Author Topic: CNC servo motor sizing  (Read 3013 times)

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Offline xzswq21Topic starter

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CNC servo motor sizing
« on: October 26, 2021, 07:24:10 pm »
Hello
I want to buy a F1210 milling machine from WABECO.
https://www.wabeco-remscheid.de/cnc%20milling%20machine%20cc-f1210%20basic%20-1889.html
the specification of the original stepper motor is as following:

NEMA 34
Bipolar current: 6 A
Bipolar holding torque: 3 N.m
Rotor inertia: 1450 kg.m2.10e-7
Theoretical acceleration: 20600 rad/sec2


Ball screw pitch is 5mm
Stepper angle is 1.8 Degree
travel speed X-axis and Y-axis: 30 - 600 mm/min
travel speed Z-axis: 30 - 400 mm/min

But I want to use ClearPath SDSK servo motors:
https://teknic.com/products/clearpath-brushless-dc-servo-motors/all-clearpath-models/

which one is suitable for the Machine according to the above specification?

Thanks
« Last Edit: November 01, 2021, 07:43:14 pm by xzswq21 »
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Offline Juha

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Re: CNC servo motor sizing
« Reply #1 on: October 26, 2021, 08:03:03 pm »
Typically you would size your motor based on mechanics. So based on the load and the speed/acceleration you want you select a motor that can achieve it. Then you would also try to match inertia etc.

Assuming the mfgt has chosen the stepper motor based on actual calculations (I doubt) then if example speed doesnt matter just pick something with the same torque.
 
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Offline Teknic_Servo

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Re: CNC servo motor sizing
« Reply #2 on: October 28, 2021, 02:40:54 pm »
Hi xzswq21,

I'm an applications engineer with Teknic.

It looks like another forum member has already touched on this point but, in general, there are two ways to properly size a servo motor:

1. Size the motor based off of an existing motor/motor specifications (such as matching the motor based on its torque and speed output) 2. Doing an in-depth machine analysis where you would calculate moving masses and motion requirements to then come up with inertia, torque, and speed numbers.

In either case, please feel free to reach out to us directly for support.  You can give us a call at 585-784-7454 or send in a contact request here: https://www.teknic.com/contact/

Best regards,

Mark D. - Teknic Servo Systems Engineer

 
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Offline xzswq21Topic starter

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Re: CNC servo motor sizing
« Reply #3 on: October 28, 2021, 05:53:32 pm »
Can I connect Three or four SDSK-3421S motors at the output of Teknik PowerHub?
Do you have any details about the PowerHub? How much power can the PowerHub deliver to the outputs?
https://teknic.com/POWER4-HUB/
« Last Edit: October 28, 2021, 05:56:23 pm by xzswq21 »
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Offline thm_w

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Re: CNC servo motor sizing
« Reply #4 on: October 28, 2021, 10:01:35 pm »
Can I connect Three or four SDSK-3421S motors at the output of Teknik PowerHub?
Do you have any details about the PowerHub? How much power can the PowerHub deliver to the outputs?
https://teknic.com/POWER4-HUB/

Its right in the datasheet: https://www.teknic.com/files/downloads/Clearpath-SC%20User%20Manual.pdf

Quote
USING THE POWER4-HUB
The Power Hub (Teknic PN POWER4-HUB) is an optional ClearPath accessory board that:
1.  Distributes DC bus power to as many as four ClearPath motors per Power Hub, thus eliminating the need for special "power chaining" cables.
2.  Delivers low voltage logic power backup to your ClearPath motors. Logic power backup keeps the motors' electronics "alive" even if DC bus power is dropped. This translates to uninterrupted communication, continuous status monitoring, and retention of encoder position.

Note: You will be unable to spin your motors when using only logic power backup. Logic power backup will keep the motor electronics alive, but is not designed to run motors.

I have some older Teknic stuff and its great.
Profile -> Modify profile -> Look and Layout ->  Don't show users' signatures
 
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Offline xzswq21Topic starter

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Re: CNC servo motor sizing
« Reply #5 on: October 30, 2021, 06:17:36 pm »
which motor is better for the WABECO F1210 with ballscrew? SDSK-3421S or SDSK-3411S?





Teknik IPC-5 power supply with cooling fan is only 500W, so how the power supply can deliver the below power to three/four motors?
Continuous power at 48V:


Continuous power at 75V:




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Offline jmelson

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Re: CNC servo motor sizing
« Reply #6 on: October 31, 2021, 08:28:12 pm »
Holding torque of steppers is an illusion.  So above very slow speed, they will not provide the same as holding torque.  Servo motors can provide the rated torque at any speed.  Also, servos can provide, typically, 4X rated stall torque for short intervals such as one second to deal with accleration.  So, a servo with about 1/2 to 1/4 the continuous torque rating can typically outperform a stepper with that holding torque rating.

BUT, these Teknik "servos" are just closed-loop steppers, so they don't provide that advantage.  This can be clearly seen in your graphs, where the torque falls off linearly with speed.  The torque at 500 RPM is half of the zero-speed torque.  That is pretty awful.  The problem with steppers is they have too many poles, requiring the winding current to be reversed too fast.  Thus, the inductance limits the rate at which current can be developed in the windings.  A typical stepper has 50 poles,  A typical brushless servo has 4 or 8 poles.  So, making some assumptions, the torque could be maintained to about 12 times higher speed.

Jon
 
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Offline xzswq21Topic starter

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Re: CNC servo motor sizing
« Reply #7 on: October 31, 2021, 08:46:14 pm »
Holding torque of steppers is an illusion.  So above very slow speed, they will not provide the same as holding torque.  Servo motors can provide the rated torque at any speed.  Also, servos can provide, typically, 4X rated stall torque for short intervals such as one second to deal with accleration.  So, a servo with about 1/2 to 1/4 the continuous torque rating can typically outperform a stepper with that holding torque rating.

BUT, these Teknik "servos" are just closed-loop steppers, so they don't provide that advantage.  This can be clearly seen in your graphs, where the torque falls off linearly with speed.  The torque at 500 RPM is half of the zero-speed torque.  That is pretty awful.  The problem with steppers is they have too many poles, requiring the winding current to be reversed too fast.  Thus, the inductance limits the rate at which current can be developed in the windings.  A typical stepper has 50 poles,  A typical brushless servo has 4 or 8 poles.  So, making some assumptions, the torque could be maintained to about 12 times higher speed.

Jon

Stepper Motor vs. ClearPath Servo Motor Demonstration Video:


The price of SDSK-3421S is $328 and the price of SDSK-3421S is $349

according to the F1210 milling machine, which servo motor in this world do you recommend me?

All Clearpath servo motors:
https://teknic.com/products/clearpath-brushless-dc-servo-motors/all-clearpath-models/
« Last Edit: October 31, 2021, 08:51:30 pm by xzswq21 »
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Offline xzswq21Topic starter

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Re: CNC servo motor sizing
« Reply #8 on: October 31, 2021, 08:59:02 pm »
One of Teknik application engineer told this:

Quote
For clarification, ClearPath is not a stepper motor with an encoder. The motor core within ClearPath is an 8-pole, brushless, sinewave-commutated servo motor.  (See more below)
The reason I’m making this distinction is because although you can put an encoder on a stepper motor and run it as a servo, the construction of a stepper motor (which was originally designed specifically to run open-loop), is not very good for servo control. The very high pole count (typically 100), which is essential to running the stepper open-loop, creates a very high electrical inductance, and reluctance torque. Both of these factors are counter-productive for a servo-controlled motor (reduced power at speed, high ripple torque, more servo “hunting”, etc.).
As an aside, you can see a cutaway of the ClearPath motor core construction here: https://www.teknic.com/products/hudson-motors/

Please contact Teknic if you have any questions.

Best regards,
Brendan - Teknic Servo Systems Engineer

for the Video on Youtube:
Quote
Stepper System Components:
•        Macron Mini Belt Stage from Macron Dynamics P/N: MSA-628 (same for both demos)
•        Schneider Electric MForce Power Drive Plus P/N: MFM1CSZ34N7
•        Oriental Motor/Vexta Stepping Motor: PK268DA
•        75VDC bulk linear power supply (same for both demos)
•        AllMotion EZ-series stepper indexer (same controller for both demos)

Servo System Differences:
•        Teknic ClearPath Integrated Servo Motor P/N: CPM-SDSK-2310S-RLN instead of the Schneider stepper drive and Oriental Motor stepper motor.
« Last Edit: October 31, 2021, 09:01:22 pm by xzswq21 »
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Offline jmelson

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Re: CNC servo motor sizing
« Reply #9 on: November 01, 2021, 02:06:00 am »

The price of SDSK-3421S is $328 and the price of SDSK-3421S is $349

according to the F1210 milling machine, which servo motor in this world do you recommend me?
The 3421 motor loses all torque at 1000 RPM.  The 3411 has SOME torque out to 1700 RPM.  What motor speed do you need to achieve to get reasonable linear speeds on the machine tool?  I don't know the the leadscrew pitch or gearing, so I can't compute it for you.
But, you would probably want to get at least one meter/minute rapid feed.  Assuming direct drive to 5 mm pitch screws, that would take 480 RPM, but maybe you'd want to go 50% faster, so 720 RPM.  With either of these motors you'd only get about 200 Oz-In torque, so go for the cheaper one.  Clearly, at lower speed the 3421 motor gives more torque, but at higher speed, the 3421 drops BELOW the 3411!
So, you need to contemplate the speeds you'd be running at.

Jon
 
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Offline xzswq21Topic starter

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Re: CNC servo motor sizing
« Reply #10 on: November 01, 2021, 06:17:41 am »
The 3421 motor loses all torque at 1000 RPM.  The 3411 has SOME torque out to 1700 RPM.  What motor speed do you need to achieve to get reasonable linear speeds on the machine tool?  I don't know the the leadscrew pitch or gearing, so I can't compute it for you.
But, you would probably want to get at least one meter/minute rapid feed.  Assuming direct drive to 5 mm pitch screws, that would take 480 RPM, but maybe you'd want to go 50% faster, so 720 RPM.  With either of these motors you'd only get about 200 Oz-In torque, so go for the cheaper one.  Clearly, at lower speed the 3421 motor gives more torque, but at higher speed, the 3421 drops BELOW the 3411!
So, you need to contemplate the speeds you'd be running at.

Jon

I really appreciate you :) I checked ClearPath SDSK motors again and I found another one SDSK-3421P.


The price for SDSK-3421S and SDSK-3421P both is $349 and for SDSK-3411S is $328. I have compared the performances at 75v and 48V as following:





What do you think about the SDSK-3421P? Which input voltage do you prefer? 48VDC or 75VDC?
Actually spindle speed of the WABECO F1210 is only 100 to 3000, someone told me it's low for a CNC, but Spindle speed of WABECO F1210HS=100 to 7500 but if you want to compare their prices:

WABECO F1210HS=WABECO F1210+$2320

Actually I should spend $500 for the accessories, $500 for the Dividing head, $1170 for ballscrews, and if I want to buy a coolant unit I should spend another $375!
plus I should spend 3x$349 for the clearpath motors+$275 for 75VDC Power supply, $100 for the cables and $377 for the controller board!
that's why I can not spend another $2320 for Motor upgrading.

there may be a solution,
I should remove some accessories and coolant unit and
I should buy a cheaper dividing head with 0.04mm accuracy:
https://www.wabeco-remscheid.de/teilapparate-und-zubehor/dividing-attachments.html
I think later I can build a coolant unit.
plus if I buy a F1200 instead of a F1210, I can save $200.  (Work table of F1200 is 50x18cm and Work table of F1210 is 70x18cm)
if I remove the ballscrews I can save another $1170! but how much will be the backlash if I don't use the ballscrews?! the servo motors are closed-loop.

if I take such a decision I can buy a F1200HS ISO30 edition.



Thanks
« Last Edit: November 01, 2021, 09:55:37 am by xzswq21 »
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Offline xzswq21Topic starter

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Re: CNC servo motor sizing
« Reply #11 on: November 01, 2021, 07:22:24 am »
a user in the below link explained some tips and I think it's useful and you see the tips as following :)
https://www.cnczone.com/forums/uncategorised-metalworking-machines/97857-cnc.html

Quote
Conventional milling (as opposed to high speed machining) says:

revs = 1000 * Vc/(pi * dia) where Vc is cutting speed of material in metres/min

which for most aluminium alloys is 60 - 120m/min giving a spindle speed of 6000 - 12000rpm on a 3mm cutter, so you are running a tad on the high side however high speed machining allows cutting rates up to 400m/min = 42krpm.

Feed rate Vf (mm/min) = Vz * revs * # of teeth, where Vz is feed per tooth in mm.

For most aluminium alloys a good rule of thumb is d/150 for roughing or d/200 for finishing, so assuming roughing Vz = 3/150 = .02 therefore feed rate needs to be around .02 * 18000 * 2 = 720mm/min.

Material removal rate Q in cc/min = width of cut * depth of cut * Vf/1000. You don't say what type of cut but assuming its 1mm wide x 3mm deep (edge milling) Q = 1 * 3 * 720/1000 = 2.2cc/min

This removal rate requires a certain power level

Pc = K * Q , where K is cutting power in Watts/cc

which for most aluminium alloys is 17 so P = 17 * 2.2 = 37W at the cutter, or around double that as input power, say 80W (~1/10HP) so its unlikely lack of spindle power is the issue.

Up your feed rate by 50%, or reduce the spindle speed, and you should get a better result...

but like all things these numbers are theoretical, you need to experiment with your own machine to see what its really capable of... rigidity becomes the key...

from Wikipedia about the cutting speed:
https://en.wikipedia.org/wiki/Speeds_and_feeds





« Last Edit: November 01, 2021, 07:45:40 am by xzswq21 »
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Offline Doctorandus_P

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Re: CNC servo motor sizing
« Reply #12 on: November 01, 2021, 03:33:06 pm »
Why do you want to use servo motors instead of stepper motors?

You can't mill faster with servo motors, because the milling speed is limited by the mill and your workpiece itself.

With servo motors you can have faster rapids, but that is usually only a small part of the total cycle time and it's not important for "hobby use". With a small CNC machine the time you can gain with fast rapids is also quite limited. (Compare this with a big router that has axis of over 2 meters long).

Fast rapids also have a disadvantage of being able to crash your machine faster then you can push the emergency stop button.

I have a small CNC machine with 112 mm long closed loop Nema23 motors and 1605mm ball spindles, and this can have rapids of over 10m/min, but I've limited it to 4m/min as this is plenty fast for me and it gives me some time to push the emergency stop if needed.

Closed loop steppers are a nice upgrade, mostly because they run quieter and the motors run cooler.

With "real" BLDC servo's you have the added complexity of mandatory fine tuning of PID loops, while stepper motors "just work" (even the closed loop versions. These do have built-in (and adjustable) PID parameters, but it's not very critical).

Also: A servo motor is not much different from a stepper motor. The main differences are that stepper motors have a much higher pole count, and 2 phase stepper motors are more common then 3-phase stepper motors. (A two-pase 200 step/rev. motor has a pole count of 50). and this explains why it has more torque at lower RPM, and more difficulty with higher RPM.
 
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Offline xzswq21Topic starter

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Re: CNC servo motor sizing
« Reply #13 on: November 01, 2021, 06:17:19 pm »
Why do you want to use servo motors instead of stepper motors?

I have a small CNC machine with 112 mm long closed loop Nema23 motors and 1605mm ball spindles, and this can have rapids of over 10m/min, but I've limited it to 4m/min as this is plenty fast for me and it gives me some time to push the emergency stop if needed.


Ballscrew pitch is 5mm so the required RPM for 4m/min is (4000m/5mm)/min=800

If I want to buy a stepper motor I should buy a Stepper driver too, most of the stepper motors are open loop, DO you know a suitable closed-loop stepper motor with a driver?

What do you think about the WABECO F1200HS? spindle speed is 100 to 7500, according to the equations and some parameters I think it's better for surface finishing.
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Offline Doctorandus_P

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Re: CNC servo motor sizing
« Reply #14 on: November 01, 2021, 07:01:07 pm »
I've bought a few Nema 23 closed loop stepper motor sets (motor + driver + cables) for around EUR 100 each.
They also need a power supply to work, and some microcontroller (I used a blue pill with GRBL).

I can not tell if a higher RPM is very useful to you.
The days of HSS tools have mostly gone by, except for cheap drills.
Carbide tools run at much higher RPM and an upgrade to 7500 rpm is probably useful, especially if you plan to run a lot of carbide tools with lower diameter.
But if this also results in lower torque at lower rpm, then it becomes a difficult choice.

Simplified version is that you need a mechanical gearbox (or (timing) belts) to change between high and low gear. If you only have electornic speed control for the whole range from low rpm to high rpm, then you always have (relative) low torque at low rpm and you can't use big face mills as they are intended to be used.

On the other hand.
If you want high rpm, you can add a 24000rpm spindle later with a bit of tinkering.
 
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