stepper, half step, servo?? which to get?

[blazer928]

Hi, I'm kinda new to cnc, but have experiance with a manual mill, so if I ask stupid questions, (about cnc )I am sorry.

The way I understand it servos are faster on the rapids, but I cant seem to figure out if they are more accurate? If so what kind of differance am I looking at? .0005 repeatability (this seems to be fairly common with stepper) compaired to ????? for a servo, and half stepper?

I have looked at several "ready to run" cnc mills and was interested to know if the servo motors are realy worth the extra $$$$$$.

I wont need to make my parts at light speed, or more acurate than .0005, (yet!)but I still have this info (mental) problem. I NEVER SEEM TO KNOW ENOUGH TO SATISFY MY NEED TO KNOW.

Any explanations about this will be greatly appreciated.

[snaggletto]

Hello,
Accuracy isn't so much the issue, both types of motors can be very accurate when properly applied. Servos do have many benefits over steppers, including top speed, smoothness/quietness during operation, and torque over a much wider speed range, but one of the biggest advantages is the feedback loop that the encoder and drive create.

Steppers, when overloaded, will lose steps, like a belt slipping on a pulley. This is lost motion and lost accuracy. You will not know when this happens, until after you measure your part and it's not the right size.

Servos, with the encoder feedback loop, is a totally different animal. When a servo starts to get overloaded, the drive can *see* that the servo is having a hard time keeping it's commanded speed and will apply more current (torque) to the motor to maintain speed/position. At the point at which the motor/drive can no longer compensate for the load the drive will fault out and the servo will come to a halt. In this sense, a servo can't lose steps as a stepper can, it will simply stop. This way, a servo is more accurate, or a better term would be more reliable.

To have a reliable stepper based system, you must oversize the steppers for the worst load conditions possible. This way, the stepper always has more torque available than will ever be needed. Say you figure you need 80 oz-in torque for a very small machine, then go out and get a 200 oz-in torque stepper for some overhead or insurance.

Geckodrives is working on a new product, the G204V. This is an "unstallable" stepper drive. Rather than letting the stepper lose steps, it slows the motor down (steppers make more torque at low RPM's) so that it maintains position. An interesting product to say the least. It should be available in a month or so, from what I gather. The bad news is that they will be a bit more expensive than the standart G201/G210 stepper drives.

Good luck.

[ger21]

Originally Posted by snaggletto Geckodrives is working on a new product, the G204V. This is an "unstallable" stepper drive. Rather than letting the stepper lose steps, it slows the motor down (steppers make more torque at low RPM's) so that it maintains position. An interesting product to say the least. It should be available in a month or so, from what I gather. The bad news is that they will be a bit more expensive than the standart G201/G210 stepper drives. Good luck.

The G204V is not the "unstallable" stepper drive. That is something else that Mariss has talked about, but no release dates have been announced for it. I'd guess it's at least a year or more away, if it's even possible. It might even be just a theory at this point.

The G204V "Vampire", is basically a G201 that can't be killed. You can do things to it that will kill a G201, and the G204V will still be fine, like wiring motors wrong, or even unplugging them while they're running. But it won't give you any better performance than a G201. It should be available in a month or so.

[snaggletto]

Hey,

I apologize for the misinformation for the new Gecko G204V. I guess I confused the capabilities of two products. My bad.

[blazer928]

Thanks for the info. Sounds like I will be ok with steppers then, as most of my work will be with plastics and brass. On the other hand, if I have it I am likely to try something stuppid,(is it just me??) like a deep cut in stainless. Thats part of the reason I want a mill that is somewhat overkill for what I am doing now. I know I will use it for bigger and bigger projects on down the road. So I will probably find out just how hard I can push before it messes up.