Print shifts at EXACTLY the same points

My Prusa i3 started having layer shifts very often recently, only in the y axis direction. The strange thing is that for a given print with given settings (speed, etc.) the shifts tend to occur in exactly the same place. That is, for one print it might happen at 4.35mm, in another at 5.15mm, etc. I do hear a slightly grating noise when it happens, but I haven't yet been able to figure out where exactly it comes from.

What I checked:
Belts are tight enough but not too tight. In fact, I've tensioned the belts by hand at the point where the shifts occur, they still happen.
Y motor is cool.
Y stepper driver has enough current and runs cool (cooled by fan).

Neither of the above would explain the reproducibility of the layer shift. Could it conceivably be a problem with the gcode? I glanced over it but see nothing that jumps out as wrong. I'm out of ideas for the moment and will keep thinking about it, but would appreciate any input.
Thanks.

Photo required so we can see how much the shift is.
Are you printing from SD or via USB?
Is anything changing at this time? Cooling fans, infill, part shape.
If its happening on different layers its unlikely to be Gcode. May be a similar time after print start so could be related to temperature on the driver, sleep mode on the laptop etc.
Loose drive gear on the stepper?

The Y axis usually is the one that shifts in printers with a bed moving in the Y axis because the moving mass is much greater than the moving mass in the X axis. You most likely have acceleration/jerk set too high and the Y axis motor can't keep up. The location of shifting varies with the specific print because it is a function of the geometry of the print. If you print hollow cylinders, it won't shift at all because there are no sudden direction changes. Print something with some gap fill in the Y axis and suddenly it will shift. Anything that causes rapid starts/stops/reversals in the Y axis will cause the shifting.

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

Attached are some example photos. The shift can happen at different times (e.g. before or after perimeters) but always seems to happen on the same layer.

I'm printing via USB from my PC, which doesn't fall asleep. Driver is cool and always cooled by a fan. Nothing changes: nozzle fan is on auto-cool and I believe off at these points, layers are 100% identical (e.g. test cube or just a z-identical shape). Tightened the gear on the stepper: no effect.

I am currently printing identical layers in the z direction, no change there. It goes fine, then all of a sudden, a shift (no change from no-infill to infill), which reproducibly happens on the same layer given a particular gcode file.

My speed is 20mm/s for everything (travel included), and my xy-jerk has been set to 0.5mm/s in Marlin, I believe that's reasonably low. Shifts still occur. Keep in mind that I used to print at 60mm/s just fine before.

Could it be a failing driver? I still don't see why it would fail on the exact same layer every time though.

Edited 1 time(s). Last edit at 01/20/2017 08:36PM by anvoice.

Try swapping drivers with one of the other axes.

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

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the_digital_dentist
Try swapping drivers with one of the other axes.

Ok, that seems to have done something. Switched x and y drivers. No shifts for now (2 prints so far), although I'm left wondering how that works. Even if y was suddenly undercurrent or the driver was otherwise failing, why did it always shift on the exact same layer? I'll report back if I find out more. In the meantime, thanks for the tip.

Does your z-axis run smooth? Maybe z-hop fails at a certain height and the nozzle doesn't come down all the way. Then the nozzle would have to deal with a higher "last layer".
I'd check the lead screws for dirt.

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o_lampe
Does your z-axis run smooth? Maybe z-hop fails at a certain height and the nozzle doesn't come down all the way. Then the nozzle would have to deal with a higher "last layer".
I'd check the lead screws for dirt.

Would make sense if I had z hop enabled... Thanks for the suggestion.

I'm still confused by how this works out, but it seems to be printing at high speeds now without shifting. All I did was swap the x and y stepper drivers, so current to motor comes to mind, but doesn't explain the consistency of where the shift was occurring.

The layer when this happens is the first layer where the outer, smaller holes start to show. The infill is somewhat interrupted and there are smaller lines. Maybe your printbed is moving in Y axis in a special speed/acceleration, where the load is a bit higher than on longer infill lines below this layer. Possibly this is the bit too much, so your bed looses a step.
You could swap back your drivers and increase the Y current a little, and/or reduce infill speed, or try another infill, to check if symptom changes.

The problem is the layers are identical, but the shift happens anyway at some constant height.

I'll try swapping the drivers again as you suggest, but I want to at least get a few good prints first. Frankly worried about breaking it again now that it works.

No worries, take your time. But from the pictures, the layers aren't identical. Can you post the GCode or the STL? Does the shift happen with other objects, too? Maybe try to rotate the object - later ;-)

Edited 1 time(s). Last edit at 01/22/2017 11:13AM by Inhumierer.

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But from the pictures, the layers aren't identical

What you see is the shifted layer, that's why they don't look identical..

If you home Z and then send a G1 Z100 does z actually move 100mm if it stops short then maybe your layer shift is due to the nozzle dragging on the previous layer

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obewan
If you home Z and then send a G1 Z100 does z actually move 100mm if it stops short then maybe your layer shift is due to the nozzle dragging on the previous layer

It doesn't seem to stop short. Plus I see the z motors move on the problematic layer, so I doubt it's dragging through the print.