Height map Z=0 point and Zhome point

zemlin

I run my machines in a mass production setup and need to have first layer consistency without resorting to manual baby stepping/tweaking at startup. I've been doing pretty well with my old machines, but just got a new rig that works a little differently and I have some questions.

My old machines are cartisian, 0,0 is at the front left. Zhome is near the front left.
New machine is CoreXY. xy 0,0 is at the also front left of the bed. Zhome happens near the center of the bed.
If bed map compensation does not consider the XY location of the home function, there could be first layer deviation if a build plate swap creates a different height map with more or less humpage in the center of the plate.

I'm thinking Zhome should be as close as possible to the bed map origin to ensure bed mapping does not impact z offset on the first layer.

I haven't studied things closely, but it seems that bed mapping may assume z=0 is at xy 0,0. I say this because when I do a 4-corner probe to check bed level on my old machines, the point closest to the origin is never way off. It's always the other three corners that show significant deviation.

That said, the first probe point isn't z=0. There's always a little deviation there. My first probe point can't be at xy=0,0 as the probe can't reach there, so there may be interpolation to XY=0,0.

So, should I change my new machine to zhome is as close as possible to XY=0,0? What happens to bed mapping if bed origin is in the center of the plate? Never tried that setup. Is Z=0 for the bed map then in the center of the plate?

Thanks!

dc42

@zemlin said in Height map Z=0 point and Zhome point:

I haven't studied things closely, but it seems that bed mapping may assume z=0 is at xy 0,0

No, it doesn't assume that. The reasons we recommend Z probing at bed centre to home Z are:

Most prints are close to the centre
If the bed has a tilt, then the height errors will be more uniformly distributed about Z=0.

It doesn't matter where you put the bed origin. I find it convenient to put it at bed centre, because that makes my prints portable to all my printers if I use the same filament type and nozzle size.

If you home Z and then immediately run mesh compensation (so that the temperature doesn't change significantly between the two operations), you should find that your height map has near zero error close to the XY point where you homed Z.