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braddock

UM2 Layer Shift

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Hi guys, my UM2 has all of a sudden started shifting layers dramatically midway through printing.

Pictures attached, the blue one is the most recent, and you can see it's shifted quite a lot.

My UM2 is only 2 weeks old, I've read this shift can be due to pulleys? the shift appears to only be happening in (I'm not sure of my axis, in 3D software, it would be Z, but I'm assuming it's Y on printers)

When people say "pulleys" they're talking about the drive belts? they seem fine to me, though I'm not sure i'd be able to tell if they weren't? would it be obvious? there is no slack, and I can slide the head around with the same amount of resistance in x and y.

I have greased all the rods now.

Also, it was very hot today and last night, over 30 degrees Celsius, and very humid. I have the aircon on now to see if that has any affect.

Any help would be appreciated, I'm pretty disappointed to be having such an issue with my new printer. This combined with the nozzle constantly blocking struggling to extrude each time I start a new print is trying my patience

 

shift 01

shift 02

.

 

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x is left to right, y is front to back. Z is up and down.

When talking about pulleys slipping, we're referring to the toothed wheels that the belts wrap around on the rods. The pulleys are held onto the rods via a single grub screw. If that has worked loose, then the pulleys can slip on the rods instead of turning (or being turned) by them. In particular, if the short belt pulleys slip, then when the motor turns in one direction, the head may not always move as it should. This can cause the print to be seen to jump as in your pictures.

However, another cause may be skipped motor steps, where the electronics has asked motors to move but they did not. This can happen due to over-heating of the stepper drivers, too little current from the drivers to the motors for the speed or acceleration requested, or too much friction in the drive system.

So in your case, if not a loose pulley, it might be due to the temperature. Cooler ambient temps might help. Alternatively, did you alter your travel speed or acceleration settings? Slower travel speed might help (150mm/s, if you've gone any higher).

You mentioned greasing the rods? You didn't put the green grease on the x-y rods did you? That might have increased friction a bit, as it can trap dirt. It's best to use light oil like sewing machine oil for the x-y rods. Only use green grease for the z-screw.

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Thanks mate, yes, I have used the grease on the x y rods, I assumed that's what it was for? I can wipe it off.

I'll check the screws, and see if the room temp helps.

Travel speed is still 150mm / s but I reduced the acceleration from 5000 to 2500

Thanks again.

 

x is left to right, y is front to back. Z is up and down.

When talking about pulleys slipping, we're referring to the toothed wheels that the belts wrap around on the rods. The pulleys are held onto the rods via a single grub screw. If that has worked loose, then the pulleys can slip on the rods instead of turning (or being turned) by them. In particular, if the short belt pulleys slip, then when the motor turns in one direction, the head may not always move as it should. This can cause the print to be seen to jump as in your pictures.

However, another cause may be skipped motor steps, where the electronics has asked motors to move but they did not. This can happen due to over-heating of the stepper drivers, too little current from the drivers to the motors for the speed or acceleration requested, or too much friction in the drive system.

So in your case, if not a loose pulley, it might be due to the temperature. Cooler ambient temps might help. Alternatively, did you alter your travel speed or acceleration settings? Slower travel speed might help (150mm/s, if you've gone any higher).

You mentioned greasing the rods? You didn't put the green grease on the x-y rods did you? That might have increased friction a bit, as it can trap dirt. It's best to use light oil like sewing machine oil for the x-y rods. Only use green grease for the z-screw.

 

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The UM1 came with a wonderful skinny screwdriver that can get to all the grub screws (aka set screws) on all the pulleys except the motors. With power off you can push the print head around until the pulley screw lines up in a favorable direction. Other than that I'm not sure what you should do.

90% of the time these layer shifts are caused by a slipping set screw. Usually the short belts as the long belts get half the force and if one pulley is loose the other one still sort-of works.

About the green grease - that is for the Z screw only. Try to clean that mostly off the rods.

Any light petroleum oil with minimum additives will work for the rods - baby oil is good. 3-in-1 is probably fine. WD-40 is bad as it has powerful cleaning agents which shouldn't be left on there for long. Try not to get any oil on the belts or under/inside the pulleys. Sewing machine oil is perfect. Car engine oil is probably fine but the lighter the better e.g. 5w30 is better than 10w40.

 

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thanks guys, I ran the print overnight with air con on in the room at a steady 24 degrees C. The nice cool, dry air seemed to do the trick, part printed perfectly, accept for the top layer which is wafer thin.

Am I right in assuming this is because I printed with no fill? meaning to achieve the flat top layer, It's basically printing on thin air internally?

No shift

 

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Yes, a very wide flat surface like that is going to be pretty hard to print with zero infill or support. If you make the top/bottom thickness larger - at least ten times your layer height, you might have more success. But the first few layers that try to bridge the distance are going to mostly fail, so it's going to take several layers to really get started laying down something solid.

Alternatively, look if there's some way that you can build in some support that can be broken away later.

 

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