Basic CNC

Okay, been doing some looking around on servos. It appears one has the choice between AC or DC servo motors.
From a hobbyist point of view, what's the difference between the two and what would be your recommendation in as far as ease of use and simplicity in set-up ?
Some of the things I read seemed to imply that it may be a good idea to use gear reduction with servos.
What are your thoughts ? Would say using a 2:1 gear reduction reduce the required servo size by half ?
Lastly, I've seen the term "Holding Power/Torque" used in reference to stepper motors. Do servos have a similar process for holding position ?
 
The torque ratings issue is quite a bit more complicated that steppers being able to "de-couple" or lose steps, all though that's part of the reason you need a higher torque rating with a stepper.

The rated torque for a stepper motor is the "holding torque" which is the torque the motor can resist when the shaft is not moving. The minute you start stepping, the torque drops by 20% or so. And as you move faster, the available torque drops even further, essentially down to nothing at high speeds. Large steppers may have only 25-30% of their rated torque at 1000 RPM. Also any vibration in the motor drive train is effectively added to the torque seen by the motor, further reducing the available torque to drive the load.

Servo motors have 2 different torque ratings - the continuous torque (this is the rating most often cited) and the peak torque which may be 2x - 3x times the the continuous torque for a large motor. With a servo motor, though, the torque does not drop off with the speed - most servo motors will run at the rated torque at speeds up to 1000 - 3000 RPM. The other great thing about servo motors is that the controller can run them a more than the continuous rated torque (up to the peak torque) for short periods to overcome static friction.

Also keep in mind that the force generated by a motor directly coupled to the lead/ball screw will produce a force (in pounds) of: (rated torque in oz-in) x (number of threads/inch) x 2 x pi x (lead screw efficiency) / 16. Therefore, 600 oz-in of torque applied to a 5 TPI ball screw (~90% efficient) would produce a force of 1059 lb. This is a fairly dangerous amount of force that would likely damage something on your machine. 200 oz-in would be more appropriate, but if you are using a 600 oz-in stepper, you may only have 200 oz-in of torque available at higher speeds.


Great explanation!
 
Okay, been doing some looking around on servos. It appears one has the choice between AC or DC servo motors.
From a hobbyist point of view, what's the difference between the two and what would be your recommendation in as far as ease of use and simplicity in set-up ?
Some of the things I read seemed to imply that it may be a good idea to use gear reduction with servos.
What are your thoughts ? Would say using a 2:1 gear reduction reduce the required servo size by half ?
Lastly, I've seen the term "Holding Power/Torque" used in reference to stepper motors. Do servos have a similar process for holding position ?

Primary differences are cost, construction, and operating principal. The AC servos are the modern equivalent of the older DC servo, normally less maintenance, but that is really a relative view. The only real maintenance on a DC servo it periodic brush replacement. Probably not something you would ever have to do on a home use machine.

From a setup perspective, AC and DC servos are comparable. Both require tuning, and the controls are slightly more complex that s stepper system. Older DC servo systems are a bit more complex to set up, but not bad.

You could do a 2:1 reduction and reduce the motor size, unless you really need 400 IPM rapid moves.;) A reduction is a good idea.

As stated by LoboCNC, the torque curve is almost flat from zero RPM to max speed. A servo system has the same or more holding torque as a stepper of similar size.
 
Thanks a ton, last couple questions.
I've read a lot about tuning, but have never heard an explaination of what it really is.
Do you have a CNC for Dummies explaination for it and is this more to do with the CAD/CAM programs and is it something I could basically ignore for now in as far as using them for what would basically be power feeds ?
What would you say is a good speed range for Rapids ?
Lastly, do you have any recommendations for a controller for use in manual operation of servos ?
 
Hmmmmm....Tuning Servos.. There have been entire books written on this subject. From a user perspective it really means getting the system, in this case your milling machine, to operate smoothly and accurately. This is normally as simple as making a few controller adjustments. It has to be done with with all drive system types.

Lets use the car analogy. You are the controller, the car is the motor, and the feedback is all of the information that you are processing (sight, sound, seat of the pants feel), and you output signals to the throttle, brakes, and steering. This is a closed loop system. For simplicity we will ignore everything except the throttle and the brakes for this explanation.

Let's say you are sitting at a stop sign and a block down the street there is another stop sign. So the ideal motion profile is to accelerate smoothly and quickly up to the target speed (speed limit), proceed down the street at the target speed, then decelerate smoothly and quickly to reach the target position (the stop sign). A new driver might accelerate too quickly or too slowly, not be able to control the speed, and brake too hard or not enough. So in the case of the servo system, you have to teach the controller how to operate the car. This is tuning.

Many modern servo drives can be operated manually with very simple controls. A FOR/REV switch, an ON/OFF switch, and a speed pot much like a VFD. The setup is easy by making adjustments to the on-board software via the front key pad or connecting it to a computer. The parameters are normally Acceleration, Deceleration, Max Speed, and the Proportional Integral Derivative parameters (PID). I'm not going to try to explain the PID here, but normally the only value that you have to adjust the P parameter. This sets the ''tightness'' of the system, set to high and the system becomes unstable and the motor will oscillate, set too low the system is mushy. For manual operation not so critical, but for CNC operation it has to be right. Not difficult to do, just takes a bit of playing around to see how the machine reacts to the changes you make.

Speed range for rapids..... I have my mill set to 100 IPM rapid. Plenty fast. Slow jog is set to 30 IPM. Some modern CNC machines run 1000 IPM rapids, that's just crazy and a lot of horsepower on the servos.

I also want to note here that the reason I suggested 1200 oz-in steppers in my post above is that you are not using ballscrews in the current setup. An Acme leadscrew requires about 8 times the torque of a ballscrew under the same load conditions. Should you decide to put in ballscrews later, you can always turn the torque down on the motor. A simple adjustment. It's better to have it and not use it, than to need it and not have it. You can also adjust the torque on servo motors, again a simple adjustment. My Z axis stepper for instance is turned down to 50% torque output, which gives me about 350 lbs of max down force on my quill before the motor decouples. My shear screws will shear at about 375 lbs in case something goes horribly wrong.
 
Think the light is starting to come on.
I understand PID, we use a lot of controllers at work (Nuclear Power Plant) for various things. As an operator, I'm trained and tested on all the system and controls all the time.
A different department works on them, but from a functionality perspective, I know how the work.
Still trying to decide between stepper and servo, but at least have a little more know how.
Decided to look at specs for a Servo Type 140 PF (recommended for my mill), here are some of the specs they list:
  • Peak Torque (half-wave series motor): to 140 in.- lb. / 15.8 NM torque
  • Intermittent Torque: 105 in.- lb. / 11.9 NM
  • Continuous Torque: 90 in.- lb. / 10.2 NM
  • Variable Feed Rate:
    .75-25 IPM / 19-635 mm/min (table & cross)
    .5-12 IPM / 13-305 mm/min (knee)
  • Rapid Traverse:
    35 IPM / 889 mm/min (table & cross)
    12 IPM / 305 mm/min (knee)
  • Gear Reduction Ratio (motor to screw shaft): 72:1.

From the suppliers I've looked at, most sell gear boxes designed for either NEMA 23 & 34 stepper or servo motors. The two models most carry are either a 5:1 or 10:1 ratio planetary gearbox.
Probably comparing apples to oranges, but thought I'd try using this data (from a mathematical perspective) to see how it would equate for servo motor drive with gearbox reduction for sizing.
Planning on spending some time in my shop today to start a small project I need to make. Figured I'd start with counting threads per inch on my lead screws and measure the torque required to move the knee, table, and crossfeed although I'm guessing some sort of fudge factor has to be used since this wouldn't be torque with a load on the table.
 
I think you have it under control. :encourage: Understanding a PID loop is half the battle.

Spend some time looking at the torque curve charts for the various motors you are looking at.
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Thanks, not in any big hurry beyond my old arms getting tired of cranking handwheels. LOL
Just seems to make more sense money wise. being able to have power on 3 or 4 axes for less than the price of 2 PF's
Not to mention being a computer and program away from a total CNC machine basically.
 
Got thinking about going ahead and putting a drive on at least one axis to test things out and get a feel for it.
As such, decided to see how much more it would cost buying the parts piece-meal vs going with 3 axis kit for the 600 oz-in servo motor kit (http://www.automationtechnologiesin...ema34-850-oz-in-72v20a-psu-g320x-gecko-driver).
At least on the Automation Technology site, I was surprised to find that buying the parts listed in the kit was actually $15 cheaper than buying the kit.
You'd think one would get a better price buying an entire kit over the pieces individually.
Huh, before posting, double checked and noticed the link says "850 oz-in, but goes to the 600 oz-in kit page. Wonder if their website messed up.
 
Huh, before posting, double checked and noticed the link says "850 oz-in, but goes to the 600 oz-in kit page. Wonder if their website messed up.

That's a bit confusing:confused:
 
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