[Mariss Freimanis]

I'm a motor drive zealot.:-) Right now I'm taking a 5-minute break from breadboarding a new DC servodrive, so I really don't have a dog in the fight.

To me, motors deliver Watts to a load. Step motors do it with the least hassle (and $$$) from zero to 100 Watts. Servos are indicated if 200 Watts or more is required. In between, either will do.

Mariss

P.S. I use extension cords between my fine Axiom speakers and snooty boutique amplifier.

[BobWarfield]

Excellent Mariss!

Now there is an engineering rule of thumb that I'll bet you've quantified very well being an engineer. Draw the lines at 100 watts and 200 watts, with some grey zone in the middle.

I'm thinking of starting a poll to talk about lost steps as well. I'm curious how many think they've ever lost steps, how they know, and what stories they have to tell in conjunction with that.

Cheers,

BW

[Oldboy]

I'm a motor drive zealot.:-) Right now I'm taking a 5-minute break from breadboarding a new DC servodrive, so I really don't have a dog in the fight.

To me, motors deliver Watts to a load. Step motors do it with the least hassle (and $$$) from zero to 100 Watts. Servos are indicated if 200 Watts or more is required. In between, either will do.

Mariss

P.S. I use extension cords between my fine Axiom speakers and snooty boutique amplifier.

I think you mean Steppers are good,cheaper,less hassle on small mills. And as you go larger use Servos for the power and speed. Since the Steppers crap out on larger mills. I don't know how many times i have heard this advice. If you wan't 4th axis or a controled position motor driving a lathe used for 4th axis? The Servo is the way to go on small or large.
The point im making is you need to understand what your Stepper or Servo will be used for.
Do your engineering calculations. For starters know what max IPM you want in your system;resistance force of the parts on your mill,amount of force you need to cut.
Then you can start understanding what mechanical parts to use. Like Acme or Ball screws. Timming Pullys what Dia. ect. Steppers or Servos?
Servos are known to have a higher RPM & good torque in there rpm range. Steppers are not as fast in RPM but do have good torque in there low to mid rpm range. Again you need to do your math.
May be this might be of help?
http://www.youtube.com/profile?user=...LC&view=videos

[sansbury]

Who said anything about a $1000 price difference, especially for your X2? Now there is a myth. I spelled out exactly the difference for a more powerful system than is needed for the X2. Isn't the difference on the x2 then logically even less expensive? Because if you think the story is different if it is a few hundred dollars premium for servos, it is very likely that on the X2 as well.

An X2 can be run just fine with a G340, $300 and $150 of stepper motors. Runs fine with a cheapo 24VDC PSU, and has a 4th axis drive too. That can be yours for $500. Now with servos, I see $130/axis for the Gecko drives--I don't know of any good control that's cheaper--plus $150/motor based on current Home Shop CNC price, and you'd need a PSU that delivers over 10 amps at 60V, right? Even if we leave the 4th axis out of it, I'll bet you spend $1000 or more to do servos.

And BTW, are you sure you never lost a step except due to a catastrophic crash of some kind?

If a stepper loses steps cutting a part and no one notices, does anyone care? I suppose I may have lost a step along the way but have yet to see evidence of that kind of problem in my parts.

BTW, what is the right sized stepper motor for an X2 that's big enough that it never lose steps unless you do something really painfully obviously wrong? How do you know that is the right size? You talk about engineering vs superstition. Where are the engineering calculations to properly size a stepper for an X2?

Same ones you'd use to size a servo. :rainfro: Neither motor will deliver more torque than the label says. On a mill the size of an X2 with its lovely sliding ways, the amount of torque needed just to offset the friction of moving the table empty is probably >90% of that needed to make the heaviest cut the machine is capable of. And I've found that the first thing to run out of power is the spindle in any case.

In other words if you can G0 from X0 Y0 to X3 Y3, then you can take a reasonable cut anywhere in there. In my experience, after a few D'oh! moments I know what sorts of feeds and DOCs the mill can handle, and I generally find other more novel ways to screw up my parts. The only times I stall my motors on the X2 is when I do something really brilliant in the program and plunge at 100IPM or something similarly clever.

I have a router table that I have lost steps for sometimes. It doesn't happen often, and it is usually because I am just cutting a little too aggressively, nothing more than that. Did I spec too small a stepper motor? Perhaps. But how would I know? It's a one-off.

Wood routers are different in that they are usually using some kind of linear rails that have a lot less friction than a dovetail way on a mill. So there you will have a bigger delta between cutting power and minimal moving power. I get this on my small wood router, which I run with impossibly-small 125oz steppers. Still, after a little experience I've found its sweet spots and it doesn't give me a lot of trouble.

How do you adjust your gibs on your X2? What if you had a closed loop way to tell if you had adjusted them as tight as possible before you started having servo faults frequently. Would that help your machine's performance? It sure does mine.

I tighten them up and then run the axis back and forth a couple hundred times, and use an indicator to measure where it comes to rest. Lost steps would show up instantly this way. As it is, I've found that it either stalls blatantly, or runs fine. There doesn't seem to be a place where it sometimes occasionally drops a step here and there.

[Mariss Freimanis]

Oldboy,

When I ask "What is diameter of the handcrank on your Bridgeport?" I usually get " 6 - 8 inches" as the answer. When I ask "How many Lbs do you apply to the handle?" I get "Never more than 15 Lbs" for an answer. When I ask "What is a fast feedrate?" I hear "20 IPM".

Let's use these numbers:

Torque on leadscrew = 16 * 15 Lbs * 8" / 2 = 960 in-oz.
Leadscrew RPM = 20 IPM * 5 TPI = 100 RPM.
Power applied to leadscrew = 960 in-oz * 100 RPM / 1351 = 71 Watts

This is not a lot of power. A square NEMA-34 motor is good for an honest 200 Watts mechanical when driven hard (6A rated winding, 72VDC supply); a near 300% safety margin.

Mariss

[CarbideBob]

Aw heck, just put some 1/10 micron feedback linear motors on it.
Course that might be a little expensive but it sure will fly.
Bob

[Mariss Freimanis]

BW,

Step motors don't lose a step here or there due to overload. They are 50-pole motors so all lost motion comes in 7.2 degrees (360 degrees / 50 poles) increments. That makes a racket that is hard to ignore at low speeds. At high speeds the overloaded motor (lag greater than -3.6 degrees) desynchronizes and flat out stops. This is noticeable as well.:-)

A servo has 2 attributes, one real and one imaginary, that gives the reputation of being more robust. The real one is reserve torque, 5:1 for a DC brush servo and 3:1 for a BLDC servo. This torque is available for short periods of time and aids in recovery from very short duration overloads.

The imaginary one is servo following error. This masks large overloads of short duration by allowing the motor to fall behind with the expectation the error will be made up once the overload relaxes. For our drives the following error is +/-128 counts which results in a momentary +/-0.0128" error (5 TPI screw, 500-line encoder). By comparison, a stepper only tolerates a +/-0.001" error (5 TPI screw, 1.8 degree motor).

The following error range is a source of specmanship mischief. Some drives tout a +/-32,000 count range; the tolerated error then is +/-3.2 inches! There must be a happy place in between.:-)

This is an imaginary servo advantage because the work being milled will faithfully record the overload as a notch or a bulge of the same dimension. The stepper's disadvantage is any missed steps accumulate as an offset error for the balance of the program.

A disadvantage of a servo is it has the same torque 'available' during a rapid as it has at the feedrate. Let's say your rapid is 200 IPM, 10 times the feedrate. The only torque needed during a rapid is what's needed to overcome mechanism friction, say 10% of the feedrate torque. You have 960 in-oz available but you can only use 96 in-oz. The motor can deliver 960 in-oz at 1,000 RPM which means you need a 710 Watt motor to get 71 Watts of work from it.

The stepper is better off here; its torque is the inverse of speed. If it has 960 in-oz at 100 RPM then it has 96 in-oz at 1,000 RPM. 71 Watts delivered at 20 IPM, 71 Watts delivered at 200 IPM.

Mariss

[Oldboy]

Hi Mariss Freimanis
Its not in-oz 960. Just 960 oz. of torque.
Yes your right in your example. Nema 34 Steppers are used a lot on mills of that size. But if your building a Plasma cutter or wood router type mill. You would want fast IPM on X,Y & Z. So Servos might work better for that type of mill. As for my example of Servo motor used to drive Lathe/4th axis.
It is a fact the Servo motor runs faster RPM'S & has good torque at the higher rpm range. Than a Stepper. But yes one could use a stepper 4th axis.
As long as they understand High RPM & Torque is not the same as a Servo would give them. You know one could install the Timming pulleys on the X,Y & Z. Then use a Fish scale to get an idea of the amount of force to just move the axis. This would be just a starting point.

[samco]

come on - apples to something totally different from apples. step/dir servo drives are evil imho. With emc - you tune the servos and set your following error. some set it as low as .001" or better depending on the tune. 128 counts? yikes. True closed loop -> encoder back to the machine controller has so many more advantages.

'I hit the estop and now I have to re-home?' Not a problem then. Homing to index? all kinds of good stuff.

(yes I am a emc lover)

sam

The imaginary one is servo following error. This masks large overloads of short duration by allowing the motor to fall behind with the expectation the error will be made up once the overload relaxes. For our drives the following error is +/-128 counts which results in a momentary +/-0.0128" error (5 TPI screw, 500-line encoder). By comparison, a stepper only tolerates a +/-0.001" error (5 TPI screw, 1.8 degree motor).

[SCzEngrgGroup]

Hi Mariss Freimanis
Its not in-oz 960. Just 960 oz. of torque.
Yes your right in your example. Nema 34 Steppers are used a lot on mills of that size. But if your building a Plasma cutter or wood router type mill. You would want fast IPM on X,Y & Z. So Servos might work better for that type of mill. As for my example of Servo motor used to drive Lathe/4th axis.
It is a fact the Servo motor runs faster RPM'S & has good torque at the higher rpm range. Than a Stepper. But yes one could use a stepper 4th axis.
As long as they understand High RPM & Torque is not the same as a Servo would give them. You know one could install the Timming pulleys on the X,Y & Z. Then use a Fish scale to get an idea of the amount of force to just move the axis. This would be just a starting point.

"Its not in-oz 960. Just 960 oz. of torque." - Ummmm.... No. Torque is not, can not be, measured in just ounces. Force is measured in ounces. Torque is measured in inch-ounces, foot-pounds, etc. In other words, force applied at a radius from the axis of rotation. Whether it's 9.6 ounces at 100 inches, or 96 ounces at 10 inches, or 960 ounces at 1 inch makes no difference. But just ounces, it ain't. Mariss' numbers and units are correct as stated.

Using a fish scale is a perfectly valid way of determining the torque required to move an axis. I did that with the knee drive on my BP clone, and it was surprisingly accurate.

Regards,
Ray L.

[Mariss Freimanis]

Oldboy,

Torque is a twisting force on a shaft. Its units are a force (oz, lb, Newtons) applied at the end of a moment arm (in, ft, meters). It is expressed as ft-lbs, in-oz, Nm, etc. 960 oz is just a weight, 15 lbs.

samco,

Yikes back at you.:-) So you have a servo with zero following error? If not, it doesn't matter if the following error is 'set' in software or in the drive's firmware. A PID loop is still a PID loop no matter where it is executed. In EMC you still have to have a command position input. The fact that the PID algorithm is executed in the PC is of no particular significance; you still have to run a +/-10V torque command wires to the amplifier and encoder wires back to the PC. It is no less evil to have STEP/DIR command wires going to a drive and execute the PID algorithm locally in the drive. Same bits and pieces, just a different distribution of these bits and pieces. Same old algorithm though.

Mariss

[samco]

mariss - your hardware is top notch. If I needed to buy step/direction drives yours would be the ones I would buy. Top Notch! That being said - you are selling step/dir drives

I did not say 0 following error. Remember that I can use encoders that are high count. Something that is an issue with step/dir drives. My current encoders are 2540 lpr or 10160 edges. With emc being the loop you can tune PID and FF0-FF2. Plus you can do cool things (that people in this thread have been touting as servo problems like run-away and such)

-Setting folloing error low so that it trips when things are over-stressed.
-Emc's pid loop now has a 'saturation' pin. So you can say if the pid loop saturates for x amount of time - throw an estop. lets say you do lose your encoder. The control will try to go to a postion but not see movement. the pid loop will ramp up (pushing harder) to get the machine in position. It will saturate. After say a few ms if that is what you have set - the machine will estop.

apples to tomatos

look at this http://www.anderswallin.net/2008/04/...th-servo-mill/ He also has some cool rigid tapping videos

His following error is below .02mm or .0008"

sam

[LUCKY13]

mariss - your hardware is top notch. If I needed to buy step/direction drives yours would be the ones I would buy. Top Notch! That being said - you are selling step/dir drives

I did not say 0 following error. Remember that I can use encoders that are high count. Something that is an issue with step/dir drives. My current encoders are 2540 lpr or 10160 edges. With emc being the loop you can tune PID and FF0-FF2. Plus you can do cool things (that people in this thread have been touting as servo problems like run-away and such)

-Setting folloing error low so that it trips when things are over-stressed.
-Emc's pid loop now has a 'saturation' pin. So you can say if the pid loop saturates for x amount of time - throw an estop. lets say you do lose your encoder. The control will try to go to a postion but not see movement. the pid loop will ramp up (pushing harder) to get the machine in position. It will saturate. After say a few ms if that is what you have set - the machine will estop.

apples to tomatos

look at this http://www.anderswallin.net/2008/04/...th-servo-mill/ He also has some cool rigid tapping videos

His following error is below .02mm or .0008"

sam

WOW, that dude has his stuff worked out very nice. Kinda impressive even.



As far as stepper-servo goes I don't know what to even add to this subject. I do feel like servo setups are worth the extra money & effort. But I do like to try and pull out all the performance I can no matter what I am working with, I guess thats the old HotRodder in me. I will be working with some steppers on my X2 thought (there free), I might thave to look into this EMC2 to control them.


Jess

[accuracymark]

Thank for all the info,, my question was answered in the first few reply 's
I have never heard of stepper's until I considered making my own machine. I seems that the hobby home machines are finicky things. Im used to figuring out the power of the machine and rigidity of the setup, getting a chip load I am happy with, order up some tooling, write a program and post to the control. Load up the machine and wait for the light to flash. very rare there is any issues with the machines.

[Al_The_Man]

Thank for all the info,, my question was answered in the first few reply 's
.

It was your heading that probably did it.
You have to watch your wording otherwise you open the flood gates
Al.

[BobWarfield]

Mariss, many questions:

BW,

Step motors don't lose a step here or there due to overload. They are 50-pole motors so all lost motion comes in 7.2 degrees (360 degrees / 50 poles) increments. That makes a racket that is hard to ignore at low speeds. At high speeds the overloaded motor (lag greater than -3.6 degrees) desynchronizes and flat out stops. This is noticeable as well.:-)

Just to clarify, a step motor can never lose a step without making a horrendous noise or stalling outright?

However, it does mean you have to watch like a hawk and do need to be able to hear the stepper motors over your spindle. It also means the machine will still keep moving in other axes. I'm not sure if a totally stalled axis is better or worse than one that just lost a few steps, but intuitively, it feels scary. I'd hate like heck if it was the Z-axis, which being the heaviest, seems the one most at risk.

A servo has 2 attributes, one real and one imaginary, that gives the reputation of being more robust. The real one is reserve torque, 5:1 for a DC brush servo and 3:1 for a BLDC servo. This torque is available for short periods of time and aids in recovery from very short duration overloads.

The imaginary one is servo following error. This masks large overloads of short duration by allowing the motor to fall behind with the expectation the error will be made up once the overload relaxes. For our drives the following error is +/-128 counts which results in a momentary +/-0.0128" error (5 TPI screw, 500-line encoder). By comparison, a stepper only tolerates a +/-0.001" error (5 TPI screw, 1.8 degree motor).

The following error range is a source of specmanship mischief. Some drives tout a +/-32,000 count range; the tolerated error then is +/-3.2 inches! There must be a happy place in between.:-)

We've agreed that the servo can't really be run at the same drive ratio as the stepper because it doesn't have the low end torque. Put another way, they are typically not direct driven as you've shown. Isn't the error then typically 1/3 to 1/6 the 0.0128" you have quoted for a fault, depending on the servo drive ratio?

So is it really fair to say the servo is going to be way off by a factor of 128 before you notice while the stepper will only be off 1 step because it'll stall completely or make that horrendous noise? And, is it really 1 step, or is it the 7.2 degree error you've suggested is reality? 7.2 degrees is an error of 0.004" by your figures.

I agree, BTW, that a really large following error doesn't sound very useful to me. I do wish the error was available to the control. It would be very helpful to be able to tell the control what error you want to fault at based on the tolerances of your parts. OTOH, some of those other drives are trying to provide the flexibility to run with higher count encoders.

This is an imaginary servo advantage because the work being milled will faithfully record the overload as a notch or a bulge of the same dimension. The stepper's disadvantage is any missed steps accumulate as an offset error for the balance of the program.

True, but you also get the servo fault if the error is too great. Not so on the steppers. And you get to see where the error happened, rather than try to guess. That bump is pretty obvious place to start looking for a problem in your g-code.

A disadvantage of a servo is it has the same torque 'available' during a rapid as it has at the feedrate. Let's say your rapid is 200 IPM, 10 times the feedrate. The only torque needed during a rapid is what's needed to overcome mechanism friction, say 10% of the feedrate torque. You have 960 in-oz available but you can only use 96 in-oz. The motor can deliver 960 in-oz at 1,000 RPM which means you need a 710 Watt motor to get 71 Watts of work from it.

The stepper is better off here; its torque is the inverse of speed. If it has 960 in-oz at 100 RPM then it has 96 in-oz at 1,000 RPM. 71 Watts delivered at 20 IPM, 71 Watts delivered at 200 IPM.

Mariss

And this is a disadvantage, how? Because you need a bigger servo as you're wasting its power at the higher feedrate?

But wait, the servo makes power to a much higher rpm too. So we're going to gear it 4:1. That multiplies its torque.

So can't we get by with a smaller servo motor relative to the stepper needed?

And what is the range of feeds and speeds the stepper can work at versus the servo? Is 20 IPM all you'd ever want to do?

A 1/2" 2 flute endmill would like to go 40 IPM in aluminum according to my speeds and feeds calculator. That 10:1 speed ratio is now 5:1. I wouldn't sell that short. Hoss has a series of videos on how fast he was able to cut aluminum on his X2 and he was definitely up there in that kind of range.

------

I think the big news for the small machines is Geckodrives has made quality stepper drives a lot cheaper, especially with the G540 form factor.

There is also a fixed cost on the servo in the form of the encoder. They're circa $30-$50, and no matter what you do motor-size wise, they don't get cheaper. Hence small servos cost relatively more than small steppers.

That says to me that a really small mill like an X2 will have a bigger delta in cost versus a servo system than a larger mill like my IH or a Bridgeport.

[Oldboy]

Thank for all the info,, my question was answered in the first few reply 's
I have never heard of stepper's until I considered making my own machine. I seems that the hobby home machines are finicky things. Im used to figuring out the power of the machine and rigidity of the setup, getting a chip load I am happy with, order up some tooling, write a program and post to the control. Load up the machine and wait for the light to flash. very rare there is any issues with the machines.

Watch the videos i posted on selecting Steppers it might help you.
Thats only if you need it.
Just tried to help.

[Mariss Freimanis]

Bob,

Good points.

1) Step motor poles are spaced 7.2 degrees apart. The motor torque over that span is sinusoidal; from 0 to -1.8 mechanical lag the motor's torque increases to counter the load (sine 0 to 90 degrees increases in value). From -1.8 to -3.6 degrees torque decreases with increasing load (sine 90 to 180 degrees decreases in value). From -3.6 to -5.4 degrees torque switches direction and moves away from the load. Finally from -5.4 to 7.2 degrees the motor settles into a stable position again.

It's all over when the motor lags -1.8 degrees. Any further increase in load causes decreasing restoring torque. That's 0.001" on a 5 TPI screw.

2) Apples should be compared with apples. In my example an imaginary 10 microstep drive was coupled 1:1 to the leadscrew. The servo in the example had to be geared down and a 500-line encoder was located on the leadscrew. This way, stepper or servo, a single step pulse moved the mechanism the same distance.

3) It is a disadvantage from my viewpoint to have to use a 710 Watt motor to move a crummy little 71 Watt load (or a 141 Watt load using your 40 IPM : 200 IPM example). :-) Servos develop their rated power at high RPMs, just where you don't need it, during a rapid.

A really nice NEMA-34 servo is good for 180 in-oz at 4,500 RPM (600W). To get 960 in-oz on the screw you have to gear it down 5.33:1. That gets you 844 RPM on the screw for a maximum rapid of 169 IPM (5 TPI screw). You still have 960 in-oz available at 169 IPM but how are you going to make use of it?

It would be nice to have an infinitely variable gearbox on your servo; high reduction at feedrates, low reduction on rapids. High torque when slow, low torque when moving fast. Otherwise the servo is like a car stuck in one gear. The speed-torque curve from your gearbox would be a torque inversely proportional to RPM. Just like what you get from a stepper.

Mariss

[LUCKY13]

Oh my, he said variable gearset!



Now we need to get that planitary set to shift when needed. Hum, I bet tuning the servo just got a little harder. LOL.



Jess