[Gareth52]

Hi, I have recently built my first cnc machine. I am an electronics dummy, so purchased a kit, comprising motors, breakout board and 3 driver cards. All the axes are free running and can be turned between finger and thumb. But I cannot get the motors to run at more than 16-20" / min. They stall easily and miss steps.
I studied the spec sheet for the motors (nema23s') and found that when wired in parallel, they have a rating of 5 amps. But the driver boards can only supply 3 amps. Using a formula from the internet I calculated that at 5 amps the motor has a power rating of 13.75 Watts, but at 3 amps can only provide 4.95 Watts. In other words, I can only get just over a third of the potential power. I sent my supplier an email with this question and he curt reply was that I had wired them wrongly (I haven't).
Can you guys with more electronics knowledge than me confirm that my calculation and resulting assumptions are correct. I understand that volts= speed and think that amps = torque. What would be the solution? The driver boards fit into the BOB like computer cards, so I don't want to have to buy a whole new set of electronics.
Sorry for the length of this post,

Thanks,
Gareth

[ger21]

Since amps = torque, running them at 3 amps will give you 60% of the torque they'd have at 5 amps.

The real issue here is probably voltage. What voltage are you running them at? And what is the motors rated inductance?

[Gareth52]

The power supply is 27 volts, but am unsure as to how much the motors receive. I don't know how to measure it either.
The inductance of the motors in parallel is 3mH, the resitance is 0.55 ohms and the holding torque 3.1Nm

[bobsch]

The power supply is 27 volts, but am unsure as to how much the motors receive. I don't know how to measure it either.
The inductance of the motors in parallel is 3mH, the resitance is 0.55 ohms and the holding torque 3.1Nm

That's part of you problem. With an inductance of 3mH, using the formula sqrt(3)*32, you'd need a 55 volt PS for maximum speed.

[ger21]

What kind of screws are you using? That could be the issue as well. Voltage should not limit you to 15ipm.

[Gareth52]

Thanks for your replies. Not sure how Bob arrived at the 55 volts-- like I said I'm an electronic dummy.

I am using 15tpi 1/2" threaded rod. I made some teflon nuts to reduce friction. Like I said before the tables run very freely. It looks as though I have bought a duff setup.

A friend of mine suggested that the ampage is limited by a transistor on the output side of the board and that it might be possible to replace it with 1 rated at 5 amps. Do you think that this might work?

Cheers,
Gareth

[ger21]

Your 15 tpi screws are probably your biggest problem. Optimally, you'd want 2-4 tpi.

Steppers lose torque as rpm's increase. by having to spin them faster, you get the motors into a range where you have less torque.

Also, 15tpi screws are probably only about 35% efficient, so at least 60% of your power is wasted off the top. 2 tpi screws can be as much as 80% efficient.

One other thing that might be an issue is resonance. A lot of cheaper drives suffer from it. Adding dampers can make a big difference in some cases.

With a stepper motor, everything needs to work as a matched system for best performance. Power supply, drives, motors, and screws. If any one is a poor match, the whole system suffers. A lot of times, these kits are poorly matched to start with. Add poorly matched screws, and you get dismal performance.

What kind of drives are you using?

[hub]

Hi, I have recently built my first cnc machine. I am an electronics dummy, so purchased a kit, comprising motors, breakout board and 3 driver cards. All the axes are free running and can be turned between finger and thumb. But I cannot get the motors to run at more than 16-20" / min. They stall easily and miss steps.
I studied the spec sheet for the motors (nema23s') and found that when wired in parallel, they have a rating of 5 amps. But the driver boards can only supply 3 amps. Using a formula from the internet I calculated that at 5 amps the motor has a power rating of 13.75 Watts, but at 3 amps can only provide 4.95 Watts. In other words, I can only get just over a third of the potential power. I sent my supplier an email with this question and he curt reply was that I had wired them wrongly (I haven't).
Can you guys with more electronics knowledge than me confirm that my calculation and resulting assumptions are correct. I understand that volts= speed and think that amps = torque. What would be the solution? The driver boards fit into the BOB like computer cards, so I don't want to have to buy a whole new set of electronics.
Sorry for the length of this post,

Thanks,
Gareth

I had/have the exact same problem.
The problem is that the steppers resonate, and stall. It's not an electric problem, but a mechanical. (There are stepper drivers that dampens resonance electrically to a degree).
Try to make a resonance damper. There are a few designs if you do a search.
...I'm still trying to fix this too..

Hub

[Gareth52]

Hi Gerry,
I agree about the drivescrews. I can't afford ballscrews, but was looking at these:-

Rondo Round Thread Screws. I would prefer a synthetic nut to bronze, brcuse grease and sawdust will be messy. What do you think?

The electronics, as I said were a kit. The links are below, if you have the time to look at them for me.
Motor :- Stepper motors
System 4c:- www.diycnc.co.uk/SYS4C.pdf

The 2.5 amp card has been replaced by the new 3 amp card:- www.diycnc.co.uk/DRV30.pdf

The machine is quite small (30"*24"). so massive speed is not important. I want to carve pieces of wood. I was carving a lithophane in plastic cutting board for a cheap trial. The cutting plane is 45 degrees. The error points were immediately obvious. They came at the edge of the picture and it was clear that that the x axis had jammed for a split second. This happened 4 times in a 6*4 picture. The cutting forces involved are minute and cannot be considered a factor. It was happening as the x axis needed to reverse direction. So that is why I consider it to be a torque problem. Maybe I'm wrong--- I'm just starting out.
I'm using Mach 3 and the driver test says that pulsing is excellent.

Best regards,
Gareth

[ger21]

Even plastic nuts need lubrication. I use a light oil and wipe off the excess.
Ideally, you want a pitch of at least 5mm. Not sure where you can get those over there.
You might want to look at the chinese ballscrews on Ebay, which are quite inexpensive.

Try lowering your acceleration, it should help with losing position. What is it set at?

You really should look at making a damper, though. My motors are just more than half the torque yours are, and with 4 tpi screws, I can get 150-190ipm with dampers.

http://www.cnczone.com/forums/steppe...tml#post256639

[Gareth52]

Yes,I read some of the thread on dampers a few weeks ago. It was very interesting. Problem is that I have single shaft motors. I'll read the whole thread tomorrow.
My acceleration is only set to 6 on all axes.

Gareth

[RomanLini]

...
But I cannot get the motors to run at more than 16-20" / min. They stall easily and miss steps.
I studied the spec sheet for the motors (nema23s') and found that when wired in parallel, they have a rating of 5 amps. But the driver boards can only supply 3 amps. Using a formula from the internet I calculated that at 5 amps the motor has a power rating of 13.75 Watts, but at 3 amps can only provide 4.95 Watts. In other words, I can only get just over a third of the potential power. I sent my supplier an email with this question and he curt reply was that I had wired them wrongly (I haven't).
...

I tend to agree with your supplier! If you rewire the motors in series and run from 2.5A drivers you should get much better performance overall. Your stall is about 4 revs/sec which is lower than normally expected from a PSU voltage issue so the stall will be from resonance and lack of low RPM torque.

You should also make sure your driver is set to 1/16th microstepping (or at the very minimum 1/8th microstepping) to reduce your resonance.

[Gareth52]

I have them set at 1/8 microstepping. Tried 1/16, but could get no movement at all. It was the supplier who recommended that I wire them in parallel configuration.

[hub]

I still think you have a resonance problem
But for ball screws (and lots of other stuff too), check out Zapp Automation Ltd - Stepper motors & Stepper Motor Drivers - Servo motors and Servo Motor Drivers - Ballscrews & linear motion products. : if you haven't already, an UK based web shop near you. I have bought some stuff from them, good stuff & prices. Their ballscrews are C7 grade, as are most chinese screws. But good value for the money

[Gareth52]

yes, I have bought some motor couplers and linear rail slides rom Zapp. He is OK.
To be honest, I am not much of an engineer and am not sure about the setup for locating the far end of the ball screw, so acme type thread is as far as I want to go for now.
There may well be a resonace problem, but I think the biggest problem is the electronics. I was showing the machine to a friend of mine, who is a good engineer. Mechanically, he could not fault it. He was amazed that I could stop the motor from turning, just by gripping the motor coupler between finger and thumb. The motor has no torque at all. Will try to wire it in series over the next couple of days.

Gareth

[RomanLini]

Yep that would be the best change to make at this point, go to series and 2.5A.

I know you have probably checked this but if not you should check the stepper driver settings, that driver can be set to "reduced current" and if you had those settings wrong your motor could be at a much reduced torque.

You can test the holding torque quite easily by clamping some lever thing to the motor shaft (motor stopped!) and putting a weight on it, so see how much weight it takes before the motor jumps steps. I like couplings with grub screws, so I just put the allen key inthe grubscrew and put a weight on the end of the allen key. It only takes a minute to get a decent idea of the holding torque (calculating by oz:inches).

[hub]

yes, I have bought some motor couplers and linear rail slides rom Zapp. He is OK.
To be honest, I am not much of an engineer and am not sure about the setup for locating the far end of the ball screw, so acme type thread is as far as I want to go for now.
There may well be a resonace problem, but I think the biggest problem is the electronics. I was showing the machine to a friend of mine, who is a good engineer. Mechanically, he could not fault it. He was amazed that I could stop the motor from turning, just by gripping the motor coupler between finger and thumb. The motor has no torque at all. Will try to wire it in series over the next couple of days.

Gareth

I'm not entirely sure I understand what you mean by "locating far end of the ball screw".. There are bearings & bearing blocks at each end. One end is "fixed", the other usually not fixed. There are ready made blocks for both ends, it's pretty simple to install. Just make sure they are at the same level.
All that would apply to an acme type screw too.

Ok, I agree that it's electrically wrong if you can stop it with your fingers. Mine have resonance problems, and stall by them selves at higher rpms. But there is no way I could stop one with my fingers. When I crashed my machine against a clamp, it bent 10mm aluminium without stalling and dislocated my gantry a bit!

[Gareth52]

I have done as RomanLini suggested and rewired the motorsin series. The torque problem is fixed, so thanks for that tip.
But now I have a new problem. I am running Mach3.

I knew before I did this that I would have less speed. The noise from the motors is unbearable. The strange thing is that every time I reset the system, the noise is at a different pitch. It is a much pitched sound than when I had them parallel. Ear muffs will cure that problem. The real problem is something in Mach 3. When I jog (jogging set to 100%), the motors really crawl. If I use the input screen they move nomally. I did not have this problem before. Any ideas please?

Gareth

[Gareth52]

Hub, I know what you are saying about bearings etc, but don't you have to get the ends machined by the maker? I was watching a video on RoboCNC and he is clearly a much better engineer than me. He said he made a big mistake when he didn't get the ends factory machined.

Gareth

[hub]

Hub, I know what you are saying about bearings etc, but don't you have to get the ends machined by the maker? I was watching a video on RoboCNC and he is clearly a much better engineer than me. He said he made a big mistake when he didn't get the ends factory machined.

Gareth

Yes, the ends should be machined. If you can't do precision turning on hardened ball screws (like me... my manual lathe at work is crap and can't do precision work anyway) I really recommend end machining done professionally.
Zapp has a couple of lengths pre-machined, but if you need other lengths zapp can do the machining, adding additional cost of course.. But it is worth it.