Circle Leveling Test

This a quick 1 minute bed leveling test. Mainly for a glass mirror bed to also test adhesion.

I was having some difficulties with a large print, due to poor leveling and nozzle height. Straight lines were drawn fine, but circles, especially small ones were dragged around which were due to the nozzle height being too high. Thus, this test was born. It's a bit more stringent than a square test.

How to use this for leveling:

Much easier to observe on a mirror or light colored bed.
Observe the four corners and the center. They should have uniform coloring. If a corner is more transparent, it's too close to the nozzle. If a corner is too solid colored/not circular/not sticking properly, it's too far away. Adjust your leveling screws until everything is uniform.

If the center and the corners differ quite a bit, your bed is warped. Get a new bed/mirror.

For example, my four corners became more and more translucent as it got closer to the edge. It was sticking quite well near the corners. The center circle was having some trouble sticking. Looks like my mirror is bowled now. Feeler gauge measured 0.10mm near the corners and 0.15 near the center. My fault for using 2mm glass and such high tension clips. Time to switch to thicker glass and maybe tape instead.

How to use this for nozzle height/adhesion:

After leveling your bed, slice several versions of your G-code, only changing the z-offset between them. For example, +0.02, 0, -0.02, -0.04, -0.06, etc. You want them to stick well enough that casual rubbing with a finger won't remove them. If they come off too easily, they'll likely get yanked around by the nozzle. But not so tight they become transparent. They should come off easily using just your nails.