Stepper motor

[RayVT]

Oznog,
I didn’t know that they are current-driven I thought if you supplied the right voltage the stepper would take the current it needed - my bad…glad you caught that.
I guess I have to something else then.

Well my overall plan is to build the CNC as inexpensively as possible without sacrificing quality; the controller I’m trying to make is just for testing the CNC at different stages as it being built to make sure it not binding anywhere and check for any lag (forward / reverse) to cut down on the cost until I’m sure the CNC is worth further investment.

I think I got carried away when I started designing this controller, I started adding more controls than what I really needed.

I was just stating that I had 6 Stepper I’m not sure of the size of the stepper’s It’s been a 5 - 6 years ago when I got them, they are still in storage at my son’s house, I do know 3 of them are good size and hopefully they will do the job and save me some bucks, I hoping to get them soon. Either way I’ll get new one’s if I have to.


I never intended to post anything until I was further ahead with my project I was just trying to help out Maverickmax with his question, I’m glad I did it probably save me from smoking my steppers and looking at a wider range of controllers, Please excuse my ignorance and I hope you don’t get to frustrated with me, I have always wanted to build a CNC now that I’m disabled I have the time, I still don’t have the money LOL but I am going to built it and I’ll get what I need to do it.

I’m all messed up now… I never heard of micro-stepping before and “current phase, rotor phase, mid-band resonance compensation” WOW I’m just going to take your guidance in the best economically path I should go.

Thank you for your time and patients,
Ray

 

[AlainB]

You could build a low tech controller for small unipolars motors using 2803 chips. This could be for a small machine that can be quite precise, perfect for instance for drilling PCB's.

Here is one that I am building now:

YouTube - 3 axis CNC for PCB drilling

I have a few other CNC machines that are working relatively well, all using very low tech controllers. But the mechanical part of them are not thight enough to do any serious work on hard material.

Alain

 

[Oznog]

Oznog,
I didn’t know that they are current-driven I thought if you supplied the right voltage the stepper would take the current it needed - my bad…glad you caught that.
I guess I have to something else then.

Well my overall plan is to build the CNC as inexpensively as possible without sacrificing quality; the controller I’m trying to make is just for testing the CNC at different stages as it being built to make sure it not binding anywhere and check for any lag (forward / reverse) to cut down on the cost until I’m sure the CNC is worth further investment.

CNC is indeed a good investment. But with a poor driver, it'll bind all the time so this won't tell you anything really.

Stepper windings have an inductance and resistance. Like this one. It's got a resistance of 1.7ohms so when you put 5.1v DC across it then you get the specified 3A. However, a router- a slow router- running at 40 ipm (at which speed you need a full minute to get from one side to the other on a 4ft table) with a 20tpi leadscrew and a standard 200 steps/rotation on the stepper= steps t 2.666kHz, 375us/step. The inductance is 7mH. Now start with 0A on an 7mH inductor and switch to 5.1v, the current will initially rise at 0.000728 a/us, and that rate decays exponentially as it nears the steady-state of 3A. Doesn't get anywhere near 3A in 375us. That's why we apply like +/-40V and the current rises or decays much much faster, but current-limit it when it reaches the target.

A linear driver (like the Linistepper) is impractical. Why? Well, the transistor driver will burn (40v-5.1v)*3A watts per motor, over 100W! 4 motors, 400W. That's an enormous heatsink and power supply, and probably will be prone to burning up.

A router that big generally must be a gantry, where the table is fixed and the router gantry rides on down the X-axis on rails and drive screws/belts on either side. Consequently, you need not 3 motors and drivers but 4. Generally, no, you cannot simply put those 2 motors in parallel even though they're stepped together. The windings are electrically coupled but not magnetically coupled and that's a problem.

The vibration from 200 steps per rotation can be vicious. Instead most drivers create partial stepping positions by driving both coils at once to a portion of their rating. This smoothes things out. It requires more steps from the CNC software per turn.

Midrange resonance is a major thing. At mid-range speeds, the rotor's mechanical position "rings" back and forth as it tries to jump to each step (or microstep). Microstepping does not really fix this. This results in a phase error between rotor and stepper winding current, and it reduces the torque output so much the motor can easily stall.

No matter how good the driver, steppers are limited to about 2000rpm (usually much less). This may not achieve satisfactory linear speeds for a large router. Ballscrews instead of leadscrews run at substantially lower pitches (turns/in), so they achieve more speed per motor rpm. Also people use chain or belt drives to make a gear ratio between motor and screw. There are belt drives which use no screw but instead mount the gantry on what is basically a conveyor belt (no leadscrew/ballscrew threads to get dust in, either).

But also a lot of machines use DC servos, just an ordinary motor really, which of course don't result in a specific position when driven with a DC voltage. Instead there's an encoder which provides position feedback back to the computer software which "learns" how the machine moves and the drive is adjusted so that it'll reach the appropriate positions at the appropriate times while it carefully counts encoder pulses so it knows the absolute position at all times. Anyhow servos can run faster than steppers and also provide much more torque.

A large router like this is a big job for the motors, because 1) the inertia is relatively high, and 2) the distances are so large that a lot of movement is needed. As such what was a good solution for a small mill may not be a good solution for a large router.

 

[RayVT]

Well it looks like DC servos is the best way to go.
Quick question is there anything special other than the encoder?
The reason I’m asking is that I have two 130 volt DC motors 6700 rpm, 2.5 hp, 18 amps and 1.0 FF what ever that is
I think I can get some more.
They also have a ¼ rear shaft; I could put an encoder on.
Now I may need help looking for a controller.

 

[RayVT]

Thanks Alain,
It looks good, I will keep that info for my smaller projects I have lined up but I need a little more kick for this project. 4’ x 4’ CNC mill.

 

[Oznog]

Are those motors reversible? Lots of DC motors are designed with brushes that only turn one way, and they'll still reverse if you flip the + and -, but it'll wear the brushes and armature badly in the long run.

 

[RayVT]

I'm not sure
They were free, I don't mind if they do ware out it will give me time to save to get good one’s.
what "FF 1.0"?

 

[Mr RB]

...
However, a router- a slow router- running at 40 ipm (at which speed you need a full minute to get from one side to the other on a 4ft table) with a 20tpi leadscrew and a standard 200 steps/rotation on the stepper ...

Nobody sane would set up a 4'x4' router table with 20 TPI leadscrews. Likewise nobody would try to run 20 TPI leadscrews at 40 IPM. These are just silly figures and not a good example when you might be advising someone on what setup to use.

...
A linear driver (like the Linistepper) is impractical. Why? Well, the transistor driver will burn (40v-5.1v)*3A watts per motor, over 100W! 4 motors, 400W. That's an enormous heatsink and power supply, and probably will be prone to burning up. ...

The Linistepper is not "impractical", there are a lot of people using them for CNC (on small CNC tables) and for more demanding uses like smooth telescope rotation etc. Just to clear up some facts, the Linistepper is recommended for 1A max motors, not 3A, and my suggestion of PSU voltage is between 15v to 20v depending on the app.

I will agree that the Linistepper is designed for SMALL motors and is probably not be the best driver for a 4'x4' table. I recently designed a big brother to the Linistepper, the SLAmStepper which is a very easy to assemble 3A chopper unipolar driver using an SLA chip.

...
Midrange resonance is a major thing. At mid-range speeds, the rotor's mechanical position "rings" back and forth as it tries to jump to each step (or microstep). Microstepping does not really fix this. ...

Wrong! Microstepping is a very good way (in fact the second best way) of reducing excitation energy and solving the midrange resonance issue.

The best way is the Linistepper, which has caps to tune the linear smoothing so the waveform to the motor looks like a sinewave (with no steps at all!) at and above the freq of the midrange resonance.

I agree with most of what you are saying, that a 4'x4' router table probably needs high speeds and good motor power and fairly large motors and drivers. But your opinion is sounding like the ONLY way to do something is to go for a high end expensive solution and that NOTHING else is any good at all.

The small stepper motors drives like the Linistepper are a very good solution for the application they suit, ie smaller motors, slower rotation speeds and very smooth operation. The linistepper is the only driver on the market what will actually run a stepper motor silently, which is pretty cool the first time you see it.

And the SLA chips which you dismissed like garbage offer a very good performance with years of proven service.

I'm not arguing with you that some larger systems are the best for large fast CNC machine, but I will argue with your attutude that anything not "the best" is completely useless, it's like someone saying you should get a Ferrari because a Chevy is not worth buying...

 

[RayVT]

I have been looking at a lot of stepping method and each one has there own advantages and disadvantages to each other (accuracy, speed, power, noise, price…) I believe Oznog was mainly referring to my application, I have been checking out some video on YouTube on different type controllers, steppers, applications and CNC designs pretty neat stuff indeed.

Well I been trying to find some shaft encoder but the only one’s I found so far are to expensive, might be better of buying a new servo motor, I’ll keep looking thou.

I’ll keep looking for a controller to build or buy for the DC servo motors I kind of like the way they run. I think I build my test controller for now.
I did find some IRF540 Power MOSFET N-Channel for 45 cents each, I was on stepperworld.com and seen that there demo board was using them, do you think that is a good one to use?

 

[Oznog]

Yep I was primarily trying to answer the question in context of his router.
And sure, 20ipm leadscrews wouldn't be a good choice for a router- but for all I know, he's operating with allthread rod out of the hardware store anyways, so what "should be done" isn't quite the issue yet.

RayVT- I should again bring up that already having a particular motor is nice, but developing an overall drive system up to the task- in fact, just the mounts to hook them up- IS quite expensive. If the system turns out to be unworkable or soon needs upgrading, the money invested in it will likely be wasted. So, do read up a bit more on CNCZone and ask people who've been making routers, and consider what's going to be salvageable if the plan doesn't work out. The encoders, yes, if they're not modified AND are compatible with new motors. They won't be especially useful if you go with steppers, but can be sold. Motor mounts, those would be less likely to be salvageable. If the power supply is the wrong voltage, then you'll probably need to sell that off.

As far as the IRF540, well, it's a transistor. With specs. You're going about this the wrong way though, you've got to start with a spec on what the motor needs, then figure out what parts fit the bill. The cost of transistors themselves is generally nothing. But yes, an IRF540 can be a useful transistor in a servo driver. First off though you need to figure out what to use for a power supply for the motors you have.

 

[RayVT]

NO NO I'm only using the IRF540 for a test controller for the stepper motors just until I’m ready for the new servo motors, I am drawing up the plans for the router I would like you to look at them as I finish each section Just about done with Y & Z section.

 

[RayVT]

Well I have part of plans done
**broken link removed**
just some screen shots.

 

[Mr RB]

Very nice design!

But there are possible impovements;
1. reduce the height to the minimum possible to improve rigidity. Most stuff you cut will probably be flat and thin.
2. you could lower the gantry linear bearing to the bottom of the gantry. the closer you get the bearing to the work the less flex.

 

[RayVT]

I see what you’re saying Mr RB, I was thinking about doing that to cut down on the weight, I’m still thinking about the linear bearing design I may have to do something else I’m not sure if I’m using is adequate enough.

I am also concerned about chain slack / tension; there is a spring in the chain adjusters so when you tighten the adjuster you actually will be adjusting the tension/compression on the on the spring.
Now that on the other hand by having all the tension on the chain I think that will increase the torque needed to move it, maybe I need to gear the motors maybe 2:1 or more?

Made some changes to the gantry, and close up on the gantry bearings.
New Link -www.ynot-diy.com/cnc.htm 'screen shot updates'

 

[Mr RB]

That looks better, you got the gantry much lower which is good.

In the second pic from the bottom, where you show the 2 rollers, they should be as far apart as possible. Same deal on the rollers for the 2 gantry sides, the further apart you can get them the less slop and flex.

Chains should be ok, they are common on the CNC plasma cutters and stuff but they don't need really high position accuracy. The trouble with a sprocket is that it may cause "cogging" where the chain travel is not totally linear to cog rotation. On a plasma cutter they only need to hold position to 1mm or so, and chains will do that.

It depends on what you are cutting and how accurate you want it!

 

[RayVT]

My main objective is to keep as much accuracy and speed as possible.
Do you think if I was to run 2 chains side by side offset them 90 degree’s and add opposing tension, would that increase the accuracy or would I be wasting my time?

**broken link removed**

 

[shortbus=]

A lot of guys are using gear racks mounted on the frame and a pinion gear on the motor.

 

[Mr RB]

There's no real way to cut corners if you want high speed AND good accuracy. You need some decent ballscrews.

If you just want accuracy you can get threaded rod, and live with a slower machine.
If you just want speed then go with the chains.
If you want both, start spending some $$ on ballscrews.

 

[RayVT]

If you want both, start spending some $$ on ballscrews.

Well I don't think I'll be using $$ballscrews!!

Could you tell me approximately how much accuracy difference between the threaded rod and chain?

I have to pick one of them so I can get started putting this thing together.

I wish I had some experience with CNC’s and different types of motor’s steppers / servos,
I really appreciate your help.

 

[shortbus=]

Chain is not real precision varies quite a bit over its length.

If you get ballsrews on Ebay the price isn't too bad, way less than a retail price.

You might want to look at this McMaster-Carr

These places also carry Acme rod and nuts:

Enco- Enco - Guaranteed Lowest Prices on Machinery, Tools and Shop Supplies

MSC- **broken link removed**

I don't know if you know about these:

MechMate CNC Router Forum - Powered by vBulletin

CNCzone.com-Machinist Community Forums - Welcome Page

A lot of good information on both of them.