I've wanted to tweak my z-offset a few times using paper, but every time I 'home' the Axis, it seems more loose than directly after auto-leveling.
Is the home position supposed to match the position the head is in after auto-leveling?
Posted Jul 7, 2023, 9:31 PM UTC
Yes, absolutely is the home position after auto level. Auto level sets ALL axes to 0. This is the machines "Home" position.
That's what I would expect, but the paper I used for setting the z offset should then have about the same friction as when I set the offset.
I had a few cases where it was noticeably higher.
So I assume it did somehow lose calibration in that time.
It depends on which printer you have. Iirc, the regular Kobra doesn't have endstops, so that one would set all axes to 0 I guess as terminator said.
The e.g. Kobra Neo and Go though do have minimum limit switches/endstops, and they are setting the boundaries.
Limit switches do not change the machine's zero. You just have switches instead of current limits. At the end of travel in any direction just lets the machine know the max movement in that direction, after which it moves to the axes 0 positions. Which is, the machine's zero.
Does your machine stop at exactly the switches position? No, of course not. The switches are just letting the machine know where the end of travel is. Once it does this, it moves all axes to the zero position. This is why you don't need feedback servos on the steppers.
Limit switches do not change the machine's zero.
I never stated that they change the machine's zero. I said they set the boundaries, as they let the machine know when it's actually hitting the travel limit and due to the setting in the firmware it then backs up XYmm to go back to zero (I'm referring to minimum limit switches in this case as already mentioned above).
Once it does this, it moves all axes to the zero position. This is why you don't need feedback servos on the steppers.
These open-loop systems don't know their actual position as there isn't any feedback as it is in a closed-loop system. In an open-loop system they basically just count the steps, but if step skipping occurs, then the machine isn't aware of that due to the missing feedback on the actual position. The only reason they are being used is the cheap price.
So, do you need feedback to make it work? Not necessarily, right.
But if you want real precision, you have to use a closed-loop system.
That's why I have always told people that they need to auto level the printer every so often. This is because of electrical "Spikes" that make the steppers 'Jump" and also, maybe the belts slip, therefore changing the Z offset.
The auto level will not always be in the same place, the Z axis can change due to the feed screw moving because of the weight of the print head and other factors.
When you start to notice a difference in the Z offset, it is time for another auto level to reset the machine's Z=0. Then do the offset again. I noticed this with my Kobra. After a while the Z offset kept getting larger.
That's what I noticed.
I suspect the magnetic sensor might also be moved over time. Bumps while cleaning the nozzle or whatever.
That's its main way of getting z position calibrated during homing.
If you notice a change in Z offset it isn't always a leveling issue that requires an auto level, it can be merely a different filament (or the same filament with different speed or temp) that you need to fine tune. You only need to do auto level and Z offset again are if there were hardware changes , if initial layer shows an uneven bed, or poor adhesion in different areas of the bed. (which could also be an unlevel z axis or dirty bed.)
Leveling has nothing to do with the Z offset. Yes, I know for a fact that if the Z offset starts changing, it is because the Z axis has changed. It has nothing at all to do with the filament.
Also, speed and temp has nothing to do with the Z offset. It is the Z axis that changes over time that requires you to reset the Z=0. Auto leveling is how you do this. Auto leveling resets the machine's Z=0. Then just redo the Z offset.
Hmm, not sure if I understood your question right actually, so bear with me if my answer isn't what you expect to hear..
Auto-leveling only measures the distance between the ABL sensor and the PEI plate at 25 spots for compensating variance of that distance during printing. It does not set your z-offset or has an impact on the home position. So when homing, the head moves down until the z-limit switch is being triggered. Then it moves up a bit again and rests there. When starting to print, it then moves down to the z-offset you set and starts extruding - at least as long as you did everything right when setting the z-offset and don't have printer which suffers form the mysterious z-offset-dementia like I have/had.. :( ;)
That is why I was asking.
I would go through the auto-leveling and set the offset with a piece of paper.
Later I'd home the head and try to slip the same paper in. The friction felt different to what it did while setting the z-offset. I'd have expected it to be practically the same.
Iirc, then you have a Kobra Go or Neo, right? If so, you have to set the z-axis to 0 before actually starting to set the z-offset value. If the head then is closer to the bed at another spot than the one you set the z-offset at, then you might have either a sligthly warped bed or a tilted x-gantry or so.
After each ABL you'd also have to set z-offset again as the probing somehow makes the value become 'forgotten'.. :(
No, it's the OG Kobra.
ABL does reset the Z-Offset indeed.
I'll check if this still occurs. I did tighten a few of the wheels after the initial post. The Z could've been slightly lower to the right with play on that wheel.