How Thick is a Piece of Paper?

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KenW
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How Thick is a Piece of Paper?

Post by KenW »

Following discussion on a Facebook group, I decided to find out what my actual Z offset gap is and ordered a set of feeler gauges.

0.05mm

But there’s a problem: the feeler gauges are protected from rust by a thin film of oil, so I expect I have contaminated the print bed in the process :angry:

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Re: How Thick is a Piece of Paper?

Post by slybunda »

clean with ipa

KenW
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Re: How Thick is a Piece of Paper?

Post by KenW »

IPA = isopropyl alcohol AKA rubbing alcohol.

Yes, although for oily substances I think white spirit might be more effective.

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Re: How Thick is a Piece of Paper?

Post by CrazyIvan »

I've had some thoughts on this.

The "thickness of a piece of paper" is a rule of thumb; the actual correct Z offset will allow sufficient gap for the full amount of filament to be extruded (as calculated by the slicer), while not being any greater than necessary so that the filament makes good adhesion with the print bed.

We will only know the gap is too small if the extruder clicks while printing layer 1, so what we need is a single layer test file for calibrating Z offset.

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Re: How Thick is a Piece of Paper?

Post by KenW »

Following repair of my 2S (see here), I set up a 100x120x0.3mm slab in FreeCAD and exported it to Cura. Then I discovered Cura doesn't like going right up to the edge (even with no brim, I guess that's reasonable) so I scaled the slab (in Cura) to 90x110x0.3 and it still wouldn't fit!

The weird thing is that the Tina2 settings in Cura has a build plate 100x120, whereas the Tina2S settings are 100x110... even though the magnetic rubber build plate on Tina2 and the texture-coated metal build plate on Tina2S are the same size (110x120)! So for expediency I set the slab to 90x100 and had done with it.

I set the z offset to my usual slight grip on a slip of A4 and ran the print, the wall started OK and then detached, but the subsequent lines were OK and it started printing infill, but the infill lines were separated so I aborted.

Then I adjusted the z offset down by 0.1mm, which is a very firm grip on the A4. The print then performed beautifully.

Measurement with the feeler gauges now suggests that equates to 0.15mm.

Image

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Re: How Thick is a Piece of Paper?

Post by CrazyIvan »

I want to take those results with a pinch of salt. In particular, it seems odd that there are gaps between the infill lines in the left example when there do not appear to be gaps between the lines making up the outer walls (OK, so they're only two lines, but...). I think you need to examine whether there are any bottom layer settings (in Cura) which might be somehow creating sparse infill deliberately.

Even if that checks out, the next thing to consider is under-extrusion. Sean Aranda's book recommends setting first layer extrusion to 105% to improve adhesion, I guess that would also ensure complete coverage. Any tweaking of extrusion settings should start with checking the extruder calibration is correct.

The bottom line is that perhaps this issue is better solved from a different direction.

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Re: How Thick is a Piece of Paper?

Post by CrazyIvan »

I think I may have some answers, or at least a starting point for discussion. I've been examining the presets in Cura, and the output it produces, as well as looking at the gcode print files supplied with the Tina2.

By default, the first layer is set to be 0.3mm thick, and subsequent layers 0.15mm ("normal" profile). In the gcode, the first layer is printed with Z=0.3, the next layer Z=0.45, etc.

The extruder commands feed filament on the expectation that the first layer is 0.3mm thick... except it isn't, the nozzle is actually flying 0.3mm plus "the thickness of a piece of paper" (whether that be 0.05mm or 0.15mm). That means under-extrusion by between 14% and 33%!

It would only be correctly extruded if the Z calibration made zero to be when the nozzle is in contact with the build plate. The trouble is, that would risk gouging the build plate if the nozzle were moved while Z=0, and any unevenness in the build plate could ground the nozzle.

What to do about it? I don't know yet, but we need to either tell Cura to over-extrude by 16-50% (for the first layer only), or get it to subtract 0.05-0.15mm from the Z on the first (and subsequent) layers.

Incidentally, an example file supplied with the Entina Tina2 has a first layer height 0.349mm, then 0.27mm, then 0.15mm thereafter.

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