Advice on eating the CNC elephant?

[2ManyHobbies]

I've got a 3-in-1 machine that I'm in no danger of upgrading in the next 5-7 years due to lack of floorspace that I'm contemplating adding basic CNC to over the next 2 years or so. Currently controls are a fwd/stop/rev switch and hand cranks. Feedback is only via the dials. No VFD, no scales, etc.

X is leadscrew, Y is cross-slide, Z is the mill/drill head when in use. I could see a situation where I might like to be able to add drive to a rotary table, but a 5th axis won't fit.

While I have used steppers in the past both programming an off the shelf controller and making my own, I feel myself being drawn towards servos and a closed loop setup using scales. I'm not aiming for tenths.

Is this a bright idea? Is it something I can buy one piece (or axis) at a time?

[jim_geib]

I've got a 3-in-1 machine that I'm in no danger of upgrading in the next 5-7 years due to lack of floorspace that I'm contemplating adding basic CNC to over the next 2 years or so. Currently controls are a fwd/stop/rev switch and hand cranks. Feedback is only via the dials. No VFD, no scales, etc.

X is leadscrew, Y is cross-slide, Z is the mill/drill head when in use. I could see a situation where I might like to be able to add drive to a rotary table, but a 5th axis won't fit.

While I have used steppers in the past both programming an off the shelf controller and making my own, I feel myself being drawn towards servos and a closed loop setup using scales. I'm not aiming for tenths.

Is this a bright idea? Is it something I can buy one piece (or axis) at a time?


That's what I did to my Smithy. A little at a time, one idea leed to another. Today it look's like this http://i86.photobucket.com/albums/k120/jim_geib/DSC00021.jpg

IT is Rude, crude and socially unacceptable but it makes parts

Jim Geib

[Black_Moons]

I like the billow way covers! Simple, cheap, And I bet they catch 95%+ of your swaff. (That would of landed on the bed anyway.. About half my swaff doesnt even land on the lathe soo.. )

[Evan]

Sure, you can do it an axis at a time. Servos aren't mysterious. It's just a dc motor with a feedback loop to a controller. Scales can be pricy but depending on the envelope required there is one very good deal I have found.

LMDC at http://www.lasermotion.com/servo.html has these linear scales for sale at $99 each. Five micron accuracy, LED light source, A quad B output which is directly compatible to servo controllers. The main limit is the travel which is 12.59 inches.

http://ixian.ca/pics8/linearencoder.jpg

I have dealt with them before, no issues.

[philbur]

I think you need to start asking yourself why, it's not possible to evaluate "being drawn towards".

Phil:)


I feel myself being drawn towards servos and a closed loop setup using scales.

[MCS]

While I have used steppers in the past both programming an off the shelf controller and making my own, I feel myself being drawn towards servos and a closed loop setup using scales. I'm not aiming for tenths.

Is this a bright idea? Is it something I can buy one piece (or axis) at a time?

With the feedback through scales you incorporate the play of the machine. Servo-drive, lead-screw, rest.

Which, if used on its own enlarges the position hunting of the machine. Computers are basically stupid and as a result, so are programmers.

[SGW]

The steppers vs. servos holy war makes an Islamic jihad look like a tea party.

You can certainly do servos if you want, but according to Ron Ginger, who does support work for Mach3, for "our" kind of stuff steppers are simpler, cheaper, and just as reliable if not more so. You just have to be sure the steppers are big enough so they can handle the loads and won't lose steps...not a difficult thing to be sure of. Do you know about http://www.machsupport.com/forum/index.php ? A good place to look for information. Elsewhere on that website is a PDF of an Installation and Setup manual that talks about servos and steppers, which you might find useful.

But if you want servos...have at it! Part of the fun of roll-your-own CNC is getting things to work.

[macona]

You dont want the feedback via scales as a primary feedback. Any, I mean, any lash or lost movement in the system makes it a tuning nightmare. I tried it on my mill, it has heidenhain scales on all three axis, it would hunt and the gain had to be set low to be stable.

Best bet is encoders on the motor as primary feedback. There was a controller called the CNCBrain that supported dual feedback. They seem to be redoing the control or something now.

I cant remember if EMC2 supports it, they do support closed loop.

[Evan]

You just have to be sure the steppers are big enough so they can handle the loads and won't lose steps...not a difficult thing to be sure of.

It's easier than that even. Steppers don't lose steps because of excessive load. If the load is too much they flat out stall and there will be no doubt when that happens. The only time they will sneakily lose a step here and a step there is when the driver setup is wrong. The main issue there is the timing of the step and direction pulses. If not correct for the particular driver used the system may lose a step every time the direction changes. If your steppers are having trouble dealing with the load you can just gear them down further and run slower.

Most of the servo systems that I used to work on were closed loop to the actual assembly position. Yes, it is more work to tune but as long as the system compliance is stiff and well damped and the error motions are properly constrained it will work. Those systems were optical focusing systems with main optics weighing in the range of 20 to 30 lbs, much like my telescope. Absolute position sensing was mandatory to ensure correct focus.

[sansbury]

You dont want the feedback via scales as a primary feedback. Any, I mean, any lash or lost movement in the system makes it a tuning nightmare. I tried it on my mill, it has heidenhain scales on all three axis, it would hunt and the gain had to be set low to be stable.

Best bet is encoders on the motor as primary feedback. There was a controller called the CNCBrain that supported dual feedback. They seem to be redoing the control or something now.

I cant remember if EMC2 supports it, they do support closed loop.

The CNC Brain was, IIRC, an effort to use Chinese DRO scales for closed-loop positioning with steppers. IIRC, it was a near-total failure, as there was some good reason to expect it to be. Macona is right about the hunting problem with this type of setup.

EMC2 has the ability to use encoders on the motor *and* linear scales. Needless to say, if learning CNC is eating an elephant, cnc servos are a whale, and dual-encoder setups are like trying to eat the moon. By the time you're there, you'd be better off just buying a Tormach or used VMC or something. It'd be easy to spend upwards of $5k on the screws, motors, and scales.

Mariss Freimanis of Gecko fame has written up a steppers-vs-servos article and basically says it's a simple matter of power output. Above a certain motor power, servos are cheaper. There's more to it than that, but it's most of the story. Anything smaller than an RF-45, servos are an indulgence. Anything bigger than a Bridgeport, you probably need them. In between it's a judgment call. The Tormach mill uses steppers, and seems to have many happy users quietly cranking out parts in their garages. That's what I'd use on your machine.

ADDED: I would do at least the X/Z axes so you have 2-axis milling and full CNC turning. Stick an encoder on the spindle and you have CNC threading. If you can do one axis, two is just four more wires to hook up, but actually useful. You can do a lot with simple G-code generators like Mach's wizards or EMC's subroutines. Get a handle on that and then start worrying about CAD/CAM, which makes the CNC part look easy.

[SGW]

FWIW, from the Gecko website http://www.geckodrive.com/ark-4/support.html?pid=33&id=124


Q.) What causes steppers to lose steps?

A.) A stepper will lose steps if it is being run at an RPM that is too high for the given load. This is the beginning of the drive stalling, and it is stalling on the steps that it is missing. Slow down the speed, and it should fix the problem.


Or get a bigger motor?

[Evan]

You can tell by the sound a stepper makes when it is about to stall. The sound becomes weaker and develops some high harmonics if it is running too fast or is overloaded. I have never seen my mill actually lose steps without stalling if it is overloaded or running too fast. I have also built a number of other stepper powered devices including a laser 3D scanner and several telescope drives. In general, all bipolar steppers behave the same way. Unipolar steppers are a little different as they are much less efficient.

I built a plotter with variable reluctance steppers and they are an entirely different animal. They can miss steps all over the place but won't stall since they don't contain any magnets, armature coils or brushes. To use them without feedback requires holding brakes and special acceleration and deceleration code.

[macona]

The CNC Brain was, IIRC, an effort to use Chinese DRO scales for closed-loop positioning with steppers. IIRC, it was a near-total failure, as there was some good reason to expect it to be. Macona is right about the hunting problem with this type of setup.

EMC2 has the ability to use encoders on the motor *and* linear scales. Needless to say, if learning CNC is eating an elephant, cnc servos are a whale, and dual-encoder setups are like trying to eat the moon. By the time you're there, you'd be better off just buying a Tormach or used VMC or something. It'd be easy to spend upwards of $5k on the screws, motors, and scales.

Mariss Freimanis of Gecko fame has written up a steppers-vs-servos article and basically says it's a simple matter of power output. Above a certain motor power, servos are cheaper. There's more to it than that, but it's most of the story. Anything smaller than an RF-45, servos are an indulgence. Anything bigger than a Bridgeport, you probably need them. In between it's a judgment call. The Tormach mill uses steppers, and seems to have many happy users quietly cranking out parts in their garages. That's what I'd use on your machine.

ADDED: I would do at least the X/Z axes so you have 2-axis milling and full CNC turning. Stick an encoder on the spindle and you have CNC threading. If you can do one axis, two is just four more wires to hook up, but actually useful. You can do a lot with simple G-code generators like Mach's wizards or EMC's subroutines. Get a handle on that and then start worrying about CAD/CAM, which makes the CNC part look easy.


The CNCBrain was using generic quadrature glass scales. But the guy got a real job so the brain got put to the side, seems he is still working on it on his off time, he recently posted on cnczone.

But even most VMC's dont have the dual loop feedback.

If you do go dc brushed servo dont use the geckos, they are awful. A friend has them on his router and they are noisy and a pain to set up right. If the unit faults it does not shut down. Had issue with that. Look at the drives from cncdrives.com

[skunkworks]

this is the first time dual loop was done in emc2 afaik.

http://jmkasunich.dyndns.org/cgi-bin/blosxom/shoptask/wichita-trip-02-20-08.html

I think it would be neat to try. Closed loop into emc sure is nice.

sam