General > General Technical Chat
CNC servo motor sizing
xzswq21:
--- Quote from: jmelson on November 01, 2021, 02:06:00 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
--- End quote ---
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
xzswq21:
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...
--- End quote ---
from Wikipedia about the cutting speed:
https://en.wikipedia.org/wiki/Speeds_and_feeds
Doctorandus_P:
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.
xzswq21:
--- Quote from: Doctorandus_P on November 01, 2021, 03:33:06 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.
--- End quote ---
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.
Doctorandus_P:
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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