Help understanding why I have trouble with setting my Z height

Threepwood

So I've tried removing the G92 Z commands from homeall.g and homez.g but no matter what number I put in to G31 P500 X0 Y24 Z-1.35 I get the same result.

I first set it to 0 to see what would happen. After homing, Z=0 would be the exact point that the sensor triggers, which makes perfect sense.

Then I tried G31 P500 X0 Y24 Z1.35 and G31 P500 X0 Y24 Z-1.35

But there is no change in where 0 is. Every time I would change the value, reset the machine, re-home and run macro:-

G28
G32
G32
G32
G28 Z

I would jog the bed down 0.2mm at a time and the moment I hit 0 in DWC the sensor would trigger, with the nozzle still 1.35mm above the bed, no matter what number I put after Z in G31.

Threepwood

@downshift64 I did try changing Z min to -5 to see what happened, but it just set the point the sensor triggered to -5 and let me move the Z to -5. Other than that, nothing changed. I guess putting in +1.35 as the minimum would set my 0 to the correct position, but I wouldn't be able to get there. I don't think the Duet would like it if a slicer told it to move below minimum.

Phaedrux

A negative trigger height would mean the nozzle tip is pushing that amount into the bed. Not what you want unless it's a nozzle contact type probe. Your trigger height would always be positive in most cases.

How are you measuring the trigger height?

You should follow this: https://docs.duet3d.com/en/User_manual/Connecting_hardware/Z_probe_testing

I also should have noticed before that your homing files are using a G1 H1 Z move as if an endstop is being used. So no wonder your G31 probe offsets aren't being applied. That would only happen if you home Z with a G30.

So replace G1 H1 Z-282 F900 with a line to position to probe like G1 X150 Y150 F2000 followed by a G30 to actually call the probe.

Threepwood

@phaedrux said in Help understanding why I have trouble with setting my Z height:

A negative trigger height would mean the nozzle tip is pushing that amount into the bed. Not what you want unless it's a nozzle contact type probe. Your trigger height would always be positive in most cases.

How are you measuring the trigger height?

You should follow this: https://docs.duet3d.com/en/User_manual/Connecting_hardware/Z_probe_testing

I also should have noticed before that your homing files are using a G1 H1 Z move as if an endstop is being used. So no wonder your G31 probe offsets aren't being applied. That would only happen if you home Z with a G30.

So replace G1 H1 Z-282 F900 with a line to position to probe like G1 X150 Y150 F2000 followed by a G30 to actually call the probe.

I'm measuring the trigger height by looking over at the hotend and watching when the red LED in the probe turns off (it also shows Z Probe = 1000), when I look in DWC it always says Z is at 0 under "Tool Position". Then I can use "Z Babystepping" under the status page and the nozzle grips an 80gsm sheet at -1.35mm.

I'll try G30 and report back! Thank you!

Threepwood

@phaedrux I've made the changes to my homing files and it now works perfectly! I can't thank you enough. I've spent months using work arounds to get where I need. Now it's finally sorted.

Previously I was using homing Z once very quickly (but at very low currect in case of probe fault) and then at my standard Z homing speed. If I keep the first instance of G1 H1 Z-282 F2400 then run G30, I assume the G30 will overwrite the G1 H1. Is that correct?

Phaedrux

YOu should probably follow the steps in here for measuring the trigger height accurately and easily.

https://docs.duet3d.com/en/User_manual/Connecting_hardware/Z_probe_testing

To do a fast probe and then slow probe try this in your homing file where you call G30.

M558 A1 F600 ; set single fast probe
G30
M558 A3 F120 ; set slower multi probe
G30

That would probe at least 3 times, and up to 4 times.

Threepwood

@phaedrux Thanks for that. I'll have a play with M558 commands. Problem is, I suspect that this will cause the bed mesh to probe 3/4 times at each point unless I add another M558 command to go back to default values.

I do have another question relating to probing and meshing. Let me know if I should make another thread about it and I will do that.

When I do quad gantry leveling on the Voron, I want quite a large dive height like 5 mm (ideally I'd like to go heigher, but that would really slow down meshing) because when the power cuts the E3D "Super Whopper Motors" cause the back of the gantry to drop slightly, or worse, if I've been working on the gantry it can end up far off parallel.

However, after this point when meshing the bed I only need to lift the gantry enough to be comfortably past the probe trigger point (the bed has a deviation of ~0.068mm). I just tried setting the dive height to 1mm and got a 121 point mesh in 50 seconds.

Is there a way I can set separate dive heights for normal probing and mesh leveling? M557 doesn't seem to have a dive height parameter, so I guess not. So I suppose I can just put M558 P5 C"io4.in" H1 F800 T27000 between the gantry leveling and the mesh leveling in my slicer start gcode, then M558 P5 C"io4.in" H5 F800 T27000 after the mesh leveling. You may have noticed that I have a thing for trying to make my Voron a speed machine (without losing any significant precision).

Does that sound like it would be the best way to achieve what I want?

It would be handy if the Duet would allow us to set a separate dive parameter for meshing. Maybe I should submit a feature request... Or maybe I've just missed how to achieve it.

Threepwood

So I've tried inserting M558 in to a "Full homing and leveling" macro like this:

G28
G32
M558 P5 C"io4.in" H1 F800 T27000
G32
G32
G28 Z

But when I do this, it messes up the offsets of the probe. It's no longer moving the nozzle 24mm forwards.

Does M558 being run reset the G31 offset values? I guess I can paste my G31 line right after. There's probably a better way to do this...

Phaedrux

I would question why you would want to put that there in the first place. What are you trying to do with that command? Which value are you trying to modify?

If you remove the P5 and C"io4.in" from that line it shouldn't reset the G31 values.

Threepwood

@phaedrux You make a good point. I was being lazy and keeping the whole line, and only changing the Z dive height, instead of finding out what values are optional in a M558 command.