Can't go below Z0 even with end stops disabled - How to calibrate then?

TBeaulieu

Hello. my procedure for setting my z probe offset involves disabling software end stops with M211 S0, sneaking up on the position that I want, setting the offset and finally re-enabling the end stops. Well, Repetier doesn't seem to let me do that. Even with the end stops disabled, it will not let me sneak past the Z home position. I have to use another app. Am I missing something? Thank you.

Repetier

Host will prevent this after homing as this is a illegal position. But this is only true for the manual move commands.
By hand you can still send
G1 Z-1
to go to negative positions.

TBeaulieu

Thank you. With Pronterface I can simply tap the .1 down control a bunch of times, not even looking at the screen and then tap .01 up to finalize, if need be. If I understand you correctly, I would need to modify the command repeatedly to get to, say 1.1, because the command is to position absolutely, versus relatively. Is that right? I'd vote for an easier way to dial this in, if so. Just my two cents as a newbie.

Repetier

This is for protection. Imagine going down repeatedly and then hitting bed because you have no z min endstop damaging your extruder.

Going below 0 is normally an error and normally only happens when your printer is not configured correctly. Z=0 should be the bed and there is no need to go lower.

For bed leveling you should have a metal block with known height and probe against this. This is more precise then paper method and has always z>0.

TBeaulieu

That's a good answer. I just ordered a metric feeler guage set. What do you typically zero to?

Repetier

I have a 6mm block, so I home, go to Z=6mm, heat extruder so no ooze nozzle exists and length is same as while printing. Then the block must fit just below without friction. I do this at all corners where bed is mounted and adjust height then it is even after 2-3 iterations. You can do this with any height > max. error, so take 1mm feeler and go to 1mm and adjust bed height to fit.

TBeaulieu

Sorry to keep bugging you, but ... I ordered a ceramic calibration gage to experiment with your technique. It occurred to me, though, that this is probably not compatible with my IR Z homing sensor. The point of my homing procedure was to determine the Z probe offset. I'd home, disable the end stops and sneak up on the "real zero". I then update my firmware with that offset to make it permanent, or just set it in memory. In fact, I wanted to experiment with piezo homing, and have that stuff on order now. It would eliminate my sensor and home with the actual extruder. I'd like to hear your thoughts on this. Thank you again.

Repetier

I think there is no ideal probing method. All have some disadvantages.
IR: - Can be unprecise if lights change or reflections on bed are not constant. Overall works good, but - has offsets.
Force Feedback: +No offset, -might bend bed until it triggers, * Needs to find the very small negative z probe height required to trigger.

For FF we also have bending correction if bed is that weak or you need forces that really bend bed. Not sure how piezos work, never used them. Guess they just have a short spike where you see the signal.

To find z probe height you can always test distance where it triggers and measure real distance with gauge or however you like. The difference is the error of z probe height you have to add/subtract to current value.