[JasonCcnc]

Hello,

I have managed to build a cnc router table. It is controlled by three stepper motors, and s mach 3 controller.

The issues I would like to work on are speed and accuracy. Currently the machine can run no faster than 500mm per minute without having the motors sound like yhey are going to blow up. At this speed my accuracy also suffers. It seems that the faster I go the further out of spec the machine gets. Is this a common cnc problem?

My drive screws are 3/4in at 6tpi. Would going to a different scew help the speed? Would it negatively affect accuracy?

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[awerby]

The speed limitation is probably due to your drivers and power supply. If you're using the very cheapest driver boards from Ebay, which can't handle more than 24 volts, then your steppers won't get the voltage they need to perform up to their specifications. This is a common problem, and replacing your screws won't help, unless they're hardware-store all-thread, in which case you should replace them with acme screws. Get new drivers that can support more voltage and a power supply to go with them, and you'll get much better performance from your steppers.

Andrew Werby
www.computersculpture.com

[JasonCcnc]

So if I am running three 75 vdc motors, and the drives are speced the same, my problem is probably my power supply. My supply is only 150 watts at 40 vdc I think.



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[KOC62]

Let's start from basics.
To travel 500mm in 1 min with a TPI of 6, single start, your screw will need to run at 118.08 RPM.
Does this speed make your motors sound like they are blowing up?
Do you have a belt or gear drive to attached to your screw?

If you have a direct drive then your motor RPM is also 118.08.
Assuming that your screw can rotate at 800 RPM before whipping, or vibrations occurs, you can then get 133 IPM or 3386 mm per min.
Your next issue is too see if your stepper motor system can cause your screw to rotate at 800 RPM.
A lower TPI screw will give less resolution while increasing your RPM for the same motor RPM you currently use. Accuracy will involve a number of variables and thus not a simple answer. e.g. screw precsion, backlash, mechanical tolerances in your build, stepper resolution, etc.

What IPM are you looking for?

For comparison, I have a 425 oz-in Keling stepper motor, a G540 and a 48V power supply. My stepper can do over 4000 RPM no-load. So getting your system to run at 800 RPM is well within reason.

[JasonCcnc]

Koc,
Thanks for the reply. I think yiu might have nailed it. I think its vibratiin im hearing, not loud motors. Im rinning acmevthread and nuts on a direct cc drive from the motor. Any suggestions to decrease vibration between the acme thresd and nut.

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[KOC62]

Mechanically tighten it?
I run my allthread with a solid coupler to my motor which has a two 4-40 screw to tighten it to the motor shaft and to the allthread screw. I also use bearings for the allthread screw, one at each end, along with a double nut at each end so that the allthread is sitting snug without being able to move forward or backwards (i.e. it can't slide) and can only rotate freely. Using my cordless drill I can rotate my allthread at full drill speed, at least 1000 RPM with no rattles but a very minor tremor.

[awerby]

75 volts DC sounds really high for a stepper motor. Is that really what it says on the nameplate? Usually the kind of 23-frame stepper motors we typically use are rated at about 2 to 4 volts for a 23-frame motor, and we use up to a 25-to-one ratio to size the power supply. So a nominal 2-volt motor would be powered by a 48-volt supply. Are you sure you've really got stepper motors on your machine? Do they have encoders on the back? That sounds more like a servo motor voltage, and servos want to be powered at pretty much what it says on the nameplate. If you're only giving them 40v, then it's logical that they'd run slow. How many amps are they rated for? If you're powering them with a 40v power supply that puts out 150 watts, that's only 3.75 amps to run all 3 motors, which sounds low to me.

Andrew Werby
www.computersculpture.com

[JasonCcnc]

Im running 3 of these. Lookd like I was mistaken its 65vdc. Is that correct?


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[KOC62]

That's what the photo says, 65VDC and it takes 7.9amps.
It can turn much faster than 118RPM, like 1350RPM.

What kind of power supply are you using?
Have you wired them in bipolar series?
what stepper driver are you using?

[JasonCcnc]

Here are some pictures of the driver plates. It is running on an unregulated 48v 150 watt power supply.

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[ger21]

65V is the max rating. Looking at a datasheet, I think it's 3.2Volts (7.9amps * .41ohms).
Those are 4000 oz-in motors, so I'm not sure if you'll be able to spin them nearly anywhere near that fast. Just a guess, though.

[KOC62]

Have you actually monitored the power supply voltage while running your CNC?
One motor at full current will disipate more than 150watts.
Maybe that is the reason for your slow motor speed? Perhaps your power supply goes in current limit mode and not provide enough power to run three motors.

[KOC62]

ger21,

I don't know either but the motor nameplate has 1350RPM written on it, so I assumed it could approach that speed.
However, he uses 3 such motors but uses a 150W power supply, which made me think it was too low for such big motors.

[JasonCcnc]

I have kind of thought it might be a power supply problem. It would be cheaper to buy three new steppers than a power supply to max out all three motors. Would those drivers run smaller motors? What are the drawbacks of the smaller motors?


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[ger21]

Isn't a 150w 48V power supply only supplying 3 amps? And you're trying to run three 8 amp motors from a 3 amp power supply?

[JasonCcnc]

I had asked that question to s couplr people in another forum during my build, and was told it would work. Maybe " it will work" meant it will work slowly.

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[KOC62]

I don't have a datasheet for any of your items so I can't clearly know. Generally speaking a stepper motor needs the current to provide the torque. Hence we use the manufacturer's current rating as the setting point. This means the driver puts out the 7.9A, in your case. If you go with smaller stepper motors then your driver will need to be adjusted to the new current requirements, which will be lower. Unless otherwise stated, I believe you should be able to use your driver if its current can be adjusted to whatever motor current is required.

I don't know what your cnc is capable of and why you have such large motors and big screw diameter. Your design must be based on some requirement that determined your parts selection, but it sounds like a heavy duty machine to me.
The cutting forces, gantry mass, acceleration, etc determine how much power you need to do the work. This power will need to be provided by your motors and its power supply.

Perhaps a design review is in order to determine what you really need vs what you desire. A dump truck requires a bigger motor than a 99 Grand Am and simply putting a smaller motor in the dump truck is not the way to go.

[JasonCcnc]

That is a good analogy. The design was based on what parts I could get for free, then I built around that. The motors and drivers were part of the free pile.

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[ger21]

I had asked that question to s couplr people in another forum during my build, and was told it would work. Maybe " it will work" meant it will work slowly.

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As a guess, I'd say about 18 amps minimum might be enough, but you might need even more than that. With a 3 amp power supply, you might only be getting 10% of the motors full torque.

[KOC62]

Can't argue with your free stuff. I'd take it too.

Well, if you had access to four free car batteries and hook them up in series to get around 50VDC but lots of available amps, you could test the idea if it then has the speed you expect from your system. Just make sure you don't short the battery cables.....