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Basic CNC

jumps4

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#31
ok i'll give this a basic shot
a stepper motor is controlled by pulses (steps) the common type has 200 steps in 1 revolution or 1 step every 1.8 degrees. microstepping is dividing 1 revolution into smaller steps with electronics. 1/2 stepping the motor would double the number of pulses it takes to complete 1 revolution 0r 400 pulses or .9 degrees per pulse. the greater number of pulses the motor uses the smoother it runs but because it does not reach full current in the time allowed per pulse the torque reduces. because the pc is limited in its speed it can send the pulses through a parallel port the controller handles the higher speeds. the higher the pulse the less torque the motor has but the accuracy of the motion increases.
if you were to use a stepper motor at the 200 steps per revolution mode connected to a 5 thread per inch shaft to move a milling table for example then one inch is only capable of being divided into 1000 steps ( 200 x 5tpi )or .001 per step. this sounds ok for most uses but the downside is that at slow feed speeds the steps are a series of hard thumps and vibration can be severe at some speed due to harmonics in the motor and drive parts. this all comes back to surface finish loss. now if you microstep that same 5tpi shaft at 2000 steps per revolution then it would require 10000 steps to move the table 1 inch. the series of thumps now is a steady buzzing. the downside is to get the torque at higher speeds you may need a larger motor or higher voltage. the pulses (steps) are too fast for the motor to reach full current in the time allowed per step.
most stepper motors are marked with voltage and amps, the amps are what is required to reach the advertised torque. the voltage is missleading and can be as much as 10 times what is marked. voltage is electrical pressure and because the step is so fast the motor does not have time to reach full current we raise the voltage ( pressure ) to make the motor reach the required amperage faster.
so a motor marked at 8v may be powered with a power supply putting out 80v.
the other advantage of microstepping is accuracy the smaller we divide an inch the more accurate we can move the axis.
steve
 

jumps4

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#32
i'll add a little more
servo motor are controlled by a slotted wheel or pickup the senses the motors movement. the sensor is located on the motors shaft or the object being moved. it is a set number of divisions per revolution or distance and not changeable. for this reason stepper motors are more desireable for very fine movements. take todays microscopes to move an object at millions of an inch would be impossible with a servo motor because of the mechanical pickup. your not going to make a pickup with millions of slots for the sensor to detect. this has to be done with electronics. thats microstepping in the smallest of uses and may not be a motor that turns at all it may be a linear motor.
steve
 

8ntsane

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#33
the more torgue you need the bigger the winding needs to be nema is the standard for interchange if the requirement excedes the frame size 17 you move up to the next size 23 and so on hey you forgot my nema42 4200 oz/in that motor has a 3/4" output shaft and is rated at over 1.5hp thats a heck of a stepper motor .
steve

Steve
Just want to verify that Nema 42 motor. You wrote 4200 oz/in, Is that accurate? Ive never seen one rated that hi. :dunno:
Straighten me out if I have it wrong. :dunno:


Edit:
Never mind, got my anwer on google :nuts:
 

DMS

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#34
i'll add a little more
servo motor are controlled by a slotted wheel or pickup the senses the motors movement. the sensor is located on the motors shaft or the object being moved. it is a set number of divisions per revolution or distance and not changeable. for this reason stepper motors are more desireable for very fine movements. take todays microscopes to move an object at millions of an inch would be impossible with a servo motor because of the mechanical pickup. your not going to make a pickup with millions of slots for the sensor to detect. this has to be done with electronics. thats microstepping in the smallest of uses and may not be a motor that turns at all it may be a linear motor.
steve
The sensor type you are talking about is called an "incremental encoder". It can be a slotted wheel, a photo-etched glass wheel, or any of a number of other configurations. The key here is that incremental encoders don't know where they are, they only know how far they have gone. So, in order to get accurate position out of an incremental encoder, you need "home switches" (a topic we should cover in more depth later). Incremental encoders give you a pulse when you move forward, and a pulse when you move backwards (it's a little more complicated than that, but we'll leave that for later). Incremental encoders tend to be cheap, and operate at at very high speeds, in harsh environments. They are also very repeatable.

Buuut....

That doesn't mean they are the only game in town. In the scenario you talked about with the microscope, you could do it with a stepper motor, yes. You could also do it with a servomotor, and an analog position sensor (like an LVDT or resolver). Neither of which you are likely to see in a home shop scenario, so this is kind of an aside.

To get back on topic (and Steve touched on this). The PC that is running your software is limited on how fast it can send pulses to your motors. And this is going to effect the encoder that you get for a servosystem or whether you can use microstepping on you drivers (assuming they are capable).
 

jumps4

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#35
to overcome the limits of the pc and windows and the fact that the cnc controller is not directly tied to the pcs internal clock there are controllers that are themselves a cpu they have their own internal clock that is dedicated to just moving the axis and reading the encoders or scales. these controllers take only the locations or parts desired and do the steps and pulses internally they are much faster, many times faster allowing for greater microstepping and less likely to make a mistake or miss steps. examples are: smooth stepper, dynomotion and uc100. each of these are very different in the way you use them but all remove the load off the pc to make the steps required. with them you can listen to music on the pc if you wanted, it wont delay or interupt your motors working normal.
steve
 

Bill Gruby

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#36
It will take a little time to absorb what is being discussed at this time. So I will not post any question from me till we are all on the same page.

When everyone is comfortable with this we will move on. If I don't hear anything in a day or so I will poet the last question givin to me. PM me or post here if you are ready to move on. Thanx.

"Billy G" :thinking:
 

Bill Gruby

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#37
I can only hope the the precedeing info was understood by all watching this thread. I see nothing posted or have received no PMs to the contrary. So it's time to move forward. There are three main components involved is a CNC set up. The Motor, power supply and encoder. Yes there is also the PC. I will let the experts pick the next component to be discussed.

"Billy G" :thinking:
 

jumps4

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#38
it's not encoder it driver
not all cnc has an encoder
are you looking for ballscrews next?
steve
 

Bill Gruby

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#39
Steve;

Pick it up where you think it should go next. I really don't know.

"Billy G" :dunno:
 
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7HC

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#40
Ballscrews, Acme Screws (can't help but think of Wile-E-Coyote and Roadrunner :lmao:), couplers, Delrin nuts, and the individual effects that each of these have on backlash when deciding on the components needed for a conversion, might be worth explaining by those who know.

I'll pick the easiest, couplers.

Two main types, the three piece 'LoveJoy' style and the single piece slit aluminum type.
Both allow for some small misalignment between the stepper motor and the driven shaft, and for a little expansion too, but the LoveJoy type has a far greater potential to introduce backlash over time as the center insert compresses.

Here are the coupler pics, and I'll leave the screw thread issues to those that know more than I do.

First the LoveJoy then the Split Aluminum:

LJ.jpg 1p.jpg


M

LJ.jpg 1p.jpg
 
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Bill Gruby

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#42
My bad Steve. I was answering a PM at just before I posted. I fixed it. Not waitin on anybody in particular. Jump righnt in there.

"Billy G" :thinking:
 

DMS

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#43
I guess I'll add a little onto 7HCs topic of couplers, including when you need them.

You want a coupler if you are using a motor to direct drive a screw of some sort (acme, ball, etc). They allow some miss-alignment between the screw and motor to exist without destroying the bearings in your motor or in the bearing blocks supporting your screw, and without introducing much backlash. If you use a rigid coupling, and the screw and motor shaft are not _exactly_ aligned (exactly), then when you bolt everything in place, the combined screw/shaft/coupler assembly is going to "dog leg" (somethings gotta give). When your motor spins you are gonna get a "cachunkachunkachunk", which probably won't be audible at low speeds, but you bearings will feel it. You can get a sense of it if you turn the shaft by hand, you will feel the resistance increase and decrease as you turn the screw. So, long story short, if you are going to direct drive, get some couplers.

Other than the 2 types that 7HC mentioned, there is also a type called a bellows coupler. As far as cost go, they tend to go (from cheapest to most expensive) lovejoy < helical-beam < bellows, which corresponding improvements in backlash with increased price.

In most cases, if you are using stepper motors, you are going to want to direct drive. This is because steppers have a lot of low end torque, and pretty poor high speed torque. If you are going with servos, they have a higher top speed, and a lower max torque, but the torque, but the torque is more even over the rpm range. That means that with servo motors you are likely going to want to gear down with a belt system, IE, no couplers.

One case you may have a stepper motor and not want to direct drive is if you have a really heavy load, like a z axis. In this case you can get a huge motor like Steve's NEMA42, or you can get a more standard motor and gear down. You will get slower rapids in your Z, but everything is a tradeoff.

Other cases where you may be using steppers, but don't want to direct drive are if you are using a direct belt drive, such as on some 3d printers (Prusa Reprap X/Y axis). These are typically really low load situations (low torque) and high speed. Incidentally, this is also how a lot of inkjet printers work. They have a head driven by a stepper motor and a toothed belt. They typically ride against an incremental encoder, and there are home and limit switches so the thing knows where to start. Pop one open some time, and you will see what I mean, lots in common with a CNC machine.
 

jumps4

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#45
I'll try a little on linear slides, ballscrews and acme screws
To be honest I didnt do much reading on items I can't afford except to note the differences.
Most people think the main reason for a ballscrew is backlash, while this is a reason to use ball screws it is not a feature of a ball screw that most hobby machinists can afford. So lets talk about friction first. The motors used for cnc are small and required to move very heavy loads in exact amounts. Friction is the biggest enemy. If you have ever tried to turn the handle of a machine that has set for a while you will feel a snap before the axis begines to move. The torque has to overcome the friction or adhesion that is between the surfaces before moving. This is everywhere in most manual machines. The thread mating in the nuts the dovetails mating to each other and the slides. Once this adhesion is overcome the motion becomes easier. Its the theory of motion things that are still want to stay still, things that are in motion want to continue moving. This plays hell on accuracy and the size of motor to overcome the initial friction/adhesion and a change in direction multiplies this as the mass has to be stopped then started to change direction (this happens fast but at one point it does stop ).
We overcome this with ball bearings. Bushings are mated surfaces rubbing against each other and the lube holds them apart so they slide on the thin film of lube instead of each other. These bearing surfaces are everywhere in a mill or lathe the dovetails are bearing surfaces and require lube to keep the two surfaces from dragging on each other. Ball bearings work different they do not rub the other part they roll against it always in contact at two points. If i put a ball on a table and put a book on top and move the book the ball will roll as i move the book not slide. There is far less surface area touching each other so there is less friction. Now if all the balls are in a straight line and "exactly" the same size they will all roll together and not against each other. Here is where precision bearings come into this, if one ball is the smallest amount larger than the rest of the balls it will catch up with the others and eventually end up pushing the entire line of balls along. This has the balls now touching in 4 locations top bottom front and back. The front and back of each ball is turning the opposite direction of the ball it is mated against so the friction and heat doubles. So ball size is very important
The surfaces the balls roll on have to be perfect also, a high spot on the table and a book that cannot be moved up leaves a tight spot in the bearing surfaces. So the balls have to be no bigger than this point in their path. Everywhere else they will be a loose fit. This is the reason for precision grinding a ball screw and it's nuts internal route for the balls to roll in.
To make the ballscrew antibacklash all the parts must be in contact at all times. but not ball against ball and they continue to roll along the thread including a passage to return them back to where they started from in the thread.
Ball slides work exactly the same way and the balls roll along the two mated surfaces and are returned through a passage to start over.
The cost of real precision ballscrews and ball slides are to high for most hobby machinist but anything that reduces friction is an advantage for our machines so even less precision ballscrews are a big improvement leaving us to just deal with the friction of our slides. this requires constant lube.
I know there is a lot to add to this and probably corrections needed

Steve
 
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jumps4

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#46
Gotta take us back for a moment or two. I just went to the Surplus Center link and they have Stepper Motors with high AC voltages and what I would consider minimal torque. What would they be used for?

"Billy G"

http://www.surpluscenter.com/
thats the same thing all steppers run on a series of electrical pulses and that could be concidered ac.
thats a problem i run into between manufacturers all the time. controller and driver are the same thing also pulse and step...
they are talking about the highest rated voltage we talked about driving the motor at higher voltages to get the amperage we need in the time allowed per step. they are giving the max.
steve
 

Bill Gruby

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#47
Question --- In a screw thread some backlash has to be present. Zero backlash on a screw thread equals zero movement. Is this the same with a ballscrew. I see many advertized as "Zero Backlash". Is it really "zero"

"Billy G" :thinking:
 

jumps4

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#48
i'd say yes because they do not slide against each other, they roll against each other, zero is possible and even some preload but as temp goes up preload increases.
steve
 

Bill Gruby

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#49
I still question it Steve. I'm a little apprehensive. If you tighten to get to zero the closer you get the slower it will rotate. I may be out in left field here with this oine.

"Billy G" :thinking:
 

DMS

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#50
Gotta take us back for a moment or two. I just went to the Surplus Center link and they have Stepper Motors with high AC voltages and what I would consider minimal torque. What would they be used for?

"Billy G"

http://www.surpluscenter.com/
Take a look at the listed RPM on these guys (3000RPM!!!). That's way fast for a stepper. You would use this on a low load system where you wanted higher speeds (3d printer, plasma cutter maybe, router).

I wouldn't use it for a mill though...
 

jumps4

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#51
the motor can turn 3000 rpm you just would not have any power out of it at that speed
my new motor said that also, they are set to 100ipm and with my screws that is 500 rpm they work fine.
if i took the screw loose and let the motor free rev and set mach to 600 ipm they would spin 3000 but i could probably stop it with my fingers
they can do it but for what reason? there is not time for them to reach full current per pulse/step so they have no power
steve
 

jumps4

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#52
I still question it Steve. I'm a little apprehensive. If you tighten to get to zero the closer you get the slower it will rotate. I may be out in left field here with this oine.

"Billy G" :thinking:
if I put 3 hardened parallels in a vise with a ball on each side of the center one and closed the screw until there was no play/backlash i could pull the center one out with out any problem because the balls would roll, they dont need play to roll to an area the same size they were in. put the parallels together without the balls tighten the exact same amount and nothing could be moved.
steve
 

DMS

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#54
Question --- In a screw thread some backlash has to be present. Zero backlash on a screw thread equals zero movement. Is this the same with a ballscrew. I see many advertized as "Zero Backlash". Is it really "zero"

"Billy G" :thinking:
There is a trick to this, and it depends on how the anti-backlash scheme is implemented. Also, it is "Zero backlash for certain values of zero".

First off, there are 3 common ways for implementing anti backlash on a ballscrew

1) Oversized balls

This involves loading the ball nut with balls that are slightly too large. The nut is forced to expand outward (and the screw slightly inward) because as always, somethings gotta give. This creates a load against all 4 contact points (2 on the nut, 2 on the screw) for each ball. This is the cheapest solution, and works really well, but is not adjustable for wear

2) Double nut, sprung

This is probably the second most common method. It involves 2 ball nuts, and a spring loaded spacer. The spacer forces the nuts apart, forcing one nut against the right side of the screw threads, and one nut against the left side of the screw threads. This can be used with lower quality screws because the spring loaded spacer allows for compliance when faced with inaccuracies in the screw pitch. It also adjusts for wear. The main trick here is that the load on your nut cannot exceed the spring load of the spacer.

3) Double nut, rigid

This involves using two ball-nuts, separated by an adjustable spacer. The spacer is adjusted to remove all play. Basically, one nut is pressing on the left side of the screw threads, and the other is pressing on the right side of the screw threads. This is the best, and most expensive solution, but requires a very accurate ($$$$) screw because variations in the screw pitch will cause variations in load on the rigidly mounted nuts. This type is adjustable for wear.

So, remember I said "for various values of zero"? Most assemblies that I have seen (that actually listed this value) show real backlash on the order of one tenth. I think we can all agree, that is pretty good, but not "zero". With a precision ground screw and a double nut, rigid mount, you could no doubt do better, but that arrangement would likely cost more than all the equipment in my shop currently..

The other thing to realize is that ballscrews and nuts, being made out of real, actual STUFF, behave like really stiff springs. If you push on them hard enough, they will move. The more the load, the more the stretch. you never get rid of it all, but you can minimize it by going to larger screws.

The other thing to realize is that these special nuts only work to a certain load, after you have defeated your pre-load (in the case of the overside ball, and double-nut arrangement), you are back in backlash country.

To address Bill Gruby's concern regarding too much force on the screw when eliminating backlash, realize that these are not sliding members. Just like setting a pre-load on a bearing (think wheel hubs), you _are_ going to increase the force required to move the bearing proportional to the pre-load, but the force required to turn a rolling element assembly is so low to begin with that the resultant force is still _really_ low.

This brings up the issue of "self" feeding. It turns out that the friction on ballscrews is so low that when you remove power from the motors, and nothing is holding them, the force of gravity can be enough to move them (think z axis). sometimes people add brakes to stop this when power is removed.
 
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Bill Gruby

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#55
While still on motors Steve suggested we discuss Holding Torque compares to Running Torque. This is past my knowledge so we will wait for someone else to explain it.

"Billy G" :))
 

jumps4

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#56
holding torque is the advertised torque of the motor sitting still, and it is used to determine use but there are differences in motors. as soon as the motor starts to turn this torque starts going down.without knowing the pulse rate and speed you will be running the motor the manufacturer cannot tell you the torque it will produce running.
the chart i posted shows a nema34 1100 oz/in motor at different speeds they are not showing pulse rate so i can only assume they are showing full step because that is the highest torgue output setting running (and they want to sell motors ). if you double the pulse (half step) you cut torque almost in half for these figures.
note that this motor at 50 revolutions per second, thats 3000 rpm is only rated at 250 oz/in.
so if you think about using a smaller motor and gearing it down to increase torque are you really doing anything
lets say we have this 1100 motor but we need 2200 torque to do the work it has to do at 100 inches per minute rapids
to start we need to know the speed we need to work with for our use so lets say - for 100 inches per minute feed rate with a 5 threads per inch screw we need 500 rpm.
so at 500 rpm from the motor we are at about 960oz/in but we need 2200 oz in
so we go to a 2 to 1 ratio to double torque, now we need 1000 rpm for 100ipm and our torque is about 1280 still to low
a three to one ratio, 1500 rpm for 100ipm and our torque is about 1260
the only way to do this and raise torque is to reduce desired speed by as much as half or more.
so before you purchase pulleys and a belt see what the bigger motor costs if you want any speed left
if your project has to have higher speeds you can only get there with a bigger motor turning slower
I'm sure i made a mistake somewhere but its all based on ideal not real world and that would make all this worse.
you have to click the image to see it the software is not displaying gif files in the thread
steve

stepper-motor-sm34-1100-curve.jpg
 

jumps4

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#58
the driver is in the basic way of thinking is just an amplifier if converts the low voltage signal (pulses) from the pc into a voltage and current the motor needs to move. as far as microstepping that is a mystery to me how it gets the motor to stop between the poles except it changes polarity constantly freezing the motor in place. and how all this is done is way over my head.
steve
 

7HC

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#60
The driver explanation works for me. This thread is titled 'basic' cnc, and following the 'basic' theme, I feel that while for instance it may be interesting to know how microstepping is accomplished, I'm more concerned to know why it's needed.

Likewise I'm more I'm more concerned to know the pros and cons of the differing ways to wire the motors to the driver board, and how to position the dip switches, than I am in how the components of the driver board actually work, interesting though that might be.

I think some discussion on the choice of driver boards, power supplies, breakout boards and the any number of auxiliary boards that are available (take a look at the products from http://www.cnc4pc.com/Store/osc/index.php for example, specifically a 'charge pump'; I'll bet it isn't what you think it might be!), would be useful to anyone just starting into CNC.

I think most people here are mechanically inclined, so the choice and physical installation of the motors, couplers, maybe ballscrews, shouldn't be any great challenge, but identification of the most suitable electronic components and perhaps the software, may come less naturally to some of us, me included.


M