Replacement servos, not sure what I need..

[yukonho]

I have been charged with the task of re-fitting a CNC lathe. It was previously CNC, andI will need to replace all of the drive electronics except for the power supply.
It is one sweet little lathe, incredibly well built.
It is equipped with ball screws, I haven't measured the pitch yet, but I am guessing 5 TPI. It is stiff to turn the ballscrews by hand, but I am certain it is just the preload. It is a tight little machine. It will never be heavily used, mainly just jog the carriage and tailstock. I may have to make a third axis for moving a forming tool into place.
I have measured the existing mounting holes, shaft length and diameter etc.
I am looking for help on the following..
What torque motors should I be looking for?
I know I can use a very lightweight motor for the third axis, can I mix servos on one driver board?
Which driver boards are the best choices these days? (money is not a big concern on this, quality is)
I will need inputs for RPM and for 5 limit/home switches if possible.
Where can I get good shaft couplers? The ballscrews are 12mm diameter.

The holes on the mounting plates are 62mm across (corner to corner) and the servos will have to be 7.5" long or less (body, not including shaft)

Thanks for any help, I have a fair amount of experience with steppers, but this is my first application for servos.

[Al_The_Man]

Your torque requirement will depend of course on the load and what kind of acceleration you need, It might be an idea to use a sizing software of which there are several free ones http://www.electromate.com/technical...morgansoftware
The common industry standard is to aim for a motor size that keeps the inertia ratio of motor to load of less than 10:1. This is subject to load and acceleration.
There are zero backlash couplers out there, one of my favorites is one made by Lovejoy/Rosta 'U' series, which resembles two coil springs contra wound, one inside the other.
What type of control system did it have before can you reuse the motors and add new amplifiers?
Al

[Al_The_Man]

BTW if you want to bone up on motor sizing there is a couple of usefull sites.
http://www.motionvillage.com/welcome...es/Inertia.PDF
and http://www.imshome.com/Product%20Man...ox_inertia.pdf
as shown on the sites, the torque required to accelerate is reduced by the square of the reduction ratio, so a simple 5:1 reduction reduces the torque required by 25.
BTW this practice should be used when sizing steppers and servos.

Load Inertia-to-Rotor Inertia Ratio
For a high performance, relatively fast system, load inertia reflected to the motor should generally not exceed the motor inertia by more than 10 times. Load inertias in excess of 10 times the rotor inertia can cause unstable system behavior and inefficient power usage.
Al

[yukonho]

Hi Al, thanks for that.
This lathe will lead an easy, pampered life. It will never cut anything, it will be used for glass blowing. The axes will simply be jogged into position and stay there while the glass is blown, then they will be moved out of the way and so forth.
All of the control hardware has been removed, so I need to essentially start from scratch on the electronics. There are no servos, no nothing!
I will cruise the online shops for servos and drivers.
Is there a benchmark for torque I should be looking for? I am guessing 300oz/in would be a good starting point.
Off I go to plug numbers into the software, I will post my results in a bit.
Cheers

[yukonho]

I guess I may be ignorant, but the software gave me nothing useful. I played with it for half an hour and came up with nothing.
I may have to just guess and hope for the best.