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Advice on buying servos.
This is probably a bit premature, but I'm thinking of fitting servos to my Fehlmann Picomax 50 CNC table. Originally it had massive Superior Electric steppers rated at 2.1Nm nominal torque.
I like the Teknic ClearPath integrated servos and I think their CPM-SDSK-3421S-RLN will do the trick - https://www.teknic.com/model-info/CPM-SDSK-3421S-RLN/
However, I like a bargain, so I've been wondering about buying used servos and drivers from eBay. The trouble is I know nothing about servos! Can anyone give me some basic guidance on what to look for and what to avoid?
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Re: Advice on buying servos.
Hi,
here i organised some info. If something not clear, please ask:
If buying new servos:
-Look for the servos to come with cables if not integrated
-www.jmc-driver.com the cheapest servos around that i know of
If buying second hand:
-cables and connectors, at least the connectors. If servo cables are needed TME.eu sells the cheapest quality cable for servos and generally speaking. Some times a great deal servo without cables could be found, just calculate what will cost you from TME. Otherwise servo cables are extremely expensive. >8-10euro per meter against ~2-3euro from TME
-Samsung and Panasonic servos are the best you could find for the buck on ebay. Light years away from cheap chinese servo, speaking of possibilities. Not that for simple CNc will make any difference, except that manual is better written.
-There are 2-3 reputable sellers from South Corea and Hong Kong/ skmh218, usedparts-pk, and others /. Avoid Yaskawa servos, some are not accepting the necessary signals so no good for us
-Read manuals and make sure you are buying servo rated for around 3000rpm, that accepts Step Dir signal and even better differential signal. Read manual and make sure if motor is general purpose servo not something special purpose
-Absolute encoder is best, but not a must. Just an added bonus
-Look at pictures carefully and buy stuff that only looks newish or not in bad shape
-Servos need or if not needed is best to be geared down 2-3:1. Why? Because gearing down gives better resolution without artificial microstepping, more power from smaller servos $$$ and at the end of the day you probably dont have a machine that could run with a servo spinning at full speed. My Samsung servos are 3000RPm straight line torque rated but could spin to 6-8k RPM without a problem.
-For your scenario 100w-200W servos geared 2:1 will be OK probably.
-Servos need a good controller as more impulses have to be send to them than normal stepper drive. Do not buy servo until you have made all calculations!. 100khz per axis is minimum, normally 400khz and up, depends on machine, feed desired, etc. My Samsung servos for example dont need so fast controller as they are rated
Example:
My 400w samsung servo could have 2000/2048/2500/10000 P/R etc.(Incremental or Absolute type) encoder. It has the 2048 one. Most of chinese servos when say they have 2048 encoder they mean 2048/4=512 Pulse Per revolution. Not so here. The samsung servo has 2048 Pulse per revolution
So i have geared it 3:2 on a 10mm ball screw. That means when once the ball screw turns 10mm distance is traveled. I wanted my machine to have 20m/min real world working speed. It could easily reach 30 or more as my servos could spin to 6-7k RPM but that's another matter. I calculated all based on 3k rpm.
For 20 000 mm/min the ball screw has to turn 20000/10=2000 times as its geared 3:2 / 30 tooth to 20 tooth HTD 5 15mm pulley / if motor spins 3000rpm then i will achieve on theory that 20 000 mm/min. for 3000rpm to happen the controller has to output 3000x2048=6 146000 pilses per minute/60=102.400pulses per sec hence controller should be at least 100khz per channel.
Do you calculation if you have the absolute controller that is 10000 pulses per revolution, you will need at least 400kz controller/ per axis. Now simply said the thing is that most controllers when say the PPR they mean /4 so that means all is multiplied by 4 , so in reality you need a controller in the Mhz not Khz
so for 1mm my motor will make 1/10 turn of the ball screw. ((3/2)* 2048)/10=307.2 pulses per mm which means my machine has electronic resolution of ~0.003mm ( 1mm/307.2 PPR). Not bad for an all purpose woodworking machine. No microstepping or so. Servo drives also support microstepping and internal gearing.
What i am saying get acquainted well with all that mentioned and you will know more or less what are you doing when buying servos
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Re: Advice on buying servos.
Are these not just closed loop steppers. They spin a about 1000 rpm A bit like the Leadshine closed loop stepper/servo
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Re: Advice on buying servos.
Quote:
Originally Posted by
Boyan Silyavski
Hi,
here i organised some info. If something not clear, please ask:
If buying new servos:
-Look for the servos to come with cables if not integrated
-www.jmc-driver.com the cheapest servos around that i know of
If buying second hand:
-cables and connectors, at least the connectors. If servo cables are needed TME.eu sells the cheapest quality cable for servos and generally speaking. Some times a great deal servo without cables could be found, just calculate what will cost you from TME. Otherwise servo cables are extremely expensive. >8-10euro per meter against ~2-3euro from TME
-Samsung and Panasonic servos are the best you could find for the buck on ebay. Light years away from cheap chinese servo, speaking of possibilities. Not that for simple CNc will make any difference, except that manual is better written.
-There are 2-3 reputable sellers from South Corea and Hong Kong/ skmh218, usedparts-pk, and others /. Avoid Yaskawa servos, some are not accepting the necessary signals so no good for us
-Read manuals and make sure you are buying servo rated for around 3000rpm, that accepts Step Dir signal and even better differential signal. Read manual and make sure if motor is general purpose servo not something special purpose
-Absolute encoder is best, but not a must. Just an added bonus
-Look at pictures carefully and buy stuff that only looks newish or not in bad shape
-Servos need or if not needed is best to be geared down 2-3:1. Why? Because gearing down gives better resolution without artificial microstepping, more power from smaller servos $$$ and at the end of the day you probably dont have a machine that could run with a servo spinning at full speed. My Samsung servos are 3000RPm straight line torque rated but could spin to 6-8k RPM without a problem.
-For your scenario 100w-200W servos geared 2:1 will be OK probably.
-Servos need a good controller as more impulses have to be send to them than normal stepper drive. Do not buy servo until you have made all calculations!. 100khz per axis is minimum, normally 400khz and up, depends on machine, feed desired, etc. My Samsung servos for example dont need so fast controller as they are rated
Example:
My 400w samsung servo could have 2000/2048/2500/10000 P/R etc.(Incremental or Absolute type) encoder. It has the 2048 one. Most of chinese servos when say they have 2048 encoder they mean 2048/4=512 Pulse Per revolution. Not so here. The samsung servo has 2048 Pulse per revolution
So i have geared it 3:2 on a 10mm ball screw. That means when once the ball screw turns 10mm distance is traveled. I wanted my machine to have 20m/min real world working speed. It could easily reach 30 or more as my servos could spin to 6-7k RPM but that's another matter. I calculated all based on 3k rpm.
For 20 000 mm/min the ball screw has to turn 20000/10=2000 times as its geared 3:2 / 30 tooth to 20 tooth HTD 5 15mm pulley / if motor spins 3000rpm then i will achieve on theory that 20 000 mm/min. for 3000rpm to happen the controller has to output 3000x2048=6 146000 pilses per minute/60=102.400pulses per sec hence controller should be at least 100khz per channel.
Do you calculation if you have the absolute controller that is 10000 pulses per revolution, you will need at least 400kz controller/ per axis. Now simply said the thing is that most controllers when say the PPR they mean /4 so that means all is multiplied by 4 , so in reality you need a controller in the Mhz not Khz
so for 1mm my motor will make 1/10 turn of the ball screw. ((3/2)* 2048)/10=307.2 pulses per mm which means my machine has electronic resolution of ~0.003mm ( 1mm/307.2 PPR). Not bad for an all purpose woodworking machine. No microstepping or so. Servo drives also support microstepping and internal gearing.
What i am saying get acquainted well with all that mentioned and you will know more or less what are you doing when buying servos
Many thanks for such a comprehensive and detailed reply. It gives me a really good starting point.
Quote:
Originally Posted by
Clive S
Are these not just closed loop steppers. They spin a about 1000 rpm A bit like the Leadshine closed loop stepper/servo
According to Teknic they're a true servo. They certainly sound like a servo when running. Teknic go to some length to make the distinction between ClearPath and stepper motors.
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Re: Advice on buying servos.
Apart from the obvious, what's the difference between AC and DC servos?
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Re: Advice on buying servos.
I see a few Panasonic and Samsung servos on eBay. However, I notice that apart from the part numbers sellers don't list any specs. Is there somewhere to check specs? Both the Panasonic and Samsung sites seem to be impenetrable.
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1 Attachment(s)
Re: Advice on buying servos.
The servos come on certain series. So you have to find the manual for the series.
Attachment 22165
https://www.google.es/search?q=Samsu...+drive++manual
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Re: Advice on buying servos.
Quote:
Originally Posted by
Agathon
Apart from the obvious, what's the difference between AC and DC servos?
The obvious. The AC servo when 230VAC does not need additional transformer. This is best. Then cheaper servos could be 80VAC then you need to feed them 80VAC. But of course depending on price availability sometimes is worth making that PSU.
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Re: Advice on buying servos.
Quote:
Originally Posted by
Boyan Silyavski
The obvious. The AC servo when 230VAC does not need additional transformer. This is best. Then cheaper servos could be 80VAC then you need to feed them 80VAC. But of course depending on price availability sometimes is worth making that PSU.
So it's not the characteristic of the motor itself but just the driver? So are all servos dc motors or are some ac?
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Re: Advice on buying servos.
Quote:
Originally Posted by
Agathon
So it's not the characteristic of the motor itself but just the driver? So are all servos dc motors or are some ac?
Ahh, the AC does not have brushes. The DC servos with brushes are older motors with older drives that may need Voltage control +-10v, not pulse control. That could make things bad, as this type of controllers are expensive cause their only purpose is to retrofit old machines. Hence thats why i told you to search for drives that support Pulse and Direction
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Re: Advice on buying servos.
http://www.ebay.es/itm/CSDJ-02BX2-CS...cAAOSw0vBUl2gB
Thats what i have / the 400w/ and where i bought it from. And what i would recommend. Just check again for the details in the manual. You will not find anything better for the money.
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Re: Advice on buying servos.
Quote:
Originally Posted by
Boyan Silyavski
Ahh, the AC does not have brushes. The DC servos with brushes are older motors with older drives that may need Voltage control +-10v, not pulse control. That could make things bad, as this type of controllers are expensive cause their only purpose is to retrofit old machines. Hence thats why i told you to search for drives that support Pulse and Direction
Thanks, sorry for being so dim about this - it's begging to make sense. Also many thanks for the links.
BTW. Apart from power, is there any difference between servo motors used for positioning ie x,y,z axis motors and servos used for spindles?
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Re: Advice on buying servos.
Quote:
Originally Posted by
Agathon
Thanks, sorry for being so dim about this - it's begging to make sense. Also many thanks for the links.
BTW. Apart from power, is there any difference between servo motors used for positioning ie x,y,z axis motors and servos used for spindles?
Yes and No. Any servo motor rightly belted could serve for spindle. But then comes the RPM. The servos that are meant for spindles could spin at higher RPM and have slightly different curve. We are talking about typical motor that will be connected to BT30, etc spindle via belt. As Yes, you could gear up any servo motor but then you need bigger more expensive motor.
these are the servo spindle motors. they gto up to 15k rpm https://www.aliexpress.com/store/pro...809932059.html
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Re: Advice on buying servos.
Quote:
Originally Posted by
Boyan Silyavski
Yes and No. Any servo motor rightly belted could serve for spindle. But then comes the RPM. The servos that are meant for spindles could spin at higher RPM and have slightly different curve. We are talking about typical motor that will be connected to BT30, etc spindle via belt. As Yes, you could gear up any servo motor but then you need bigger more expensive motor.
these are the servo spindle motors. they gto up to 15k rpm
https://www.aliexpress.com/store/pro...809932059.html
Thanks again. Not cheap then.
While I'm here. Am I right in thinking that the advantage of an absolute encoder over incremental is that the motor position is known after power-down?
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Re: Advice on buying servos.
Yes, but there are different types of absolute. you have single turn and multi-turn, the name is self-explanatory.
Also, Absolute encoders have a much higher resolution than incremental.
Quote:
Originally Posted by
Agathon
Thanks again. Not cheap then.
While I'm here. Am I right in thinking that the advantage of an absolute encoder over incremental is that the motor position is known after power-down?
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Re: Advice on buying servos.
Quote:
Originally Posted by
Gary
Yes, but there are different types of absolute. you have single turn and multi-turn, the name is self-explanatory.
Also, Absolute encoders have a much higher resolution than incremental.
Thanks. I notice that you have both DC brushless and AC servos on your website - what are the advantages and disadvantages?
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Re: Advice on buying servos.
Quote:
Originally Posted by
Agathon
Thanks again. Not cheap then.
Servo controller for me is a must for a metal working machine, a heavy gantry router or a high precision machine for miniature machining, industrial production machine that needs to be fast as possible For all else a stepper will do just fine job.
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Re: Advice on buying servos.
Quote:
Originally Posted by
Boyan Silyavski
Servo controller for me is a must for a metal working machine, a heavy gantry router or a high precision machine for miniature machining, industrial production machine that needs to be fast as possible For all else a stepper will do just fine job.
What I really meant was that servos for spindles weren't cheap. I'm a great believer in "getting what you pay for". I can live without a servo spindle, but I think I need to invest in servos for the x and y axes of my mill.
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Re: Advice on buying servos.
Quote:
Originally Posted by
Agathon
Thanks. I notice that you have both DC brushless and AC servos on your website - what are the advantages and disadvantages?
One take DC and the other takes AC, so the AC servo driver has a power supply built in that is rated to the nominal and peak rated current of the driver.
Also Just because it's a DC servo does not mean its brushed, a DC servo can be brushed or brushless.
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Re: Advice on buying servos.
Quote:
Originally Posted by
Gary
One take DC and the other takes AC, so the AC servo driver has a power supply built in that is rated to the nominal and peak rated current of the driver.
Also Just because it's a DC servo does not mean its brushed, a DC servo can be brushed or brushless.
OK, that's a very concise reply, but why buy a DC over and AC and vice versa?
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Re: Advice on buying servos.
There are a few reasons, but this one normally gives you an indication.
Ok, lets say you have a machine with 8 axis and each axis needs a servo, that is not too large.
you can get 8 AC servo motors and driver and you will be paying more because each driver has its own power supply.
with the DC option, you will have 8 DC servo drivers and a large power supply.
Also DC servo motors are typically low voltage DC, so the current is higher, so typically the motors dont go too big (under a KW)
Personally, i would not even consider a DC servo system, unless you know why you need it. AN ac servo system is typically a better option.
Quote:
Originally Posted by
Agathon
OK, that's a very concise reply, but why buy a DC over and AC and vice versa?
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Re: Advice on buying servos.
Quote:
Originally Posted by
Gary
There are a few reasons, but this one normally gives you an indication.
Ok, lets say you have a machine with 8 axis and each axis needs a servo, that is not too large.
you can get 8 AC servo motors and driver and you will be paying more because each driver has its own power supply.
with the DC option, you will have 8 DC servo drivers and a large power supply.
Also DC servo motors are typically low voltage DC, so the current is higher, so typically the motors dont go too big (under a KW)
Personally, i would not even consider a DC servo system, unless you know why you need it. AN ac servo system is typically a better option.
Right, so AC servo drivers aren't typically 240vac SP input then and need a step-down transformer?
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Re: Advice on buying servos.
No, AC is AC, regardless of the voltage.
Typically single phase goes up to about 1.5KW
For a hobby machine, its unlikely you will need anything that big.
Even a mill like Bridgeport MDI /Interact can use 750W easily.
Quote:
Originally Posted by
Agathon
Right, so AC servo drivers aren't typically 240vac SP input then and need a step-down transformer?
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Re: Advice on buying servos.
Quote:
Originally Posted by
Gary
No, AC is AC, regardless of the voltage.
Typically single phase goes up to about 1.5KW
For a hobby machine, its unlikely you will need anything that big.
Even a mill like Bridgeport MDI /Interact can use 750W easily.
OK, so just to be clear. A 400W AC servo motor would typically be coupled to a driver powered by AC at a voltage less than 240v? So is the power supply simply a step-down transformer or is there more to it?
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Re: Advice on buying servos.
it totally depends on what the bus voltage of the motor is.
To make it easy, if and when you buy them get a matched pair with cables, or talk directly to the company and get advise on what they suggest for the application.
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Re: Advice on buying servos.
Quote:
Originally Posted by
Gary
it totally depends on what the bus voltage of the motor is.
To make it easy, if and when you buy them get a matched pair with cables, or talk directly to the company and get advise on what they suggest for the application.
Thanks - I'll email you in the next few days.
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Re: Advice on buying servos.
Quote:
Originally Posted by
Gary
Personally, i would not even consider a DC servo system, unless you know why you need it. AN ac servo system is typically a better option.
Me too. And honestly valuing my time i will not consider separate PSU but a pure plug and play system.
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Re: Advice on buying servos.
Quote:
OK, so just to be clear. A 400W AC servo motor would typically be coupled to a driver powered by AC at a voltage less than 240v?
No, most of the currently available 400w AC servos are 220-240V
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Re: Advice on buying servos.
Quote:
Originally Posted by
Ger21
No, most of the currently available 400w AC servos are 220-240V
Nice.
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Re: Advice on buying servos.
What do you think of this: http://www.ebay.co.uk/itm/Samsung-RS...-/282340382590
Might it be suitable for my app or am I barking up the wrong tree?
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Re: Advice on buying servos.
Is it important that the servo driver and motor are from the same manufacturer? Or is just that the driver and motor must match electrically speaking?
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Re: Advice on buying servos.
The most important is that the encoder connections on motor match what the drive requires.
Some encoders/drives use 12 or more wires, some use as few as 5 or 6.
Imo, unless you really know what you're doing, it's best to but a matched set from the same manufacturer.
Not sure if they are available over there, but here inthe US, and Canada, DMM are the best bang for the buck.
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Re: Advice on buying servos.
Quote:
Originally Posted by
Ger21
The most important is that the encoder connections on motor match what the drive requires.
Some encoders/drives use 12 or more wires, some use as few as 5 or 6.
Imo, unless you really know what you're doing, it's best to but a matched set from the same manufacturer.
Not sure if they are available over there, but here inthe US, and Canada, DMM are the best bang for the buck.
Many thanks Gerry. Those DMM servos are very good VFM.
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Re: Advice on buying servos.
The Samsung servos need a Samsung drive. Or better said, you could not hook other servo motor to a brand drive. As it auto detects motors and will give " Faulty" all of the time. Samsung motors have 9 wires to encoder, some 11. The one on link is with 9 wires.
The only way you could make a similar motor work is Universal servo drive like Granite Devices.
And yes, that motors are good price and good ones, but you can check the Samsung PDF manual link i send you earlier and see that for yourself. But remember what i told you about the servo cables. Dont make decisions without knowing what will cost you when you include them. The good thing is these motors are in Europe. You could buy the drives for around 130 each, used.
DMM are out of question in Europe. Going that way is better to buy Chinese drives or the second hand Samsung. I have pondered over that many times last years and every time money and price prevails.
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Re: Advice on buying servos.
Thanks for the reply.
I've taken a punt on these: http://www.ebay.co.uk/itm/322584699618
I spoke to the young lad whose selling them and it seems his grandfather worked in the motion control field. They may not work of course, but not much lost if they are US.
Will keep an eye out for motors to match.
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Re: Advice on buying servos.
I think you misunderstood what i said. Drive and motor combo from different manufacturers will NOT work!!!
You need a drive that lets you program the motor parameters, not a drive that lets you choose from certain motors!!! Check that in manual.
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Re: Advice on buying servos.
Quote:
Originally Posted by
Boyan Silyavski
I think you misunderstood what i said. Drive and motor combo from different manufacturers will NOT work!!!
You need a drive that lets you program the motor parameters, not a drive that lets you choose from certain motors!!! Check that in manual.
Thanks for the clarification. I'll be looking for used Omron or Yaskawa motors for the drives I've just bought (it appears that they had a joint venture).
It does seem that the issue of whether different manufacturers motors can be driven from different drives is not straightforward: http://www.cnczone.com/forums/servo-...vo-driver.html
However, I take your point. At £90 for two drives I'm willing to have a play and see what happens.
You mentioned cables in a previous post. Is it impossible to make up cables given the correct plugs and sockets?
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Re: Advice on buying servos.
According to the specs this is the correct motor for the drivers I just bought - http://www.ebay.co.uk/itm/OMRON-R88M...-/182634177958
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Re: Advice on buying servos.
Just out of interest, via a socket screw axial to the ballscrew, I used a precision torque screw-driver (usually used for very carefully setting the torque on small screws) to see how much torque was required to move the table. As might be expected the initial torque required to overcome the stiction was higher that what was required to keep it moving. However the maximum torque required was surprisingly low - 0.5 Nm. I might just pop a 16mm slot-drill in a collet and see how much torque is required to make it cut steel...
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Re: Advice on buying servos.
Having thought about this for a few mins, I am now a little unsure that a 200W servo will be powerful enough for my table. The Omron R88M 200w motors produce 0.637 Nm continuous torque with a peak of 2.2 Nm.
I think I noted in one of my first posts on this forum about stepper replacement that Fehlmann now fit Omron 400W servos to the P21 which is a similar machine to mine (but with a slightly larger table) - I'd forgotten about this until doing some searches earlier on.
I suppose that if I do as you suggest Boyan and fit the servos with a 2:1 reduction a 200W Omron servo might be adequate. Being more optimistic, a reduction of 3:1 would produce the best part of 2Nm continuous torque - which will probably do the job.