New guy questions

[wizard]

Hi tom;

Look below for a few comments.

Originally Posted by tr4252 Hi, My name is Tom, and I'm just getting started. I have a few questions, and would deeply appreciate some advice. I run a Tool & Die shop, but have little CNC experience except a programming class and several years as a draftsman, using autocad. I'd like to get myself started with a sort of entry level set up, 3 axis, something not too expensive. I have a small vertical milling machine, and a somewhat specialized grinding machine; more on that in a moment.

If you are interested in keeping expenses low consider using Linux and EMC as your CNC controller of choice. EMC is good due to the source code being available, thus if you have the back ground you can adapt it to unusual implementations.

EMC is not however the simplest of NC programs to run on a PC. In a nut shell that may not matter as you cna easily set up a dual boot machine to run EMC/Linux and what ever you need on the Windows boot side.

As far as I can tell, I'll need an extra computer, software, step motor driver(s), and servos. For the sake of simplicity, let's consider the rest my problem. If I may, I'd like to categorize my questions;

While an extra computer is nice, if you already have one that is fast enough you may be able to get buy with it for awhile. This can keep expenses low during the learning curve. YOu may however not be able to use that computer for other things while it is doing your NC work.

1. I need to educate myself; what books, websites, and other resources would you recomend?

Considering that application you mentioned in a following reply I'd suggest getting your choosen NC software /PC combo to gether and implement a cheap mill to learn on. The good books have already been mentioned. As to Web sites www.linuxcnc.org is a good one even if you don't choose it asyou controls program. Sherline has useful info and web searches can bring up others.

2. What software would serve a newcomer like me? I've been "lurking" and get the impression Mach3 is pretty good. Any suggestions for a first timer on the software? Is there anything inexpensive I could use to get started? I'd like to use autocad files, of course, where applicable.

The best software is the package that best suits you needs. Frankly I can't answer that right now. You make mention of a grinder as being you application, I have to wonder a bit about how automated you are going to be able to make this machine with low cost hardware.

The issue as I see it is dressing the grinding wheel and constantly updating the NC control as to its size. This can be done of course, but I just don't know of examples at the low end of the spectrum. You may be able to come up with a way to do this manually. Of course if you are using some sort of belt grinder then I just wasted a couple of paragraphs.

3. I've seen a lot of driver components advertised and discussed, as well as step motors. Where does a guy start the selection process? I know what the mechanical requirements would be, but as to compatability, etc., I'm a bit lost. Would a motor/driver package from one source be the way to go?

There are advantages to single source purchases. The big disadvantage being cost, though this is not always an issue.

As I see it you have three low cost driver choices.

The first is steppers which are completely open loop and are normally driven by step and direction pulses. This is generally as cheap as one can go, further you have broad compatability with PC hardware and software.

Then you have servo motors that in effect emulate steppers by being driven by step and direction signals. In most cases these would be considered open loop. They are also about as flexible as the stepper solution and slightly higher in cost.

Finally servo motors that close the postion feedback loop through the NC controller are available. Generally this is a bit higher end and requires additional PC hardware. There is the potential for better performance and more flexibility going this route. The big disadvantage is the cost is not as controlled. You also have to be much more careful here about hardware and software compatibility.

4. As a first project, I'd like to turn two lead screws simultaneously, on the above mentioned grinder, in increments of about 6 degrees per pass. The unit is already equipped with limit switches. Can this be done with step motors, without the need to have a computer controlling the operation?

You could certainly do so with out a computer. Some have already mentioned hardware to do that. I'd suggest though that a computer might be the easy approach to generating manual pulses. That might imply a need to program a bit to turn the computer into a manual control pendant.

In any event if you are just in the prototype stage consider implementing a CNC contoller with free software. Just about all of the CNC software can be operated in MDI mode to allow manual postioning of the axis. As your prototypeing progresses you can start to use the CNC controller as more of a NC control and less as a pulse generator. The only downside is if your prototype work leads to the discovery that you need a different CNC package then the controls cabinet might require a minor retrofit to support the software change.

I know how irritating ignorance can be, and am openly displaying my own, but hope some of you guys will find a few moments to respond.

You got it wrong I and I'm sure others, have already learnt something new as the result of your quesitons. So fire away.

The nice things about these forums are the many perspectives that come to play. My exposure to CNC was years ago in the plant I work in, almost all of that on precision lathes. It has now been a few years since I worked on such equipment, later I got interested in some of these DIY-CNC projects. It is very interesting to see how people have implemented different solutions.

Tom

Thanks
Dave

[tr4252]

The response to my first tentative inquiry has opened up a wealth of information for me, and I'm deeply grateful to you all.

So far, you've saved me a lot of trial and error time, in the following areas:

1. I should bypass the intermediate step of going sans computer control for the sake of simplicity and lowest cost. For the slight additional price, another inexpensive desktop won't be any problem. And since I want to learn from this experience, I might as well jump in with both feet.

2. The references provided have already given me a basic knowledge of what I'm going to do, saving me from wasting effort on the typical dead ends a guy like me would no doubt pursue before he got more familiar with the subject.

3. Finding a source of expertise and advice. I don't even remember what convoluted path actually led me to this website, but I'm really glad I found it.

Thanks,

Tom

[NC Cams]

The following should answer pretty much most of the inevitable questions that may arris re: servos versus steppers:

A reply from Mariss Freimanis in CNCZone.com that addresses the recurring "stepper versus servo" inquiry as posed by Freak Brain (with editing for spelling and apparent typo’s by NC Cams):

Step motors and servo motors service similar applications, ones where precise positioning and speed are required.

The biggest difference is that steppers are operated "open loop". This means there is no feedback required from the motor. You send a step pulse to the drive and take on faith it will be executed. Seems like a problem but it's not.

If you have a quartz watch with hour and minute hands, then you have a step motor on your wrist. The electronics generates 1 step pulse per second, driving a 60 step per revolution motor which turns at 1 RPM. It keeps nearly perfect time. Any errors are due entirely to the electronics timing accuracy (quartz crystal oscillator).

With apologies to David Letterman, here's some "Top 10" lists I came up with for choosing between steppers and brush DC servos. Others can add more, I'm sure.

Top Ten Stepper Advantages:

1) Stable. Can drive a wide range of frictional and inertial loads.
2) Needs no feedback. The motor is also the position transducer.
3) Inexpensive relative to other motion control systems.
4) Standardized frame size and performance.
5) Plug and play. Easy to setup and use.
6) Safe. If anything breaks, the motor stops.
7) Long life. Bearings are the only wear-out mechanism.
8) Excellent low speed torque. Can drive many loads without gearing.
9) Excellent repeatability. Returns to the same location accurately.
10) Overload safe. Motor cannot be damaged by mechanical overload.

Top Ten DC Servo Advantages:

1) High output power relative to motor size and weight.
2) Encoder determines accuracy and resolution.
3) High efficiency. Can approach 90% at light loads.
4) High torque to inertia ratio. Can rapidly accelerate loads.
5) Has "reserve" power. 2-3 times continuous power for short periods.
6) Has "reserve" torque. 5-10 times rated torque for short periods.
7) Motor stays cool. Current draw proportional to load.
8) Usable high speed torque. Maintains rated torque to 90% of NL RPM
9) Audibly quiet at high speeds.
10) Resonance and vibration free operation.

Top Ten Stepper Disadvantages:

1) Low efficiency. Motor draws substantial power regardless of load.
2) Torque drops rapidly with speed (torque is the inverse of speed).
3) Low accuracy. 1:200 at full load, 1:2000 at light loads.
4) Prone to resonance. Requires micro-stepping to move smoothly.
5) No feedback to indicate missed steps.
6) Low torque to inertia ratio. Cannot accelerate loads very rapidly.
7) Motor gets very hot in high performance configurations.
8) Motor will not "pick up" after momentary overload.
9) Motor is audibly very noisy at moderate to high speeds.
10) Low output power for size and weight.

Top Ten DC Servo (brush type) Disadvantages (besides higher relative cost):

1) Requires "tuning" to stabilize feedback loop.
2) Motor "runs away" when something breaks. Safety circuits required.
3) Complex. Requires encoder.
4) Brush wear limits life to 2,000 hrs. Service is then required.
5) Peak torque is limited to a 1% duty cycle.
6) Motor can be damaged by sustained overload.
7) Bewildering choice of motors, encoders, servo drives.
8) Power supply current 10 times average to use peak torque. See (5).
9) Motor develops peak power at higher speeds. Gearing often required.
10) Poor motor cooling. Ventilated motors are easily contaminated.

Mariss

Subsequently another question was posed by Freak Brain:

One question....

On the Top Ten DC Servo Disadvantages you wrote, Requires "tuning" to stabilize feedback loop. Is this done in the drive or is this done in the encoder? and the safety circuits, are these in the drive or is this something I would have to buy separately. Like your drives, are these items in your drives?

Mariss’ reply to this inquiry:

Tuning refers to adjusting the PID coefficients to cause a critically damped response from the motor/load when adjusting to a disturbance. Sounds complicated but it's not.

PID stands for Proportional, Integral and Differential. The "difference" error (where you should be versus where you are) is separated into 3 channels (PID), then recombined. You perform this algorithm unconsciously when you drive a car.

Say you take a road trip from LA to San Francisco up I-5. Your task is to stay side-by-side next to the "command position" car on this trip. It can accelerate and stop instantly, you can't. Your gas pedal and brake (torque command) only adjusts acceleration and deceleration like in a real car.

At the start of the trip, both of you are stopped. The "command car" instantly accelerates to 85MPH (average to slow for what you see on I-5).

The first thing you notice is a lot of distance has opened up between you and the "command car". This is the Proportional component. You press on the gas and away we go. Your speed builds up and after a while the distance begins to close.

Your rate of closure is the derivative or Differential component. As long as the distance to the "command car' keeps opening, you press harder on the gas. As it closes, you ease up.

To close the distance, you have to go faster than the "command car". Otherwise you will never catch up.

You are now getting very near the "command car". Both the separating distance and the rate of closure decreases towards zero so they are no longer of use. You have come off of the gas enough to nearly match its speed.

This is where the Integral component comes in. You are now side by side. You now adjust your speed based on inches of separation. If you slightly edge into the lead, you ease off. If you slightly fall behind, you make up the difference. Rate of closure (differential) or separating distance (Proportional) are too small to use.

Using this method (PID), you will arrive at your destination simultaneously even though hundreds of miles and hours of travel have elapsed. You do it naturally and unconsciously.

A mis-tuned servo (not enough D or too much P) by this analogy would have you overshoot the "command car", hit the brake, fall behind, over-accelerate and overshoot the "command car" again, over and over (sometimes called "hunting" or "oscillating"). Tough on your passengers and car (400 miles, 4 hours) and equally hard on your servo motor.

All sorts of other stuff works with this analogy. Two things come to mind.

1) Feed-forward compensated PID servos. This is where you are told ahead of time what the "command car" will do. You don't have to sit and watch in surprise when it suddenly takes off or changes direction; you are fore-armed with its future intentions. This somewhat makes up for time otherwise lost in catching up.

2) S-shaped acceleration/deceleration profiles. This keeps the 2nd derivative of velocity (namely jerk) finite, minimizing the "jerk" factor. Again, you do this naturally driving a car.

(The rate of change of the motion is velocity or first derivative of motion. The rate of change of velocity is acceleration or the first derivative of velocity and second of motion. The rate of change of acceleration is jerk or the first derivative of acceleration, or second derivative of velocity or third derivative of motion. Got that??? If you do, you understand elementary calculus.).

Imagine you are cruising down a boulevard when the light up ahead changes from green to red. In a simple CNC program, you stand on your brake until you come to a stop. This would be very uncomfortable in real life.

When you decelerate in a car, you tense your muscles to balance against the deceleration G-force until you just counteract that force. When you come to a stop, deceleration abruptly disappears along with its G-force. Your tensing against it does not though. The result is head and body bobbing back and forth until you find the new balance. Not comfortable.

What you actually do when coming to a stop is to tail-off on the brake pedal. Before coming to a stop you lessen the rate of deceleration (brake pedal pressure) to make it become zero as your speed approaches zero. This is an S-shaped deceleration curve.

Where it matters on your CNC machine is it eliminates ringing (head bobbing) at the beginning and end of acceleration and deceleration. This decreases wear and error.

Mariss

Additions by NC Cams:

The safety circuit inquiry response is sort of a yes and no answer.

Limit switches are mounted externally on the machine. These cut power via drive disengagement should "runaway" occur.

Most servo amps can output S/D (shutdown) signals from built-in protection circuitry that tell the controller a fault occurred in the drive (IE: over voltage, over current, rpm runaway, loss of feedback, etc).

The machine tool builder is the one who is charged with installing limit switches. These are the last defense for stopping a runaway servo at some end limit point of the table/quill travel.

Further elaboration by Arvidb:

The way acceleration and deceleration is handled is up to the controller (Mach2 for instance). You won't get oscillations by setting these wrong (in the drive), but S shaped acceleration is easier on the mechanics than constant/linear acceleration.

PID parameters are set in the servo driver (e.g. the Gecko drive). Get these wrong, and you might get very nasty oscillations (vibrations or "buzzing") from the motor. These will wear out the mechanics quickly - and sound awful! This is the "not enough D or too much P" part described by Mariss.

(Mostly to Allen): PID adjustments are done to get the machine to respond as quickly and accurately as possible to a command, while still making sure the machine is stable (no oscillations).

First the difference between actual and commanded values are calculated (differences = "error"),
then a control signal is calculated as the sum of proportional (= factor * error), integral (= factor * sum of all errors measured from machine start), and derivative (= factor * rate of change of error) terms.

The three factors are the things one adjusts (usually called Kp, Ti, Td). It can be quite difficult to get them right.

Response by TORSEN:

The difference between a stepper and a servo in simple terms is the fact that when not moving a stepper is held in position by current -- servo is not.

(NC Cams comment: this is technically true BUT if the cutting or other forces try to move the table against the servo, the encoder shows motion and the servo will apply force via the application of current to return the table to the prescribed position. For a snapshot in time, there is no current flow, however this can rapidly change as the cut is made - read on)

The full current is flowing to hold a stepper motor in position this is why they are rated for a certain amount of holding torque. As the stepper starts to rotate, the torque diminishes as the speed increases.

(When) No current is flow(ing) on a stopped servo motor, its power is regulated by a error register in the driver. When a force is encountered (like somebody trying to turn the shaft by hand or cutter force tries to move the servo) the error of position on the shaft will cause a current to be send to the motor to correct the error.

The amount of current is in direct relation to the amount of error so a small error will produce only a small current.
This is like a rubber band effect when trying to move the motor and should be taken into consideration when the resolution of a system is considered.

With the ever moving improvements of the technical community both technologies have been working its way towards each other. Servos can now be controlled just like stepper systems and Multi-stepping systems with current reduction at stand still make them behave much more alike.

[tr4252]

Thank you all for the advice so far.

It seems I've had four step motors and a bunch of other hardware all along; stored away since the late 90's are two pen plotters I had used in a graphic arts business of which I was a partner. They're Calcomp Design Mates, Models 3024 and 3036. As I recall the 3024 ran well, the 3036 was always getting out of register and needed a lot of service calls. They ran on Windows 95.

I opened up the 3024, and saw that it has two beefy little unipolar steppers. They're marked;

STH-56D215
1.8 DEG/STEP 2.8V 1.1A
NO. 23552
SHINANO KENSHI CO. LTD JAPAN

They've got neat little circuit boards attached to their backs; the motor leads (8) plug into the boards and ribbon cables run from there to the main board. I have no idea what these circuit boards attached to the motors are for. In addition, there's a component which raises and lowers the plotter's pens which might be useful. Not to mention an abundance of other mechanical parts such as linear bearings mounted to pairs of rods, timing belts/cogs, etc.

So, I'm guessing that the motors, at least, might be useful. Also, the plotters must include drivers, which I might be able to use. Unfortunately, I haven't located the manuals and software yet. I have to believe that I could derive 2 axis systems from these components, if I knew how. Any ideas or comments?

Tom

[WhiteTiger]

A site that offers a pretty full range of CNC components is www.microkinetics.com You can get everything from tiny sherline turnkeys and retrofits to individual components.

The products are all compatible within power and type, and they have some affordable software for both CAD and CAM that sync's perfectly with the hardware.

The stuff seems pretty robust, considering the components I purchased from them 9 years ago is still all ticking right along and has never presented any problems.



Tiger

[tr4252]

Again, my sincerest thanks to you all; the advice I've recieved has saved me a lot of time and effort. I'm attaching (I think) a recent photo of the machine I was building when I joined this forum, and initiated this thread. I designed and built this unit for the grinding of knife blades, and it works really well. The step motor drives were the piece of the puzzle I needed to fully automate it, and I've followed the suggestions made here in that element of the design. The motors are surplus (I bought several spares), the drivers are made from kits (modified), limit switches and trimmer pots proved to be satisfactory means of control.

Now I'm looking at a Microkinetics Benchtop Mill to do some of the other operations in my projects.

Thanks,
Tom