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Author Topic: What size motors?  (Read 15051 times)

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Re: What size motors?
« Reply #30 on: April 22, 2009, 03:34:29 PM »
Just a comment on torque values for the respective technologies, Steppers, DC servo DCBL and AC servo generally all have maximum torque at zero speed, if you study their performance specs.
There have been many instances of machines being manufactured by high end users like  Mazak using Fanuc and Mits systems  using direct drive ball screws using DC servo's, agreed many of these were large motors, but torque is torque, whether stepper or servo.
On a similar note, Galil, which is one of the largest manufacturers of Motion cards do not appear to use steppers in any of  the dozens of application examples that they display on their web site, despite their cards being capable of step/dir.
Recently they won a face off for a very high speed, high precision positioning competition with Schneeburger mechanism.
http://www.galilmc.com/support/servotrends/st_10_08.pdf
Nosmo
 
« Last Edit: April 22, 2009, 07:02:58 PM by NosmoKing »
Re: What size motors?
« Reply #31 on: April 22, 2009, 04:01:42 PM »
One thing that no one has mentioned is that with a stepper system if everything is ok it works but if anything goes wrong it stops.
A servo system on the other hand can fail in such a way that the servo takes off at full speed and goes until it hits something that can stop it.
That is the reason for the high cost of commercial servo controls. They have to be able to detect and stop any such failures.
Systems that only rely on the driver to recognize out of position moves don't have the safety of a system that also depends on the control.
Speed is not the most important thing when most of us are at best high end hobbyists. A mill that can move at 500 -600 IPM would scare a lot of us.
It is not as if shaving 30 seconds off of a part run would save us millions like it could in a major factory.
« Last Edit: April 22, 2009, 04:06:13 PM by MachineMaster »

Offline simpson36

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Re: What size motors?
« Reply #32 on: April 22, 2009, 05:32:14 PM »
Torque over time = power. Power moves things, not torque.

The biggest difference, power wise, between steppers and servos is that the Stepper's torque drops off markedly with speed, whereas a servo does not. Also a stepper is rather severely speed limited. I just noted a document on the forum that shows a stepper loosing 75% of it's torque at high speed (high for a stepper).

Here is a comparison of the different motor types (when they are running) using Horsepower as the measure (universally understood).

HP = torque x speed / 5252

We start with equal torque stepper and servo, we'll be very conservative and say the stepper loses only half of the torqe and gets up to 1,000RPM (that would be a very good stepper)

Servo is not going to loose torque and will be running at 3,200 RPM (that would be an average servo)

Stepper 1.5625 ft-lbs * 1000 /5252 = .298HP

Servo    3.125   ft-lbs * 3200 /5252 = 1.904HP

Servo generates a bit more power. Those are the numbers.

As to the runaway issue with servos, in the case of the Gecko drive I am using, the drive provides the power to the motor, if somethign oges wrong, it simply faults and shuts down the motor. In the event the Gecko failed in some way that gave full juice to the motor, the setup can be wired in such a way that the limit switches shut down the current to the motors.

None of this means steppers are bad. It's just not a complete picture to simply compare a torque spec and draw the conclusion that the performance is anything near similar. 

It is what it is. Steppers are fine for some applications, but it is inaccurate to say that they are equal to servos in performance. My rig has steppers on three axis and they do just peachy for what I need. On the other hand, even a relatively large stepper was not up to the task of running my 4th axis, so I'm messing around with my forst servo motor now.













Re: What size motors?
« Reply #33 on: April 25, 2009, 12:21:27 AM »
I just got my Sharp mill working on cnc. I used the Keling KL34H2120-60-4B  steppers 1200in.  Direct drive with Ruland couplings only 2 brackets for the x ,z and 3 for the y.
The Sharp is about a third larger than the Bridgeport. If you have ball screws I thank that the 1200in steppers would work just fine. I have not been able to find ball screws for the Sharp yet so am somewhat limited on max speed but it is much faster than hand cranking.  :)

Offline simpson36

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Re: What size motors?
« Reply #34 on: April 26, 2009, 09:44:29 AM »
I put ball screws on my mill as a prelude to the planned CNC conversion. I still cranked by hand, but 5 turns per inch was SOOOOO much better than 16 . . .  8)

It is not very difficult (famous last words) to make ball screws for any machine tool.
Re: What size motors?
« Reply #35 on: April 26, 2009, 10:20:09 AM »
It is not very difficult (famous last words) to make ball screws for any machine tool.

'If any machine tool' you mean manual machines as well, It has been my understanding that the main problem is back-feeding when the much lower friction B.S.'s are fitted?

Offline simpson36

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Re: What size motors?
« Reply #36 on: April 26, 2009, 10:39:31 AM »
That's an interesting point.

Certainly with a fast lead and ball tracks that would be the case. With a 5tpi el cheapo ball screw and a crappola asian mini mill like mine, not a problem. It took the effort from intellerable to bearable  :D