Y axis jump during print job

Hello,
I was printing 3 off 20mm dia x 15mm Z(long) feet for a project and about 7mm into Z the printer suddenly made a Y axis jump/step of about 1.8mm.
The result you can see in the attached picture:
Printer a RR Prusa clone from china and most of the time works really well.
Software is Cura 15.04 and Printer is driven by PC not the SD card.

Running the job again works fine and I have seen same problem on Y axis on couple of times..
Not really a big issue on small jobs but I have a STL to print that is estimated at 22hrs, so be a pain if happened in last hour.

STL file was produced Alibre and PC is Win 7 pro.

Any idea please what might cause these random axis jumps?
Many thanks IMK

I had the same problem. I built a printer with a 12 x 12 bed and the y belt path is really long. I tighten the belt and had y skipping one in a while. Loosen the belt and the same thing happen Now I have the belt set just right and no problems. Could also be voltage on driver.

Ya in think the belt was too tight, also judging by the amount the print was displaced about 1.6mm and the belt teeth pitch is 2mm I think it must have jumped a tooth.
So I have backed of the tension by a tooth so let see how it get on.

BTW how often do you need to change the belts, do they stretch?

I'm having an issue very similar to this on my CoreXY. I know it isn't the voltage of the drivers since when the motors are engaged I can't actually force the motor to move by hand without risking damaging my printer in some way. I know it isn't the pulleys shifting on the stepper motors as I've triple-checked their set screws. The only thing left that makes sense to me is the belt. Mine doesn't feel too tight, I'm not able to pluck it like a string, but it also doesn't really feel lose either.

Does anyone know of a video or guide or something that demonstrates how tight your belts should be?

Rein,
Not seen a guide but one would be a good idea.
But since reporting this I have had an idea.
Create a STL of a narrow but long object say 1mm z 5mm wide and 100mm or more long (whatever you printer can accommodate)
Now slacken off the belt on the long (say Y) axis and do a test print, if it jumps or slips tighten it up some.
Repeat until this axis does not slip or jump, then do it on the other axis X
imk....

There is one major difference with my problem: It doesn't seem to happen unless the hot end is traversing a gap either between parts or between sections of a part.

I've attached an example.

EDIT: added another example.

Edited 1 time(s). Last edit at 11/12/2015 06:34PM by Rein_Aurre.

Ah,
Just looked in Cura Advanced setting and Travel Speed may need to be reduced.

That was at 150mm/s travel speed and 60mm/s print speed. The speed doesn't really seem to affect it.

Q: What is the difference between the X and Y axis?
A: The bed moves in the Y axis and the extruder moves in the X axis.
Q: Which weighs more, the bed or the extruder?
A: The bed.
Q: What causes the more massive axis to skip steps?
A: Too high acceleration and/or jerk settings.
Q: How do you stop the Y axis from skipping steps?
A: Turn down the acceleration and/or jerk settings.

---

Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

Quote
the_digital_dentist
Q: What is the difference between the X and Y axis?
A: The bed moves in the Y axis and the extruder moves in the X axis.
Q: Which weighs more, the bed or the extruder?
A: The bed.
Q: What causes the more massive axis to skip steps?
A: Too high acceleration and/or jerk settings.
Q: How do you stop the Y axis from skipping steps?
A: Turn down the acceleration and/or jerk settings.

I'm 90% certain it isn't skipping steps. I'm using a CoreXY with a bowden setup so my hot end carriage has very little weight to it. I've set the movement speeds up to insane levels and tested them using the manual controls and it moves faster than my ability to try and mess it up. The only thing I can think of is that my belts are either too tight or too lose, and I'm not sure how to determine which it is.

I was responding to the original poster.

---

Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

Quote
the_digital_dentist
I was responding to the original poster.

Ah, apologies. Your response is completely correct in that context.

Try turning your print and travel speed down, mine are about half of yours and I get some really good prints.
The other thing I do is when the job is printing the bottom/contact layer I reduce print speed to 50%.

Looking again at your prints pictures it looks like you might have under and over shoot.
Is the pitch/shape of the belt correct for the sprockets you have? A was reading up on it yesterday and looks like GT2

[reprap.org]
[reprap.org]

I'll try turning the speeds way down and see what happens, and I also have my first layer at 50% speed with 150% extrusion to help with adhesion.

My belts are GT2 and I can only assume the pulleys actually are the correct ones for them since I bought them as a set (and it's been a while since I installed them).

I've watched these shifts happen while it prints and it seems very much like the head gets caught on the print somehow when traveling (despite my z-steps/mm being correctly calibrated and not over-extruding) which causes it to act like the motor skips a step or two. I fully believed that to be the cause until I tried moving the print head while the motors were engaged and couldn't, so there's no way the part is adhering to the bed *that* strongly (especially when I can remove it by hand without tools).

I did some research last night and from what I can tell you can't actually over-tighten your belts (unless you get to the point where it would exceed the strength of the motor's driving rod), you just need to make sure they're all the same. Taking all this known information into account, the only thing I can imagine is that there is *just* enough force and the belts are *just* loose enough to slip on the pulleys once in a while.

I'm going to try getting my pulleys tighter and if that doesn't make a difference I'll back way off on the travel and print speed.

EDIT: This was the belt/pulley set I bought: [www.amazon.com]

Edited 1 time(s). Last edit at 11/13/2015 01:57PM by Rein_Aurre.

I believe you can over-tighten the belts as remember when you tighten the belts you increases friction on the bearing.
I think my x axis belt was over-tighten as backed it of a notch and moves much freer now