Servo or Stepper for CNC?

[bestsigns]

I am having trouble adjusting my servos.
The arcs that I rout are not very smooth. I can feel the table vibrating on curves. When I turn the damp and gain I get no resulting change in the motor oscillation. Any ideas?

[NC Cams]

Servo or stepper???

Slow down the feed (servo) to reduce "hunting" from overshoot.

Try slowing down your feed or use microstepping (steppers) - sounds like you're running into an oscillation problem that microstepping might solve.

Look for RF noise in cableing - this causes all kinds of buggaboos.

[Mariss Freimanis]

Servos vs. steppers. Nice list, I wrote and I still like it.:-)

Let me add a little to it that may be otherwise practical:

If you are designing a machine and you get to motors, the first thing you should do is calculate the power you need. Never buy a motor (stepper or sevro) first and then figure out if it will fit what you need. That is the sign of an amateur or hack.

Motors are motors. They couple power to your mechanism and power is what makes things happen. The choice of a motor comes after you know what's needed.

Power is velocity times force or torque times RPM. It doesn't matter if the motors are steppers, servos or a gerbil in a spinning squirrel cage at the start.

To seperate what motor need (neglect the gerbil), is the power your mechanism needs.

Rule #1: If you need 100 Watts or less, use a step motor. If you need 200 Watts or more, you must use a servo. In between, either will do.

So, how do you figure the power you need?

Method 1: You have a plasma table, wood router or some other low work-load mechanism. You have a clear idea of how many IPM you want but your'e not sure of what force you want at that speed.

Pick the weight of the heaviest item you are pushing around. If it weighs 40lbs, use 40lbs. Multiply it by the IPM you want. Say that's 1,000 IPM. Divide the result by the magic number "531". The answer is 75.3 Watts so use a step motor.

Eq: Watts = IPM * Lbs / 531

Method 2: You have a Bridgeport CNC conversion you are doing. The machine has a 5 TPI screw and you need a work feedrate of 120 IPM. 120 IPM on a 5TPI screw 5 * 120 or 600 RPM.

How about force? Not a clue? Use your machinist's experience on a manual machine. The handcrank is about 6" inches in diameter. How much force would you place on the handcranck before you figure you're not doing something right? I hear about 10 Lbs.

!0 Lbs is 160 oz, 160 oz on the end of a 3" moment-arm (6" diameter, remember?) is 480 in-oz (3 times 160) ot torque on the leadscrew.

The equation for rotary power is: Watts = in-oz * RPM / 1351

For this example, Watts = 480 in-oz * 600 RPM / 1351 or 213 Watts.

213 Watts is servo territory. You have to use a servo motor to get that, about a NEMA-34 one.

OK. Long post, late night. If anyone cares, let me know. Proper application of servo motors is an entirely different topic, it's involved but not particularly difficult. Servos are not steppers and they are not interchanchable. Let me know if I should continue.

Mariss

[CJL5585]

Mariss,

Please continue with the Servo information. It is an asset to this site and to all members and visitors alike.

Jerry

[imserv]

Servos vs Steppers for benchtop machines





Fred Smith - IMService
http://www.cadcamcadcam.com/hobby

[NC Cams]

Please add/continue whatever pearls of wisdom you have on applying servos.

I've already appended my cheat sheet with post 15 and I'll append again with whatever you post after that.

Surely, the subject will come up again and these highly valuable replies surely need to be linked to - good information should NEVER go to waste or go un-recorded/un-documented.

At least if you post it, it will be on record and linkable to henceforth. Not posting the info will only lead to idle conjecture and the dissemination of "sage wisdom" that is often anything BUT.

[jingham]

Don't forget the use of gearing or toothed belt drives for increasing the mechanical advantage the motor may need to push around a heavy cut. A NEMA 23 servo (Moog 53) I have at 2:1 reduction will not even strain on a 1/2" Dia cutter .375" deep in oak. I haven't challenged the unit any more than that so really don't know where the limit may be.
I really prefer servo's and have found no more difficulty setting up the PID routines than one would have in tuning the steppers.

[Kipper]

Excellent posts NC and Marriss. Keep up the good work guys

[walter]

Please continue Mariss, your effort is much appreciated.

[Mariss Freimanis]

Sorry for the lapse; I've been up to my neck in alligators while draining a swamp.:-) I've bookmarked the thread; give a few days and I'll pick it up again after I've turned a few of these nice alligator hides I have now into shoes or belts or something...

Mariss

[digits]

How does the use of gears/timing belts effect the overall accuracy of the system?

Say I wanted 1Nm at 1000 rpm, I could do it with a 4:1 reduction on a 4,000 rpm, 0.25Nm servo, or by gearing up a 4Nm stepper running at 250rpm by 1:4.

I'm guessing that the geared down servo will have more positional accuracy due the the gearing effectively quartering the step size, wheras the geared up stepper will effectively quadruple its step size, but won't both systems be a lot less accurate due to backlash in the gearing?

I guess what I'm asking is does it actually make sense to use ballscrews if you're not going to drive them directly from the motor?

[NC Cams]

In our situation (2:1 ratio belt driven preloaded ball screw, gibb based Bridgeport Eztrak mill with with high $$ true ball screw support bearings) we found the "error" contributed by the OEM "gilmer" style square tooted timing belt to be inconsequential.

Granted, we spent a small fortune on the ball screw bearings and weeks tuning and adjusting the machine to remove/eliminate slop. During the tuning process, we also spent a ton of time playing with belt tension and the like.

A belt in good condition had virtually NO effect on net accuracy once we got the slop out of the rest of the system. The BIGGESt contribution came when we had a pro tune the servos so as to get the follower error of the servo's "tuned out".

We tested via cutting perfectly round circles via a canned circle milling program. We then checked the form for roundness on our camshaft measuring machine (measures to 0.0000025" resolution).

When the machine was properly tuned, the circles were round within 0.0003" or better and the "flats" at the 4 direction changes were 0.0001" or better - the "flats' are nearly IMPOSSIBLE to eliminate due to gibb slop needed to allow the table to move without binding. We ultimately learned that the above "errors" were due to other hysterisis issues that simply weren't practical to resolve on a gibbs based "bridgeport mill".

Does the average guy need such accuracy? No.

But we CONCLUSIVELY showed that "belt induced errors" are much less in reality than what one might "expect" IF you have a well adjusted/tuned system made of parts with decent integrity.

Worn/tired crap isn't worth spending the time on. Get good sprockets and premium belts, properly tension them and make sure they "wrap properly" and spend your time and money on other stuff that provides a better cost/benefit ratio - like a good servo/stepper tuning regime and the best damn ball screws ans ball screw bearings that you can hardly afford.