CNC router occasionally makes jarring noise and home jumps when it does this. Help!!

Hi Guys I was hoping that you could help me track down the problem behind why my CNC 6040 router now and then (but pretty much most jobs) it will happily be machining away. When at some point the machine will make a louder 'jarring' kind of noise and it appears to be along the Y axis by what it does to my work pieces (the longest axis which the gantry moves across). The machine doesn't appear to stop but it looks as though it jumps by about 10mm. I haven't been able to physically see this by eye as everything is always too quick and it's not spectacular enough. But I can see it jumps on my work.

You can see from the photos attached what it is doing.

The machine appears to be operating just fine until this happens i.e circles are all correct as are square shapes.

I know that they have lots of quality issues, but I would be really grateful to you guys for advice on how to diagnose this.

Upon starting up the CNC router and running mach3 on my desktop all 3 stepper motors lock as they should and the router happily runs/jogs along all axis. I can turn the stepper knobs by hand freely and cannot feel any rough parts of travel in the ballthreads.

Please help me diagnose this with simple steps. Is it more likely that it's wiring or a stepper motor or driver?

Just guessing possible Y axis wiring issue. But would love to see what you all think.

As you can see this error has made it into all 3 jobs below. Looking forward to hearing what you guys think.

Cheers andy

Attachment 11267Attachment 11268Attachment 11269

On the second photo it looks like it jumped then came back into line again, did it do that itself or did you intervene and correct it ?

Is it direct drive or via toothed belt ?

I think in the second photo it jumped twice by different amounts, as it's not gone perfectly back into line.

It could just be the Y-axis motor is very occasionally stalling. I'd try running this code to test it more thoroughly and report back.

There are way better guys on here than me to advise, but here goes....
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What you are describing sounds like a motor stall on the y-axis, which is likely due to 2 causes:
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1. You have a mechanical problem, such as the rails, ball or lead screw(s) or bearings being out of true and causing binding i.e. a resistance the stepper motor is having difficulty overcoming or;
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2. You have an electronics issue where data or pulses are getting lost and the steppers are not moving when they are supposed to be.
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For 1. I would see if you can first disengage the ballscrew/leadscrew and see if the gantry can be moved across its full range of travel with only a little pressure by hand - my own gantry weighs a fair amount but I can push it easily with a finger. Or failing that see if there is any adjustment possible on the ball/leadscrew alignment and redo that - not sure how your ballscrews/motors etc. are attached, so can't really advise on the method of doing this.
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For 2. Only advise I can give at this stage is if your wiring is not using shielded cable, then replace that with shielded CY cable and ground the shield at the control box end only and to the same ground point all the other earths on your machine should be going to, which should in turn be wired to mains earth. Make sure the ends of the shielding at the stepper motors are isolated i.e. not contacting anything. Other than that you are looking at driver, control board/BoB replacements, but I can't advise on diagnosing your existing components. Also I have seen some stories of inadequate grounding within the control box itself, so check that out.
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Oh also if the machine came with a cheapo parallel cable for connection to your PC then try another one. I had a problem with my machine getting spurious e-stops that turned out to be a shoddy parallel cable. Replaced it with a chunky shielded one and that problem went away.
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There'll likely be other guys that can advise further, but having tripped over most of the pitfalls in building reliability into my own machine that's where I would start.
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Hope that helps.

If it's a sudden jump straight after the Jarring noise and it's just on the Long axis then I suspect it's the ballscrew end bearings floating.?
Grab the gantry firmly at both sides and try to move back n forth. If you have end float you'll feel it clunck.

If it's losing position over time then I'd suspect a lose coupling first, after that then I'd look at the wiring. These cheap machines come with crap wire on the motors which gets brittle and breaks internally but still works and causes all sorts of strange things like this. Same with the connectors there rubbish and need replacing.

From you saying it works fine then does this then I don't think you will have it over tuned or anything so wouldn't go messing in that direction just yet.

With the noise your saying it's making then 90% chance it will be mechanical 10% wiring or electrical.

It would appear to me that the problem occurs during rapid moves and not whilst cutting. if this is the case you may be able to work around the problem by changing G0 to G1 as a short term fix until you are able to resolve the problem. G.

Thanks guys loads of ideas to look at here! eddy its a direct drive stepper motor without any drive belts. As for the photo and it correcting itself. I think it did the top text line in a different order I.e do a few letters and go do something else then come back. I haven't corrected anything in any of the pics.. hope that helps?

I think it is most likely a mechanical issue as is suggested. so I'm going to take a few videos while its cutting to see if I can narrow it down further.

@ Jonathan I think you are correct about it having jumped different amounts, it looks like that to me too. I will run the code tonight when I get in from work. thank you!

@ washout and jazz cheers guys I will look into your suggestions after I can nail this this down on video.

and Geoffrey I shall indeed try changing those settings as it does look like its only doing this whilst moving fast and not actually cutting. cheers guys I will report back. hope I haven't missed anyone out trying to type this from my mobile!

cheers andy

Check your pulse train speed as I had a sort of similar issue and found I was running too fast. If you want to post up your axis settings I'll check them against mine. I've got a 3020, but I think the settings should be similar?

Thanks Richie I will post up some screenshots of my settings tomorrow it could be that the stepper motor is running to fast in one direction and trying to reverse too quickly (if this makes sense?) I have managed to capture it doing this on video twice and have attached a youtube video. You will be able to hear the 'jarring/grinding noise' briefly. Sorry for the crappy fastening down I did the first clip without the spindle running. The second part of the video is just to highlight that it's doing the same thing with the spindle running too.

I am a beginner to all of this and appreciate all of the help you guys have offered. I tried rocking the gantry back and forth and left and right and I was unable to find anything loose as such.. no real obvious play. I will look into this more tomorrow night.

http://youtu.be/hqf3sHZVPjE

Any more thoughts welcome? does this rule anything out or more clearly point to any of the above suggestions?

Thanks again Andy

100% the noise is motors stalling from being over tuned and trying to cut too fast.

Looking at the speed you where cutting at I think your being too ambitous on the Feed n speeds for this little machine and the motors are running out of torque and thats why your getting stalling motors.

Drop the feed rates down and try again and I think you'll find it works fine.

You could play with motor tuning which would help but I think your just pushing the little thing too hard.!! . . .. Try lowering the Accel first if you adjust motor tuning has that will have the biggest affect and would cause this kind of stalling/missed steps.