Weird issue, CNC is doing some random stuff

Hi Guys,

After hibernating for quite a while, I decided to put my MPCNC back to giant printer configuration, to see how much I could improve the printing quality.

But it seems like I have a strange issue: the printer seems to do some random stuff.

Sometimes it will print fine for an hour or more, then becoming suddenly crazy, going in a direction, then coming back. Sometimes it will just do crazy stuff after 2 minutes, or 5, or 10, or… well there is no real pattern. It is quite weird, since the printer will go straight in X or Y direction for something between 10 to 100 mm or more, and then it will come back again to its original position and continue printing as if nothing happened. This happens also with the extrusion motor, it will sometimes retract for 100 or 150 mm (yes, there is no typo) and then prime back again. I don’t understand why.

It happens randomly at different times, even with the same gcode.

I tried to swap my arduino mega for a new one, which didn’t solve the issue.

I’m wondering, what could it be? It used to print fine a few months ago, so I don’t really know what could suddenly have gone wrong. Any suggestions?

Check you connections, I was having a weird binding issue that turned out to be a loose connections. When it would free up it would turn back and forth and chatter. I took the machine apart a good 4 times today and yesterday. I made a mod to it and cause a cable to be moved and caused it to kind of float so it was not making the best connections. After I got that worked out, another cable did the same thing as it got hung on something when I was moving stuff around.

TLDR: check your connections for power, I spent almost two days troubleshooting something that was not the cause.



I’m not sure this is due to the connections, for two reasons:

-First, because most of them are either soldered, or are using good quality connectors

-Second, because in my opinion, bad connexions would either result in step losses or total crash of the machine.

Here, the machine goes in some random direction at some random points, but then comes back to its original point and continue printing as if nothing happened. Which means that I can actually finish the print and have a somewhat usable part, if you except the fact that the part will look super ugly with a lot of stringing because the printer still extrudes a tiny bit of filament while doing its random travels.

I think it looks like something is wrong with the Gcode interpretation, or with the gcode transmission to the arduino. Maybe it does change a command coordinate to another, so the printer goes there and then goes back at the next command.

Actually, after doing a bit of research, it looks very similar to this issue:

So I guess I will investigate my screen and screen wires first. My screen has been a bit damaged during my first tests with the plasma cutter (a full line of pixel is blown), so I guess it makes it a good suspect.


Oh, yeah, I recognize that one, had it last year. And it seems 302tt from that thread you linked is right, IIRC the problem went away when I changed the location of some cabling. (Note : I was reluctant to point out possible interferences, since you have experience with them :frowning: )

Note : If you’re sending the gcode via SD Card, there seems to be a # to enable the checksum checks. (checksums seem to be ignored on the serial link though… Wonder why ? perfs ?)

Interesting, I didn’t know that.

My plan is to first try without the screen, to see if this is actually the problem.

Then try different cable arrangements and maybe (again…) more shielding…

I’ll order a few new screens anyway, they’ll be useful no matter what.

Well, apparently the issue seems to be fixed.

My screen used to have a full row of dead pixels, which I thought was one of the symptoms of something going wrong, possibly impacting the data transfert from the SD card too.

So I disconnected the screen, threatened it to replace it with a newer, sexier screen. Then I plugged it back again. It seems like it got the message, all the pixels came back and the next prints were completely successful. So, apparently, sometimes a good talk can solve complex issues.

I’m pretty happy with the print quality, especially given how far it was from the center carriage (about 30 cm deep, which is quite a huge distance considering the Z axis stiffness) I think after a few more tweaks it might be perfect.

Next step will be to implement the bed autoleveling, since it is quite mandatory on such a big bed surface.

The part during printing:

[attachment file=53013]
[attachment file=53014]

I printed quite slowly this time, to make sure my settings were right. took around 2 hours to complete

I think printing quality was close to perfect, especially since I was not using the part cooling fan at all.

[attachment file=53015]

The part is a bracket which will support my electronics box. Not a big fan of this color, but that’s all I had left, I’ll probably just paint it later. It is very strong and tough, the big nozzle really produces parts with an incredible amount of strenght.
[attachment file=53016]

As always, a nice print from that giant printer of yours!

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