Servo selection question...

[BWay]

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 a friend that own's 2 cnc routers (one with steppers and one with servo's) he strongly recommended using DC servo motors. He 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 (direct belt drive) that translates to a .551in/rev,the y axis (ballscrew) is .5in/rev. and the z axis (ballscrew) 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.

What should I be concerned with using servo's and actuators with low teeth per inch ball screws? Can the servo's perform well with low RPM requirements or will I be required to further reduce the drive ratio by adding a pulley reduction type setup?

Any thoughts?

[Al_The_Man]

Both Servo's and Steppers have maximum torque at zero speed, with servo's the torque curve is fairly flat or taper down very slightly up to maximum speed.
With steppers, if steps are not taken to maintain the current with increase in rpm the torque will drop off drastically, this is usually overcome in modern drives.
Steppers are traditionally higher torque for a given NEMA size, NEMA is not however an indication of Torque, just a mounting, for example, the servo's I use come in three torque ratings in each NEMA mounting size, which is usually dependent on the body length of the Stepper or Servo.
Servo's have traditionally performed very well in direct coupled applications, but as you can see they would have to be sized larger to be comparable to the stepper within the stepper rpm range.
For both types of motors, the biggest influence on deciding on a suitable size, or whether reduction is needed, is on the required load, of which, the required accel/decel rate has the biggest influence, this is due to the motor-load inertia ratio that will increase as the acceleration rises.
So it is usually a decision whether to increase the motor-drive size, or use reduction and a more economical motor-drive.
If you plug in some numbers into one of the motor sizing programs, you can get a feel of what different factors affect the sizing.
Al.

[BWay]

Originally Posted by Al_The_Man If you plug in some numbers into one of the motor sizing programs, you can get a feel of what different factors affect the sizing. Al.

Al...thanks. Your answer was exactly what I was looking for. The good news is that I am planning a 4 axis router which allow me to use 2 motors on the x axis which is also the largest load to move.

You mentioned motor sizing programs....where can I get such a program?

Thanks again.

[Al_The_Man]

Many of the servo manufacturers have them, this is one that has been around for a while.
http://www.electromate.com/technical...morgansoftware
Emerson and Allen-Bradley still have one I believe.
http://www.macphersoncontrol.com/navbar/mcptools.html
Al.