Weird structure in outer shell...

[Julian2801 1]

Hey guys,

Today I noticed a weird finish of my outer shell. As you can see in the picture, there are some layers that seem to be shifted... The axles are calibrated just fine. I think that this is caused by the Bang Bang bed heating technique, due to that the temperature always is 1 or to 2 degrees too high, before it cools down and so on... I will change that to PID later... Do you have any other ideas what could cause that?

[geert_2 331]

It could be a Z-screw issue too, but then it should always be at the same height in all prints.

 

One of my UM2 has this too a little bit, the other has not. Both have a different board: the affected one has a newer board with controlled nozzle fan (switches off below 40°C), the unaffected one has an older board with nozzle fan always on. But there doesn't seem to be any Z-issue.

 

So I have been thinking in the direction of the bed heater too, although from a different viewpoint: if the heated bed (or the nozzle heater) draws a lot of current through the board, this might shift the ground level a few millivolts. And this ground-shift might influence the temperature measurements, which are probably in millivolt too. So the temperature regulator would adjust the temperature incorrectly because of this ground-shift, although it would still display the correct temperature value.

 

This is a very wild guess as I don't know the board layout, but it does not seem impossible? Are the ground-lines of measurement circuits (temp sensors) totally separated from the grounds of power-circuits (heaters and steppers)? Or do they use the same printed circuit traces? And are these circuit traces really thick for lowest resistance (e.g. 5...10mm), or are they rather thin?

 

If this would be the cause - if - then soldering thick wires parallel to the ground circuits involved might reduce the ground resistance and thus this effect. However, that could lead to the wires acting like an antenna and picking up or radiating HF-noise. Like a simple transistor-amplifier with long open input wire sometimes acts like a radio, playing all stations together (had that long ago when trying amplifier circuits on a breadboard for the first time).

 

If this would be the cause - always *if* because I don't know - then another option would be to never heat or move steppers and measure temp at the same time: switch off all heaters and steppers, let transient currents settle down, measure temperature, and then resume heaters and steppers.