The best way to check a motor for good linearity is to listen to it run as its speed is slowly increased. The smoother the sound, the better the linearity.

I was lucky. I had a bunch of PK268-02AA motors on hand that have excellent linearity when used with a G203v stepper driver. (They can also be driven by other Geckodrive products with excellent results.)

AFTER the stepper drivers were tuned to the motors (read the instruction sheet that is available on the Geckodrive website for instructions on how to tune the stepper driver), those motors ran about 2X faster with much better torque than they had run with the Oriental Motor Unipolar driver that had been packaged with the motors. Unfortunately, those size 23 motors are too small for the MechMate, but they were a standard to which I could compare size 34 motors.

I had several different size 34 stepper motors on hand. I found that it was hard to tune the round motors to the Gecko stepper drives. The "square" motors worked much better. My favorite motor is the PK296-F4.5 followed by the PK299-F4.5. Both of those motors can be wired half-coil or parallel. Both of those motors, WHEN DRIVEN BY THE CORRECT VOLTAGE had advertised torque and excellent speed. Both motors had almost no 'dead spots' on the speed curve, but I'm almost certain that both motors have resonance in the lowest speeds even when stepper driver tuning has been completed.

That's why I've always recommended Oriental Motor stepper motors and Geckodrive stepper drivers. That combination has always worked for me (as long as the motors were the "square" design). Other motors have worked very well. Mike (Metalhead) had me test the motor that he recommends. It was an excellent motor. Based on the test of that single motor, I would recommend it to anyone who either couldn't get the Oriental Motor steppers or where the nearly double price of the Oriental Motor steppers was a big factor.

As far as I understand stepper theory, linearity is highly dependent upon the load and the acceleration rate used. Ramping too quickly or too slowly can affect linearity. Trying to drive too heavy of a load can affect linearity.

According to the experiences of others, a motor that has about 600 oz*in will work very well with the MechMate. If a belt-drive is used, then a motor producing 300-450 oz*in of torque works very well, depending on the gear ratio. Those motors are "middle of the pack" motors. When you study the torque curves of those motors, you'll see a very smooth torque curve EXCEPT of the resonance part of the curve. In traditional applications, the pulse generator would be configured to ramp through that resonance part of the curve as quickly as possible to avoid resonance problems. In a CNC application, unless you are cutting lines along the x-axis or y-axis or if you're cutting a 45-degree angle, you may face resonance issues, i.e. possible chatter when one axis is moving in the resonance range of the motor.

(Read the discussion above, especially the test that Gerald recommended to test for resonance to see how we may be able to 'quantify' the amount of resonance in a motor and the speeds at which that resonance occurs.)

Seeing it on a scope would be pretty, but not essential. For me it is enough to know how many pulses the encoder output lags behind the input under certain conditions, and to know what conditions cause the max lag.

When cutting a gentle curve with two motors, one of the motors may not actually be driving - that motor could be acting as a brake, counteracting the sideways cutting force of the spinning cutter. The braking motor changes position by "slipping" or "releasing" a microstep at a time....in theory that is. On top of this we have the motor wanting to go into a natural detent position (What we feel when we turn a motor by hand). I have always been suspicious that microstepping drives could really produce smooth motion under these conditions.

I've been playing with a spreadsheet to show the number of steps that a MechMate would move as a circle is being cut. The PDF shows the first few lines of that spreadsheet when a 5" circle is being cut. Remember, a stepper motor can only make full steps. Just as Gerald said, one axis is not moving (and possibly going to its natural detent position).

The top portion of the spreadsheet shows how far an axis moves for each step when a PK296A2A-SG7.2 geared motor is used.

The bottom portion of the spreadsheet shows the distance that each axis moves per angle increment (as specified in the top portion of the spreadsheet).

From this little example, it looks like the "chatter" could easily be caused by "natural detent" + multiple "moves" with no actual movement on an axis.

I had to travel about 100 miles to fix a problem at one of my computer customer's installations, so I had time to think a little more about "chatter" and about full-step vs micro-stepping.

Let's assume, that under certain conditions, that the 0.000273" micro-step position can be out of place so that it turns into an 0.00273" error (although it would probably be 1/2 of that amount because the motor would probably "slide" to the closest natural detent).

Let's assume that BOTH axes could "slide" into full step positioning under certain conditions.

Then, looking at the height of the "chatter", does it look like the "hills" and "valleys" are about 0.00273" high (or low)?

I no longer have a CNC machine to play with, but when I had my Shopbot PRT-Alpha with retro-fitted 7.2:1 geared motors that had 1,000 steps per revolution instead of 2,000 steps per revolution, I remember that the "chatter" was minimal, but still clearly visible. Just how much chatter was there? I don't know, but there was enough that I had to do some hand sanding before I could send the work out for painting.

Given the fact that we may have discovered what causes most of the "chatter" and given the fact that no one is going to throw away his stepper motors and stepper drivers to solve the problem, what can be done to minimize the problem? Think of motion pictures and persistence of vision. When we watch a motion picture, we're really seeing "still" frames racing by at 24 or 25 frames per second. Our brain can't process 24 or 25 frames per second, so we have "moving pictures" caused by persistence of vision.

Perhaps that same basic principle could be used when cutting curves and circles. Most stepper motors have listed (by symbol on the torque charts) the maximum starting speed for that motor without ramping. Let's say that that speed is 400 full steps per second. With a Gecko stepper driver, that means that we could try running that motor, without ramping, at 4,000 steps per second and see if it stalls or if it runs smoothly. If it runs smoothly, then, in theory, we could run that motor at 4,000 steps per second without ramping. Looking at the chart in my post above, that means that would could cut at 1" per second without ramping. Using the "persistence of vision" principle, we could try cutting at 1" per second and hope that the natural vibration of the machine, of the motors and of the cutter would produce the "smoothest" possible cut, that that speed would let everything work together.

There is one more thing that much be addressed. The CAM program (Vectric V-Carve or whatever you're using to create the tool path) should be calibrated so that the minimum move is equal to the distance that the stepper motor moves an axis each time the stepper is "stepped". Trying to get 0.0001" moves from a geared motor that could only produce 0.000275" of resolution will not work. On the other hand, setting the software to create arcs with 0.01" segments would not let the motor do all that it is capable of doing.

Mike

Where in Vectric software does it allow you to calibrate the software to the motors? I use Aspire.

I don't believe that is available.

Nils,

I don't know. I'm 100 miles from my home so I can't check things from here. I'll post when I have a chance to look at the V-Carve settings.

This is an interesting discussion on motors. Thank you for sharing your expertise with us.

Nils,

I've got egg on my face. The Circle Segment Resolution is part of Shopbot's SB3 program. I was certain that V-Carve Pro also had a parameter setting, but my old mind failed me, proving, once again, that memory is fleeting.

However, I ran some circle tests on my test bench with my two "best" motors, a PK299-F4.5 and a PK296-F4.5 attached to the X-axis and the Y-axis. I drew a single circle in V-Carve Pro and then created a tool path file. I exported the file as a Mach-2/3 compatible file. The circle had an 8" diameter. V-Carve Pro created a file that used four arcs to cut the circle, using the "I" and the "J" parameters.

When I ran the cut file, there was a very distinct "stutter" from the motors at the ends of each arc. The motors ran smoothly, as expected, when they approached the 45-degree portion of each arc.

Of course, this test was not scientific, but my fingers and my ears detected harmonic problems when cutting the circle, which probably would have caused "chatter".

I looked some saved posts from the Shopbot forum where I had posted some circle test cuts (years ago). The "chatter" on those pieces fairly well matched what my fingers felt and my ears heard.

Mike,

Since this has been an interest I have had for some time, where is the chatter coming from. It sounds as though you have determined that the software it self is part of the problem.

I also notice there is a jerking or stutter while cutting but I have attribited to the machine.

If this is the case how do we fix this problem? Pick a different post processor in Aspire.

The Vectric people are not eager to admit there are problems in my personal experience but I am willing to address the problem.

What do you suggest or do we need to do more testing.

MetalHead or domino11, may be this motor topic needs its own thread and we release it from Kim's build thread.

Nils,

I don't think that there is much that can be done with either the software or the motors.

We use stepper motors because they are accurate and inexpensive. One of their inherent problems is that they "resonate" at certain low speeds. That is a characteristic of stepper motors that can be minimized, but not eliminated, at least if we need both low speeds and high speeds.

The CAM software (tool pathing) can be optimized (by the software engineers who wrote the software) so that it generates arc segments that are suited to the minimum step distance of the stepper motors.

The big problem, at least as I see it, is that the tool path for cutting arcs (circles and curves), if written as straight-line segments, would require partial steps most of the time. A stepper motor cannot make partial steps. It either makes a step or it doesn't. Add the "resonance error" to the equation, and sometimes that stepper motor will make or not make that step because of electrical/mechanical "bouncing".

In normal cutting (straight lines), resonance is not a problem. The stepper motor shoots right past that portion of its speed curve and no problem is ever detected. However, cutting curves and circles keeps one or both axes in that partial-step and "resonating" portion of their speed curve - and we get chatter.

What about using a servo-motor? Well, there are some huge problems with servo motors. I have one large servo motor that is about 3X longer than a PK299 stepper motor. It only develops 225 oz*in of torque, so it must be geared at least 3:1 to handle the expected load. It requires a 7A, 70VDC power supply to operate. It requires an encoder. If something happens to the encoder, it will move the axis at the fastest speed possible until it crashes against the stop. So, it would take 4X the power supply, at least 2X the cost, and it would only solve the "resonating" problem. A servo, just like a stepper, cannot make partial steps (at least not the brush type steppers that most of us would use).

One of my computer customers had a new sign made that had acrylic letters glued onto an acrylic base. Those letters were cut on a "big iron" machine that cost over $300,000. Guess what? Those letters had about as much "chatter" as the same sized letters would have had on my $10,000 Shopbot.

So, I wouldn't worry too much about eliminating "chatter". My goal would be to see what depth of cut and what speed minimized the problem. I would also see if a "clean-up" cut would help reduce the "chatter". After finding the best possible solution, I would just live with it and use sandpaper as necessary to handle the remaining "chatter". That's basically what I did when I was cutting. I gave my customers the choice of sanding the work or having me sand it at $30 per hour. Most of them sanded the parts themselves, after all, they had to do some hand work before painting. As long as we agreed on the quality that they could expect and the amount of hand work that would be required, everybody was satisfied.

Every machine has limitations. We can try to make the machine do something that it cannot normally do, or we can find the limitations of the machine, and then use another process to handle the remaining problems.

I had a situation about two month`s ago where I had a client that wanted some arcs cut.I did the job and had some chatter that can be sanded.The client wasn`t happy and went to another chap that cuts with a multicam machine with vacume and all story`s .They had a lot to say about the chatter but while back at the Ranch there is 3 workers sanding away.Moral of the story anything to say when competition is lurking around the corner.

Sean D. I think you would know who I am talking about.They do what you do in your part of the world.

Consider the money & effort to build MM range of machine. Asking for 100% chatter free cut all the time on any material...
I consider that irrationally paranoid.

This discussion has been repeated many times. We have yet to prove, and accept, that micro-stepping of stepper motors simply doesn't perform perfectly under all conditions. All of our theory on cut quality is based on about 2000 microsteps per revolution. If that is not true, then it will be useful to know how far our motors can drift away from the theory.

If I understand what Mike is saying - chatter occurs at the resonate frequency/rpm of a given motor. Any attempts at minimizing chatter, therefore, should revolve around keeping that stepper from turning at that rpm. This resonate rpm, again if I understand his reasoning, tends to be on the low end of the motor's range, hence we see chatter in an arc or circle when the requirements of the cut dictate that one motor or the other moves very slowly. Is that correct?

If so, how about finding a way to never allow a motor to spin below that resonate rpm - i.e., always keeping it at or above whatever sweet spot it might have? I don't know how practical this might be, but how about using an electronic clutch that could be programmed to "slip" when slower cut speeds was needed?

Brian
Taxachusetts

It often can be heard, that rough motion. In those cases I up the feedrate a bit to avoid the spot where it resonates. It helps a lot to listen carefully sometimes on test cuts.

Brian, when you cut circles, the speed of a motor has to go from zero to max, and it has to cover all speeds. You cannot decide to stay away from a certain speed.

PS. I don't think this has anything to do with a motor's natural resonance. I think we are talking about a motor's ability to use micro-steps to fight its inherent detents. We see an oscillating battle between the micro-steps and the magnetic detents and this oscillation sometimes resonates with the external machine's structure (gantry, etc.).

Classic resonance of a motor is internal to a motor and independent of external structure stiffness. No?

Well to just let you all know.

I got the 2 Leadshine AM882 Drives today, hooked them up, and viola, no more resonance in the motors.
I actually had to take a look at the motors to make sure they actually moved when I tested them off the rack..

Incredible drive, which has a lot of settings. The fact that it can be fine tuned via the computer, just makes for an even more versatile drive.

I can highly recommend this drive for those having high humming sounds from their motors at low speeds.

Whether we use a Geckodrive that has 10:1 microstepping or whether we use the driver recommended by the stepper drive manufacturer (and get 200 steps per revolution), we really have very little control over how a stepper motor operates.

IF the motor "slides" to its nearest "detent" position (+/- 5 microsteps), there is really nothing that we can do about it. It is a characteristic of the machine.

We need to learn the machine's characteristics and then we need to realize that EVERY machine has limitations. My Delta Unisaw is NOT 100% repeatable. When I move the fence, I might be "off" by 1/16" or 1/32" because of the way that I see the numbers OR the way that the fence "settles" when I push the handle down. My bandsaw is even worse. I can clearly mark the cut line on the material, but the bandsaw blade may "wander". Even when I'm sanding with my 10" disk sander, I may push too hard and get a "flat spot" on the material.

A CNC machine has inherent "characteristics". Ignoring those characteristics will not improve the cut. Blaming someone else for those characteristics will not improve the cut. Learning that there are limitations to what the machine can do will let us tell the customer what to expect and then bill accordingly.

I wish that stepper motors did not have "resonance". I wish that every step of a stepper motor moved the axis exactly the distance expected; but, that is fantasy. Peter Pan lives in fantasy land; we do not. We need to learn the limitations of our tools and then accept those limitations. Endlessly trying to make things perfect will destroy our budgets and take longer than the time allotted to each of us on this earth.

And Mike I agree with you! Why does everything always need to be perfect?

Mike

That might be true, but never the less.
The Drives we opt to use, makes all the difference.

I used cheap Chinese Drives from Wantai motor, and those have a poor microstepping control I found out. My motors made heaps of resonance at low speeds.
Now after I mounted the Leadshine AM882 Drives, same configuration, same microstepping, and same 5% of maximum speed, the motors now have no resonance at all. That proves to me, that the quality of the drives do have something to say.

So it comes down to expense, how much do you wish to spend on getting a smoother driven motor on your CNC.

"Quality only costs the difference" is a Danish saying "I think", and it proves itself right this time atleast.

Kim,

Yes, it's always best to buy as much quality as possible.

I know nothing about your first set of drives or of the drives that you're using now; but, I have had the same experience with non-gecko vs gecko drives. The Geckos give much better performance than the older drives that I was using.

In my world of process control applications, the Gecko drives made everything easy and smooth. In the world of CNC routing, even the Gecko drives don't seem to be able to completely compensate for the stepper motor's characteristic resonance; but, because I have no other stepper drivers that work better than the Geckodrives, I would still use those drives if I built a MechMate, simply because I have those drives on hand.

My old drives were from longs-motor on ebay, the DM860A drives.
Apparrently they should be good, if you ask longs motor, and wantai motor, who are the manufacturer. But these drives are far from good, the microstepping or chopping in the drives are just horrible, they do make the motors move, but the motors resonated like hell.

The new drives are Leadshine AM882 Digital drives, which comes with a setup of 9 different types of motors from leadshine already programmed into it. If that's not good enough, then you can finetune the drives from the computer with a program that Leadshine applies free of charge.

I only hooked up the drives, and then set them for a Leadshine motor similar in size and frame as the ones I had. I powered up the system, and the motors ran ultra smooth and almost completely quiet. best buy ever. Easier to setup than the geckos, no need to fiddle with resistors for current control. All can be setup using dipswitches on the drive, both current setting, and misrostepping setting can be set using the dipswitches to... All in all, a good drive, at around the same price as a Gecko. They are somewhat larger than the geckos, but ever as good, and as far as I'm concerned, easier to setup...

http://www.leadshine.com/Product_Show.aspx?ID=44

This is the drive.

Here's a picture of the cut quality of my machine with only the new drives on the x axis. Notice that this is an eliptical small sign. I have only touched up the edges to remove the flossed sharp edges.

Hence my suggestion to use an encoder and measure the quality of a drive system.

Gerald,

I agree that running some tests with a good quality encoder and the other proper hardware and software devices would show whether or not the stepper driver was filtering out the resonance, but the tests would not change the way the cuts look.

I thought long and hard about building the things that you suggested, but when I put pencil to paper and added up the costs (minimum of $300 for parts) and the time (proabably 5 to 10 days), and then knowing that the cut quality would remain the same. I couldn't justify the expense or the time. That's the reason that I've written those posts about finding the limitations of the machine and then living with the limitations.

Only Gecko can change the resonance problem. Mariss has done an excellent job for the money that he charges. I talked with him on the telephone twice and visited his home in California once. He's as knowledgable a guy as I have ever met, but CNC machines are not the main market for his stepper drivers. He builds stepper drivers by the thousands for process control, where various axes move the same distance 24/7.

The least expensive test would be to put some Geckodrives on Kim's machine, tune the drives, make some cuts, and then repeat the process with the drives that he uses now.

If anyone has four G203v drives in Europe that they could send him for the test, it might help everyone decide which stepper driver to buy.

I don't notice much in the way of stepping on a 4:1 belt driven system (G540 and Keling 960 oz.in motors driven parallel) at any speed. I phoned Mariss years ago when I was building a Mechmate and he said to hook up a mirror to the shaft and map a laser pointer's position against a wall tracking the position of hte micro-steps.

The KFlop does stepper feedback and with 1000ppr encoders popping up on Ebay it's looking like a good option. The really juicy bit however is that you can compensate for the micro-stepping non-linearity of the steppers even with the active feedback present, but the holding torque of the stepper will still be strongest around the full step so for slow high load cutting or acceleration I could see why the position would deviate.

Hi Kim
I am very inspired with your builds.

I know it's been awhile since you posted this. But do you still offer this set?
Or have I read it all wrong.
Best
Mac

No, I do not supply these anymore.

They would probably be atleast twice the price if I were to order them again.