Replacing Servo motors

[m_c]

Without going into too much detail, what would you replace some 3.4Nm (continuous rating) 138V DC servos with?

Budget won't allow for new AC servo replacments, and I'm not sure how much of a sacrifice steppers would be.
I'm wondering what the frugal members of this forum would suggest.

[Jonathan]

Hard to say without knowing what machine they're going on, the mass of the axes they are driving, the pitch and diameter of the ball-screws and what speed and acceleration you got with the DC servo?

[m_c]

Well the complete machine weighs about 1500kg and has been stripped for parts, so until I physically get it, that's all I know. I'm just wanting to know possible options for now without getting into too much detail.

[i2i]

I have a couple of 1200 kg bedmills that have triple stack nema 34 motors running on 85v power supplies. The tables are 42" x 9" bridgeport, and will happily run at 3m/min on rapids,

[m_c]

That sounds reasonable. Any idea what Current rating the steppers are?

This machine is a couple hundred kilos heavier than a bridgeport, and the table will most likely be a bit heavier. I think the servos are all connected via belts, so gearing shouldn't be too hard to alter. I'm still trying to aim for some reasonable speed though, as the X travel is just short of 2 feet, and I'd quite like closed loop.

I've been pricing up a few options, but I need to decide on what controller to use before making any decisions.
Plus there's the issue of geting the thing home and into the workshop!

[i2i]

They're running at half step on a 2.5:1 reduction on 5mm pitch ballscrews. Current is 3amp.

[Jonathan]

If you can afford servo's from DDM they would be excellent, since you'll get high rapid feedrates and much higher resolution:

http://www.dmm-tech.com/Files/DHT_Servo_Motor.pdf

Their 60EM-DHT-36, 400W servos would be plenty, or even the 300W servos. Send them an email and they will calculate which is best suited to your machine if you know the mass each axis is moving. I've not bought from them yet, so can't comment on the quality first hand, but from reviews I've seen on CNCzone they seem to be excellent...but it will cost more than a Nema 34 stepper motor system.

Or there's some on eBay, but you said you can't afford servo's so...

If using Nema 34 stepper motors I would get mains voltage drivers since this will greatly increase the torque at high speed. For example:
Zapp Automation Ltd - Stepper motors & Stepper Motor Drivers - Servo motors and Servo Motor Drivers - Ballscrews & linear motion products. : 2M2280N (High Voltage Stepper Driver) [2M2280N] - £176.40
Although they've gone up in price since I last looked, so maybe:
CNC Cut Nema 34/42 stepper motor driver DQ2722M,7A 110-220VAC www.wantmotor.com | eBay
or
2MA2278 CNC Stepper Driver For Nema34,Nema42,Nema51 Stepper Motors 7.8A 110/220V | eBay

If I actually had something useful to do with my micro lathe I'd get one of those drivers for the spindle.

[Gary]

So the machine had 1Kw servo motors on and a 300W servo motor will give you plenty enough power? Without getting more info on the machine it would be wrong to say that....
Even i am assuming that the motors are 1Kw, so i am unable to suggest the right motors without making an assumption.

If the motors are rated at 3000 RPM, they are 1Kw motors, that would give you about 3Kw of peak power. that is over three times what you suggested.
Going to steppers would be a big step backwards, even running them at 230V. While its a big step back, it will still work, but dont even think it will be close to what it was with the servo motors on.
Oh and the price of the 2M2280N has not gone up in price in 3 years. VAT is shown in the price at the moment, so an easy mistake to make.

[Jonathan]

So the machine had 1Kw servo motors on and a 300W servo motor will give you plenty enough power? Without getting more info on the machine it would be wrong to say that....

Fair enough, that's why I suggested contacting the supplier as my estimate may well be optimistic and yes I didn't spot the VAT which was pretty bad of me...

[m_c]

From the figures I've found, a similar mill using an older posidata drive used 3/4hp motors combined with a 1.1hp drive.
The motors that are currently on the mill, come in around 1kw from what figures I've found (they're SEM) but the owner wants them as spares for the bridgeport interacts he's got.

I know steppers will mean a hit on performance, but it's how big a hit I'm trying to quantify.
The DMM 750W servos should come in under £500 each before shipping (depending on how customs class them), comparable in price to the HBS86 closed loop steppers. Performance wise I doubt there will be that much practical difference. Steppers will produce less torque at speed, but this mill won't be cutting at speed, so low speed torque is more important.

I really want something closed loop, and all the options start around £500.
Options I've priced are Dynomotion K-flop + Kanalog, combined with Nema 34 steppers and US digitial encoders, and high voltage drivers which is around £1700 all in.
HBS86 closed loop are around 500 each plus power supply.

Thanks for the suggestions guys.