Need some remedial nozzle height assistance

Hey there,

I am trying to get my Prusa I3V setup with the Itty Bitty Double Flex extruder and I'm struggling with nozzle height.

I was hoping to get some remedial instruction to ensure that I am fully understanding the relationship between nozzle height and layer height.


I am using auto-bed leveling and have completed the basic height adjustment where after running the G28/G29 commands I lower the nozzle until it just grips a piece of paper and adjust the Z-offset accordingly.

I am printing with .2mm layer heights, .35mm first layer and trying to print clough42's .35mm calibration print.

When I print I get a measurement of about .65 - .8 in thickness, but when I use a feeler gauge and measure the distance between the nozzle and the bed I get less than .35. (The LCD shows a z position of .3 (doesn't show two significant digits but I see in the g-code it is .35))


I'm using the E3D-V6 .4mm nozzle with 3.0mm filament (PLA), printed at 195.

So, my basic questions assumptions are:

- When properly zeroed the nozzle should be just touching the surface
-- When printing, if the LCD screen indicates a specific height, that nozzle should be that height above the bed (so for .35 first layer height, g-code sets the z height to .35 and that should be the distance between the nozzle and the bed)
-- For a layer height of x the nozzle should be x height above the last layer (or alternatively the bed)


Are these assumptions correct?

Since I'm struggling so much I'm leaning towards I have a misunderstanding somewhere of what is supposed to be happening.

Thanks for any feedback.

Thanks,
Bill

I'll start by saying that I don't know much about this. Two things come to mind:

(1) If subsequent layers are printing .65-.8mm thick, is the Z-Axis stepper correctly calibrated? Could a requested movement of .2mm turn out to be the stepper moving 3-4 times that amount?

(2) Is the extruder calibrated correctly? Maybe too much filament is being put out?

Once you see it print correctly, you will know what you are looking for. But probably the right place to start is set the #define Z_PROBE_OFFSET_FROM_EXTRUDER value a little bit larger (less negative) than you need. Maybe by 1mm. This will cause it to air print. Depending on how high you are up above the glass, adjust that number so you have half of the air space between the nozzle and the bed. Print again. See if it still air prints. Keep adjusting it a little at a time until the plastic starts to get squished into the glass.

You do need a small gap between the nozzle and the glass. But it is very difficult to measure and get right. It is much easier to adjust the #define Z_PROBE_OFFSET_FROM_EXTRUDER number until the plastic is getting pressed into the glass the right amount.

Quote:

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Originally Posted by Bredmond

So, my basic questions assumptions are:

- When properly zeroed the nozzle should be just touching the surface
-- When printing, if the LCD screen indicates a specific height, that nozzle should be that height above the bed (so for .35 first layer height, g-code sets the z height to .35 and that should be the distance between the nozzle and the bed)
-- For a layer height of x the nozzle should be x height above the last layer (or alternatively the bed)


Are these assumptions correct?

Since I'm struggling so much I'm leaning towards I have a misunderstanding somewhere of what is supposed to be happening.

Thanks for any feedback.

Thanks,
Bill

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Your assumptions are essentially correct, however this is ignoring the paper thickness.

1) When properly zeroed the tip of the hotend (nozzle) should be at the shimmed paper height, which is not touching the surface but very close. The gap is usually around .1mm, or whatever paper thickness you are using is. Your LCD should read 0.00 at this point.

2) When printing, the tip of the hotend (nozzle) should be at the LCD height PLUS paper shim height. If LCD reads .35mm then your nozzle height should be ~.45mm.

3) This assumption is correct. Deltas between layers should be exactly what the layer height is set too. In practice, the first layer height on the bed is hard to pin down to an exact number...it's more important that you just achieve good adhesion.

I've been suffering from issues similar to these lately too. My Kossel is running RC Marlin and it's G30 A Dnnn auto-cal didn't build a very good map of my bed heights. There could be a lot of reasons for it (which I'm slowly eliminating) but when it comes down to it, my first layer print is usually only .1mm-.2mm thick despite my kossel reading .35mm on the display. I've calibrated it to this point by adjusting my Z Max Height (seen as Z-Height in RC Marlin) to be larger than it was, which offsets the head a lower from the Endstops. Essentially your just tuning your machines build height (which auto-cal doesn't always figure out well).

If subsequent layers are the correct height then I would recommend you increase your Z Max Height. Adjusting the Z Probe Offset will not work if your machine/firmware has Z negative protection enabled. Theirs a G code command to disable it, but I don't have it handy right now. Anyway if I were you I would just increase Z Max Height, or Z-Height or whatever it's called in your machine, and give it a try.

In the couple days that I've had my printer I've come to find that auto-cal is not a magic bullet that works perfectly. Manual calibrations are necessary for most machines.

I'm having a similar issue with the ABL. My Z_PROBE_OFFSET_FROM_EXTRUDER is -0.70 with the paper loose between glass and nozzle. I load a print, it runs g29, starts print and the nozzle is hitting the bed. I change the offset to -0.50, upload and run the print again with the same result. Nozzle is still hitting the bed. I adjust the offset multiple times till I'm finally at 0.0 and still hitting the bed.

Update.. Changed the offset from -0.70 to 0.70. This should get me high enough to "air print". I saved and uploaded. It is still touching the glass. I can bring the Z up manually .20-.30 and it's off the bed. Could I have another setting off in Marlin or could it be Simplify3d?

Eqn coefficients: a: -0.00 b: -0.00 d: -2.24
planeNormal x: 0.00 y: 0.00 z: 1.00

Bed Level Correction Matrix:
0.999995 0.000000 -0.003181
-0.000001 1.000000 -0.000409
0.003181 0.000409 0.999995
echo:endstops hit: Z:-3.01
echo:Active Extruder: 0

Quote:

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Originally Posted by WatsonGravy

I'm having a similar issue with the ABL. My Z_PROBE_OFFSET_FROM_EXTRUDER is -0.70 with the paper loose between glass and nozzle. I load a print, it runs g29, starts print and the nozzle is hitting the bed. I change the offset to -0.50, upload and run the print again with the same result. Nozzle is still hitting the bed. I adjust the offset multiple times till I'm finally at 0.0 and still hitting the bed.

Update.. Changed the offset from -0.70 to 0.70. This should get me high enough to "air print". I saved and uploaded. It is still touching the glass. I can bring the Z up manually .20-.30 and it's off the bed. Could I have another setting off in Marlin or could it be Simplify3d?

Eqn coefficients: a: -0.00 b: -0.00 d: -2.24
planeNormal x: 0.00 y: 0.00 z: 1.00

Bed Level Correction Matrix:
0.999995 0.000000 -0.003181
-0.000001 1.000000 -0.000409
0.003181 0.000409 0.999995
echo:endstops hit: Z:-3.01
echo:Active Extruder: 0

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Try this and report back:

After just turning your printer on do a G28 to home. Send a G1 Z20 F2000 command to place your head at 20mm above the bed. Slowly move the head down .1-1mm at a time until it slightly grabs a piece of paper. What is the Z Height reading? It should be 0.00. If not then your Z-Height is set incorrectly. Note that I never instructed a G29 command. This process was just to check that your build height is correct. If it's not, then you need to adjust your build height until it slightly grabs paper at 0.00.

After you have set the correct build height you can try ABL. Pop your Z Probe down and send G29 Z5. Once it completes the ABL, send a G1 Z20 F1000 and see what happens. Does it look like 20mm? Do you need to raise or lower your G29 Zx number?

Remember that ABL is not necessary on a machine with a decently level bed and good manual calibration. More time should be spent achieving an ACTUALLY level bed then trying to get ABL to compensate. A nice metal ruler and some aluminum foil will help shim that bed to level in no time!

Quote:

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Remember that ABL is not necessary on a machine with a decently level bed and good manual calibration

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So far I agree. My steel Createc i3 does not vary at all, even when removing the glass, cooling overnight, reheating, putting glass back on, etc.

I think silicone thermal (Gino) pads (no metal clips) allow the glass to set, heat, remove, and generally just be independent of the bed's contractions. That, combined with rock-steady frame (steel has half the thermal expansion of aluminum), means that the nozzle height just does not change.

So, so far at least, no ABL for me.