G32 inconsistent with G33

Sander

Hi everyone,

I know there are loads of threads about autoleveling issues. But I haven't seen my particular issue yet. 

Printer: Anet A8 (Zonestar type)

Modifications: inductive probe. No Z endstops.

Procedure: 

- Power on

- Home X+Y

- G32

- G33

Symptoms:

After a G32, I would expect that the average bed height + rotation is calibrated. So doing G33 after G32 should result in a bump map that is close to 0 (I know my bed's warp is <0.5mm). However, the bump map generated from G33 has a bias - almost -2mm on each measured point. How can this be? It seems to me the G32 probed measurements are not consistent with G33's measurements.

Second thing which I found strange: once G33 is done, as expected, Z is moving with X and Y. But when I re-home X, it moves the Z axis down a lot. This would be OK seeing as the bump map does have a big offset. But it does it every time I home X. In other words, if I keep homing X, Z will eventually crash into the bed.

Is this a consequence of not having a Z max endstop?

Thanks a lot for the help!

Sander

A bit of extra info:

- Bed heater is not on during probing (I read that this causes measurement errors)

What I also find interesting:

- If I power on, home X+Y, G32, then lower the nozzle to 0 at one of the G32 probing points, it is perfect. In other words, the G32 measurement seems legit.

- After doing G33, storing the bump map, restarting, then doing G32 and lowering the nozzle to 0 anywhere, it is also perfect. So even though G33 has an offset of 2mm, somehow it still works. That seems impossible - I would expect that the bump map's bias is either "fixing" or "breaking" the Z height...

MartinH

I have noticed that while X or Y are 'homing' Z correction does not alter, then as soon as homing is complete, Z correction catches up to where it should be for X Y position.

I now disable bed levelling before sending.

Sander

MartinH said:

I have noticed that while X or Y are 'homing' Z correction does not alter, then as soon as homing is complete, Z correction catches up to where it should be for X Y position.

I now disable bed levelling before sending.

I noticed that too, and I guess it is correct behavior. But I think if you home twice in a row, it should only compensate the Z once. It shouldn't keep going in the same Z direction after every home. Or am I missing something?