Stepper vs. Servo Question for Turning Center

I'm at the critical tipping point and would like some input and advice on my choices.

I've done a few stepper based builds with EMC, years ago. All is fine there.
This will be my first turning center conversion with LinuxCNC. It is on an Emco 220P if it matters. ( I'm thinking not )
I would like your input and experiences on the decision of whether to use steppers or servos for this conversion, and this is why -

The turning center has the OEM stepper motors in it. They are 5 phase Berger Lahr steppers. I can acquire drivers for them and leave them in place, and use them with the Mesa cards I will purchase.

Orrrrrrrrr...

I also have several AMC 25A20 DC brushed servo drives on hand, and I could just buy two servos to replace the existing steppers and by appropriate Mesa cards for those. This would allow me position feedback which is appealing to me for a few reasons. The first is the obvious benefit of having positional feedback as opposed to the possibility of losing steps and position with steppers. The second is that I have not done a servo build yet and would enjoy the experience and education.

The cost of the stepper drivers vs. the servo motors is negligible. I imagine the same will hold true for the Mesa cards. The relative gains in speed from steppers to servos will also be negligible in this case, as the work envelope is small enough that it won't really have much effect. The Berger Lahr steppers ( RDM 596/50 ) are approximately 110 oz/in and the servo motors I am considering are 400 peak oz/in - 55 continuous oz/in at 60V.

I would like to purchase the Mesa cards and whichever choice from this thread in the next few days so I can get to building. So, which would YOU use, and why? Thank you.

I'll have to buy the encoders, but yes, the costs are about the same. Yes, space isn't a concern.

andypugh wrote:

Look at this another way, What reason is there not to go with the servos?

I don't know, Andy. That's why I am asking you guys.
I just now tripped across these which would work, also - ( last two on the page ) www.kelinginc.net/ServoMotors.html

Some of my hesitance comes from my own ignorance - I don't know what I don't know, so don't know if it will be a PITA or not. I've actually never had to set up servos from scratch. I don't know if it's simple or complex. For instance - which encoder resolution does one want? Many things will come down to my simply being completely ignorant about WHAT to choose.

I've already ripped the old control off. The spindle motor will be out soon, as well. I have room for NEMA 23 size servos. I have five 25AT20 drives. Those will operate from 40 - 190 VDC and 12.5A continuous/25A peak. If I want to make some adapters to cantilever the servos I can put some NEMA 34 size motors in there.

I'm getting the feeling that the servos would be a better choice, functionally... I just don't know what it will take to make it all hum though....

Would this be a 5i20, 7i48, 7i37 type of deal with Mesa cards? Or would something else be better?

Thanks gents.

andypugh wrote:

Well, you need to tune the PID, and you will always wonder if you could have tuned it better.

Ha! You've just described my whole life in one sentence.

andypugh wrote:

If the drives take +/-10V then the 5i25/7i77 combo ($230) looks like a winner.

They do. Thanks, Andy.
< runs off to check Peter's website... >

What about encoder counts and the such?

andypugh wrote:

If the drives take +/-10V then the 5i25/7i77 combo ($230) looks like a winner.

I don't see it mentioned specifically, but since you and John actually have turning centers going, will that combo allow using the spindle's encoder for threading and possible future use of positioning for live tooling?