Official Luthiers Forum!

I am in the process of building a CNC router. I would like to finalize my motor selection in the next few weeks but am more confused than ever. After trading some emails with John Watkin he strongly recommended using DC servo motors. He's used servo and steppers and feels strongly that the extra money for servos is worth the expense to maximize accuracy and speed. It all sounds good to me.

Here is the problem I am wrestling with. I currently have all my linear actuators (I picked them up used) and the x axis is a belt drive setup that translates to a .551in/rev. The y axis is .5in/rev and the z axis will be .393in/rev. All the actuators are equipted with Nema 23 motor mounts which makes attaching the motors very easy. It is my understanding that servos perform best at high speed. Since my ballscrews and belt drives have a low teeth per inch then will the servo's still work at lower speeds?

I am going to post a similar question on CNCzone but thought a few of you might tell me your thoughts on the issue.

Thanks!

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Brad
Avon, OH

I'd be interested to hear what you find out Brad.
Of course you can use larger servos if you're concerned about it. You also can do a gear ratio with your belt drives to give you extra umph.
I agree with John, small open loop steppers stink. Larger steppers with encoders (closed loop) are probably alright and closed loop steppers are best. I would really try to avoid an undersized open loop stepper system if you can. Of course money does enter into the equation.

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Jim Watts
http://jameswattsguitars.com

Servos get higher torque with higher RPMs. The goal is to be spinning the servo at 80% of its max RPM or so at the top speed your machine will be moving. Odds are that when the machine is moving slower you don't need as much torque, anyhow. Not necessarily true in a metalworking machine (slow == steel) but in woodworking your torque requirements generally scale with the feed rate (slow speed = small cutter).

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Edit: I meant to "closed loop servos are the best"
Bad typing on my part, as usual!

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Jim Watts
http://jameswattsguitars.com

I have retrofitted my two CNC's with servo drives due to poor performance with steppers.
With that said, I think that the stepper technology on the market today is improved considerably.
I have noticed running servos under Mach3 that I will usually be off by a few thousanths on X and Y after running a group of parts.
I see this when homing to the axis limit switches again. Not sure if this would be caused by accumulative rounding errors in Mach3 or what.
Usually see not more that 3 or 4 thou error so it's not a show stopper. Can always "rehome" in the middle of a run if necessary.
Nelson

Brad there is another way to go, if you have the money and want to part with it!

If you want to use the steppers, and have the assurance of total accuracy, well within limits, look at the CNCBrain. You will need the Brain, encoders for your motors or a way to get them in the motion of the motors, and some linear scales. These can be expensive or you can build your own, which is what I am going to have to do due to the size of my machine. By using both types of encoders you can get a dual loop feedback with position correction, even with steppers.

The Brain is still in beta and there is not a lot going on with it right at them moment, but that is about to change. There is a forum for it on the CNCZone so just do a search. While you are at it you can do a net search for Safeguardrobotics....dot com I think.

Mike