am using a vfd and 2hp 3700rpm Motor and can get more rpm than needed. Lathe is a 13x37. I can cut half mm with minimal bogging I can hear the motor kick in and push back. Probably would’ve gone 3hp if I had to do again. I’ve seen other guys add an encoder and use a 3hp setup and essentially have a c axis. What I’ve noticed is when I drop the rpm’s down 25-75 I have a difficult time rigid tapping and have to use a floating holder and higher speeds because the drive doesn’t keep the torque up enough. That is if I leave it geared 1:1 so I can however manually change gear levers and compensate allowing for higher motor speeds/torque and lower rpm and then rigid tap. This presents another issue with a c axis in that the gear box has too much backlash. I understand from your drawings the intention is to build without a gear box and why I mention as you’ve stated you don’t have 220v. That’s an issue. A good ac servo is going to need 220v 15a as others suggested. I’ve got a large 30 amp Servo that would make an excellent spindle at 2.6kw however in my case I’m looking to have a smaller 750w ac servo 1:1 gear driven directly off the back of the spindle using two 75t modular 1 gears to keep backlash at a minimum and use this only when I have c axis work I want to perform. Engaging and disengaging the servo is the challenge here so I can switch back and forth. The idea is something I may engage electronically yet I have some exploring myself with the endeavor. It may be something you could consider where power source is limited. Unfortunately 220/240 is a preffered power source. Most homes these days are well equipped with single phase 220/240 at the breaker panel unless it’s oldwr than 1970 and hasn’t been updated or you’re in some type of rental complex. I’d be curious to hear if anyone has explored using a separate c axis servo and a engagement method so not to back feed the amplifier by running the standard spindle motor. Interesting topic.