[accuracymark]

Im planning on building a x2 benchtop cnc machine up for fun. I am a trained machinist and cnc programer, I haven't worked in the industry for 5 years. theres been lost of changes in the home machines in the last 5 years. I worked with fadel and haas machines some okk's and okuma's aswell. I believe they all run closed loop type servo systems. why do the stepper systems dominate the home shops?

[Al_The_Man]

I believe they all run closed loop type servo systems. why do the stepper systems dominate the home shops?

Mainly Cost I would say.
Al.

[accuracymark]

Mainly Cost I would say.
Al.

I guess that makes total sense as hobbies uses up the spare whats left after the bills are payed.

[SCzEngrgGroup]

Im planning on building a x2 benchtop cnc machine up for fun. I am a trained machinist and cnc programer, I haven't worked in the industry for 5 years. theres been lost of changes in the home machines in the last 5 years. I worked with fadel and haas machines some okk's and okuma's aswell. I believe they all run closed loop type servo systems. why do the stepper systems dominate the home shops?

For a small machine, like the X2, servos are more expensive, and have absolutely no functional advantage over steppers. Servos are good for high performance, and heavy machines, but for a machine the size of the X2, you can't use more performance than a good stepper system can provide. Plus, with steppers you don't have to worry about the one disadvantage of servos - potential damaging runaway if you lose an encoder, or have a driver failure. That can't happen with steppers.

Regards,
Ray L.

[accuracymark]

For a small machine, like the X2, servos are more expensive, and have absolutely no functional advantage over steppers. Servos are good for high performance, and heavy machines, but for a machine the size of the X2, you can't use more performance than a good stepper system can provide. Plus, with steppers you don't have to worry about the one disadvantage of servos - potential damaging runaway if you lose an encoder, or have a driver failure. That can't happen with steppers.

Regards,
Ray L.

On mach 3 can you use dro mode with steppers?

[Al_The_Man]

.........you don't have to worry about the one disadvantage of servos - potential damaging runaway if you lose an encoder, or have a driver failure. That can't happen with steppers.

Regards,
Ray L.

Only one disadvantage, thats not bad
I use servo's regardless of system size and I can overcome that with a true closed loop system by keeping track of the following error at all times and issue a shut-down.
Most Commercial systems work this way, to detect both encoder loss and following error.
Al.

[SCzEngrgGroup]

On mach 3 can you use dro mode with steppers?

If you're asking if you can use encoders to read machine position, then yes. If you think this provides closed loop, the answer is no. It simply displays what the encoders are reading - same as for servos. Mach3 is NOT a closed-loop controller, and with a properly-designer stepper system, there is no need for closed loop. That's one of the nice things about steppers - if you design the system correctly, you tell it where to go, and you can be confident it WILL go there, as long as you operate within its capabilities.

Regards,
Ray L.

[Mariss Freimanis]

Small servomotors have to turn at high speeds to generate power. Power is torque times RPM and servo torque is typically 15% of what you get from the same size step motor. That means the motor has to turn 6 - 7 times faster to get the same power. Some numbers:

NEMA-23 servomotor: 6,000 RPM, 30 to 60 in-oz of torque.
NEMA-23 step motor: 1,000 RPM, 200 to 400 in-oz of torque.

You have a 5 TPI leadscrew. You can direct couple (1:1 ratio) the step motor to the screw. To get the same leadscrew torque, you have to gear the servomotor down about 7:1. This makes the mechanism more complex.

Mariss

[NC Cams]

Check sht eFAQ section of CNC ZOne, There was a detailed dissertation there on steppers vs servo's. LInkes anyone? I lost mine.

[BobWarfield]

Performance is one aspect, but I wouldn't say it's the only advantage of servos over steppers.

Consider what it means to know your servos are tracking as expected via closed loop (doesn't have to go all the way back to Mach3, just has to fault when its wrong). On my IH mill, I will get a servo fault if the servos couldn't keep up with the commands by more than 128 encoder counts, which amounts to 9 thousandths. Not only will it fault if it gets off by that much and stop the machine, but if it gets off by less, it will endeavor to "catch up".

Now what happens with steppers? Remember lost steps are not all or nothing. You can lose a few here, a few more there, and so on, especially on a light machine. All that has to happen is for the cutter to bog down temporarily.

With servos, it has a shot at getting back on track. With steppers, that lost motion is forever lost. The machine will keep trying, and it will be further and further off as it loses steps. This may cause it to veer unknowlingly into clamps, vises, tables, or whatever.

Putting aside the catastrophic crash, even just figuring out where you went wrong (perhaps your feeds were too aggressive at one point) is going to be harder. Once you lose the steps and the program keeps going, who knows where it got into trouble by the time it ends? If you fault, you can be pretty sure something bad was happening right where the cutter was when it faulted.

Not trying to say you can't do great work with steppers--I have both steppers and servos on different machines. Just that there are some more advantages for servos.

Cheers,

BW

[sergizmo]

If there is no mechanical binding and the machine is being used within the capabilities of the structural rigidity of the machine itself, HP at the spindle and power provided by the steppers, there will be no lost steps.

Let's get real here, it's an X2. This is a 110 pound Chinese made machine, not a Haas. Steppers will not be the limiting factor to the machine's performance.

Plus with steppers you don't have to worry about:

1) Having both home and limit switches. These are a must with servos. If one of them fail, it could be a disaster. Steppers just torque out against a hard travel limit.

2) Encoders, tuning, etc...

3) Runaway.

4) Extra cost.

5) Extra time to get all the above sorted out.

[BobWarfield]

Yeah, I know sergizmo. It's not like anyone would ever stick an X2 in a full enclosure, run flood cooling, and then build a rotary toolchanger for one, is it?

Oh wait. I guess they do. Forgot about hoss.

Cheers,

BW

[sergizmo]

Yeah, and what's Hoss running? Steppers.

[accuracymark]

Thanks for the input, things to think about. I am used to just worrying about my setup sheet, vises or fixtures, tooling and hight offsets, The machines where all good to go. Never really thought about stepper or servos and how they work. I have some learning here for sure.
Brent

[cyclestart]

EMC2 can track an encoder and shutdown when missed steps are detected. Mach3 can probably also do this although an extra piece of hardware might be needed, at least for less than a full rotation (?).

MicroProto has a closed loop system for steppers but their site has removed much of the information. Anyone using this ?

Then there is the cncbrain and possibly smoothstepper(?)

In any case, I agree a simple open loop system is fine for an X2.

[SCzEngrgGroup]

Performance is one aspect, but I wouldn't say it's the only advantage of servos over steppers.

Consider what it means to know your servos are tracking as expected via closed loop (doesn't have to go all the way back to Mach3, just has to fault when its wrong). On my IH mill, I will get a servo fault if the servos couldn't keep up with the commands by more than 128 encoder counts, which amounts to 9 thousandths. Not only will it fault if it gets off by that much and stop the machine, but if it gets off by less, it will endeavor to "catch up".

Now what happens with steppers? Remember lost steps are not all or nothing. You can lose a few here, a few more there, and so on, especially on a light machine. All that has to happen is for the cutter to bog down temporarily.

With servos, it has a shot at getting back on track. With steppers, that lost motion is forever lost. The machine will keep trying, and it will be further and further off as it loses steps. This may cause it to veer unknowlingly into clamps, vises, tables, or whatever.

Putting aside the catastrophic crash, even just figuring out where you went wrong (perhaps your feeds were too aggressive at one point) is going to be harder. Once you lose the steps and the program keeps going, who knows where it got into trouble by the time it ends? If you fault, you can be pretty sure something bad was happening right where the cutter was when it faulted.

Not trying to say you can't do great work with steppers--I have both steppers and servos on different machines. Just that there are some more advantages for servos.

Cheers,

BW

Bob,

By definition, if you lose steps, you've either under-designed your steppers, or you're over-driving them. On a small machine like an X2, it is very easy, and inexpensive, to put on steppers that FAR exceed the capabilities of the machine itself, and lost steps become a non-issue. Many people just don't take the time to understand how much power they need, and end up with either too-small motors, or weak drivers, or too-low supply voltage, etc. With a properly-designed, properly-programmed stepper system on an X2, you should NEVER lose steps.

Regards,
Ray L.

[sansbury]

Now what happens with steppers? Remember lost steps are not all or nothing. You can lose a few here, a few more there, and so on, especially on a light machine. All that has to happen is for the cutter to bog down temporarily.

Steppers don't "lose a few here, a few more there" the way an old car occasionally backfires. Lost steps are a sign that you're doing something wrong. I realize you don't mean it this way, but this sort of talk promotes the myth that "steppers are bad because they can lose steps."

With servos, it has a shot at getting back on track. With steppers, that lost motion is forever lost. The machine will keep trying, and it will be further and further off as it loses steps. This may cause it to veer unknowlingly into clamps, vises, tables, or whatever.

This reminds me a little of those late-night infomercials for kitchen gadgets where they show someone trying to flip a pancake with a spatula and it always ends up splattered all over the wall. "What a mess!" Realistically if you are a newbie, you're going to crash for so many reasons other than lost steps that you should never be more than a few seconds away from the Oh-$#@! switch.

Putting aside the catastrophic crash, even just figuring out where you went wrong (perhaps your feeds were too aggressive at one point) is going to be harder. Once you lose the steps and the program keeps going, who knows where it got into trouble by the time it ends? If you fault, you can be pretty sure something bad was happening right where the cutter was when it faulted.

Theoretically this is true, but I really wonder if this actually comes into play in practice. Usually you can look at the workpiece and see where things went pear-shaped pretty easily. If it's a first-time program, then the easiest thing is to sit there and watch it run.

I'm finishing hobby machine build #4 (an X2) and of the 9,743 different things that have caused me to temporarily hate this choice of hobby, servos might have helped on about seven of them. On an X2, the only thing servos might offer is a higher G0 for bragging rights on this forum. The sliding ways are not meant for 100IPM nor is any other part of the machine.

My take is that if you're a newbie and you and your buddy can lift the machine off the bench, don't even think about servos, it's a complete waste of time/money/energy.

[The Blight]

The good thing about having some sort of feedback on each axis is that if it crashes, the machine will stop.

I get 80IPM on my X2 with stepping motors. The limiting factor is TurboCNC. Servos might be overkill for this machine, but some kind of feedback would be nice. Because this machine has dovetails, it can stall if you forget to oil it once in a while. The bearings on the screws can break. Anything like that will put the machine out of position. If the machine has no way of knowing about this, it might ruin something. Like plunging down into the vice. Breakdowns do happen. So if I had a choice, I would mount encoders on all axis. Just as a way of telling if something goes wrong. Might even be able to save the workpiece.

But in the end. I think you should go with stepping motors.

[Mariss Freimanis]

The only encoders that "cannot lose position" are absolute encoders. As an analogy, take a linear slide potentiometer and put an ADC (analog to digital converter) on it. You now have a half-baked absolute encoder. Move the slider back and forth, turn power off and move it around some more. Turn power back on and the digital output still reads the correct position. Real absolute encoders cost an arm and a leg.

Most encoders are incremental. They require a "counter" that keeps track of position. By analogy, imagine you deposit and withdraw $1 at a time from a bank account. The bank accountant (bean counter) keeps track of the deposits and withdrawals to maintain an accurate balance. The problem is this accountant keeps the sums in his head. If he loses his memory, you lose track of your balance.

Every time you turn power off, the accountant loses his memory. You turn the encoder while power is off, the deposits and withdrawals are not recorded. You trick the accountant by withdrawing $10 instead of the expected $1 (electrical noise), your account is and stays out of balance thereafter. All these things can and do happen with incremental encoders.

Mariss

[Al_The_Man]

This whole argument about Steppers versus Servo's is trotted out every so often and by now I would have though that some kind of common sense would have prevailed, yes there IS a difference between the two and for Your particular application and needs, your technical ability and your budget and based on the experience of others using the same machine you intend to build, Steppers will most likely fill the bill, if so, for you the problem is solved.
The fact that I personally use Servo's exclusively does not mean that my intention is to dissuade anyone from using Steppers if they indeed do the job for you.
What I would recommend however, is for those that attempt the foray into CNC control as more than just a means to an end to obtain a working machine, and want to explore all the possibilities out there and have the wherewithal to be able do it, is to become much more familiar with both technologies.
This way you can truly make an assessment based on your personal preference. Instead of me or anyone else telling you that Steppers/Servo's are better.
There are technical difficulties to overcome with both methods, so it behooves you to make sure you are up to the task.
But that is what this Forum is for.
Al.