Use of G30 Z0 to achieve a perfect first layer (with grid level and nozzle based probe)

Okay so recent issues getting my corexy printing a consistent first layer with new re-arm board and learning smoothieware for the first time (full details here [forums.reprap.org]) and it was suggested to me (by sdavi - genius) to do grid levelling and then go to bed centre (although assuming levelling grid is working it doesn't matter where on the bed) and then do G30 Z0. This sets the nozzle (since I am using a piezo hotend probe which is nozzle based and very accurate) at exactly z=0. Then just print with correct first layer height i.e. actually 0.3mm for a 0.3mm first layer rather than 0.3mm+thickness of paper etc.. It is the equivalent of doing a grid probe and then homing z afterwards, which I did not assume was possible as this would clear or reset the grid.

All I can say is this is a really clever way of doing away with all that messing around trying to get a good first layer, you know - paper tests, feeler gauges, raising and lowering the head and calculating the distance, nozzle offsets (there is no probe offset) etc... This renders babystepping pointless and since not all firmwares have it this is a good thing. I accept it is only possible with a probe that uses the nozzle and is so sensitive that z=0 is z=0 rather than compliance in the probing method generating an offset. But it seems to work, I can print an even first layer at any point on the bed (which is not very flat or level) and its all automatic.

This seems to work with reprap firmware on Duet boards as well, but I haven't tried it with Marlin/repetier.

Does everyone know about this already or did I just miss z-probing 101? Or has it previously not been that useful as a very precise nozzle based probe wasn't available?

Edited 3 time(s). Last edit at 02/18/2017 11:32AM by DjDemonD.

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Simon Khoury

Co-founder of [www.precisionpiezo.co.uk] Accurate, repeatable, versatile Z-Probes
Published:Inventions

This is something I've been trying to wrap my head around for a while. Does the slicer set the nozzle at exactly Z=0, or does it know to set it at Z=0+paper thickness? Is this what the Z offset is effectively doing in S3D? I don't mind using feeler gauges to level my bed, I just disable the XY motors and move the head around manually to each point until a 0.10mm feeler gauge has just a little bit of resistance under the nozzle. What does bother me is having to guess my Z offset for different bed temperatures, whilst also trying to set the Z offset to a full step rather than a microstep. I have a DC42 IR sensor lying around that I should really hook up..... It would remove a lot of hassle with my bed levelling routine, even if it was just to get the height map feature of the Duet.

Well the issue with paper test is that you're setting z=0 in the coordinate system, but physically your nozzle is at z= 0.1mm. So a 0.3 first layer is actually 0.4. I presume this is a residual technique from the time when probes were non-existent or not very accurate, we all had a piece of paper lying around and beds weren't as flat so you needed a bit of give and take.

Thermal expansion is an issue but I'm probing with a hot bed and a nozzle just below ooze temperature so I imagine there will physically be a slight discrepancy between what's shown on the Lcd and the actual nozzle to bed gap.

The ir sensor has an offset so you have to work that out but once you're certain of the offset it should work with this technique, however thermal expansion can't be automatically compensated for when changing materials as the sensor is mounted on the carriage/effector not the hotend itself. You can measure it for each temperature and manually adjust offset.

With the piezo probe the nozzle is the probe, hot, or very hot doesn't matter. So there's some testing to do, but in theory this approach, grid levelling, plus the new probe mean no more first layer nozzle height issues to worry about.

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Simon Khoury

Co-founder of [www.precisionpiezo.co.uk] Accurate, repeatable, versatile Z-Probes
Published:Inventions

Using the nozzle as probe has a logic conflict IMHO:
You have to heat the nozzle and wipe it clean before you know where Z=0 exactly is. With a stiff frame and solid bed, you can easily damage the bed surface.
What's your way of doing that?

Any method of setting the height level requires that you clean it first - even if this is the old fashioned piece of paper.
I have used piezo disk nozzle contact probes on two printers for nearly a year now and have no damage caused to the bed or anything else by this probing. The contact force on my Delta printer is of the order of 10 grams and the time in contact is very brief, possibly as short as 10ms.

Mike

Quote
o_lampe
Using the nozzle as probe has a logic conflict IMHO:
You have to heat the nozzle and wipe it clean before you know where Z=0 exactly is. With a stiff frame and solid bed, you can easily damage the bed surface.
What's your way of doing that?

Hi O_lampe these are good points but its really not a problem.
-Yes I heat the nozzle but only to just below "ooze temp" for your filament so 130-160 deg C which should NOT damage your bed if contact is brief. Do not probe at full hot end temperature.
-I wipe the nozzle with my usual wire clippers.
-This works as the piezo probe triggers with minuscule force, I will put my jewellery scales on the printer and measure the probing force if it works.
-Your bed material has to be able to tolerate filament being deposited onto it at whatever temperature you print so I cannot think of many surfaces that won't tolerate 250 deg C briefly and we are using 160 at most.
-In the start gcode (or probing macro) you should have G30 (or G30 Z0) then immediately G1 Z5 or G1 Z50 whatever height you prefer to move to after probing, so you don't leave the nozzle on the bed hot.

As for the thermal expansion I did a few calculations:
The thermal expansion effect from say 160 to 235 is very small, even if we assumed the entire hot end was heated, an e3d v6 would go from 62.40345mm @ 160 deg C to 62.46092mm @ 235 deg C so only 0.06mm and that's if the whole thing was hot. In reality the heater-block and heatbreak up to the heatsink is 19.5mm, so the change is only 0.02mm. If you probed with hotend cold you would be expecting 0.06mm change in dimension from 20-235 deg C. So if worried probe with hot end cold, but bed at full operating temp, but add 0.06mm when printing to compensate for expansion.

Edited 2 time(s). Last edit at 02/19/2017 05:34AM by DjDemonD.

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Simon Khoury

Co-founder of [www.precisionpiezo.co.uk] Accurate, repeatable, versatile Z-Probes
Published:Inventions

Quote
DjDemonD

Quote
o_lampe
Using the nozzle as probe has a logic conflict IMHO:
You have to heat the nozzle and wipe it clean before you know where Z=0 exactly is. With a stiff frame and solid bed, you can easily damage the bed surface.
What's your way of doing that?

Hi O_lampe these are good points but its really not a problem.
-Yes I heat the nozzle but only to just below "ooze temp" for your filament so 130-160 deg C which should NOT damage your bed if contact is brief. Do not probe at full hot end temperature.
-I wipe the nozzle with my usual wire clippers.
-This works as the piezo probe triggers with minuscule force, I will put my jewellery scales on the printer and measure the probing force if it works.
-Your bed material has to be able to tolerate filament being deposited onto it at whatever temperature you print so I cannot think of many surfaces that won't tolerate 250 deg C briefly and we are using 160 at most.
-In the start gcode (or probing macro) you should have G30 (or G30 Z0) then immediately G1 Z5 or G1 Z50 whatever height you prefer to move to after probing, so you don't leave the nozzle on the bed hot.

As for the thermal expansion I did a few calculations:
The thermal expansion effect from say 160 to 235 is very small, even if we assumed the entire hot end was heated, an e3d v6 would go from 62.40345mm @ 160 deg C to 62.46092mm @ 235 deg C so only 0.06mm and that's if the whole thing was hot. In reality the heater-block and heatbreak up to the heatsink is 19.5mm, so the change is only 0.02mm. If you probed with hotend cold you would be expecting 0.06mm change in dimension from 20-235 deg C. So if worried probe with hot end cold, but bed at full operating temp, but add 0.06mm when printing to compensate for expansion.

Just a slight correction from your earlier post but an IR probe can help with different bed heats as it will just trigger sooner if the bed has expanded, giving you the same offset. It won't take into account nozzle expansion but as you said this is minor. The next problem is that you can't set an offset in the slicer of 0.06mm unless your Z axis can move in those increments, you can rely on microstepping but it's not perfect when you really want to rely on multiples of full steps for the Z Axis.

I wasn't concerned about the nozzle heat damaging the bed surface, but the nozzle wipe move scratching into the surface.
My Prusa is in a separate room and I control it by Octopi and a WiFi camera. Since I'm using an inductive probe the nozzle doesn't touch the bed and I don't do the paper test anymore.
That's my ideal scenario, which wouldn't be possible with a piezo-nozzle probe.

There is no nozzle wipe, the z axis lifts between probes. All I'm saying is if your current setup works fair enough but I've tried inductive, capacitive, ir, fsr sensors and this is the best by a country mile. And I'm not selling it.

Edited 1 time(s). Last edit at 02/19/2017 03:08PM by DjDemonD.

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Simon Khoury

Co-founder of [www.precisionpiezo.co.uk] Accurate, repeatable, versatile Z-Probes
Published:Inventions