Ah, that is really interesting and it explains a lot. I guess I should just quit obsessing with the line width but focus on the visual. Or if I want to calibrate, I should just make sure the extruder's steps are correct.
For calibrating the extruder steps, is there any accurate way to do it? I mean I did it by measuring the distance travelled by the filament when extruding. However, as I have mentioned before, the filament is a bit curled so measuring it using a caliper will introduce an error.
Considering what goes on in the little space under a nozzle reminds me of considering what goes on in the little space of the combustion chamber of an internal combustion engine. For such small spaces, it's complicated.
When printing conventionally the stripe of plastic is often trapped on one side by a previous stripe of plastic. The new stripe can't flow that way because the previous stripe has created a wall and so it pushes the other way. The line width is the "Index" movement between one stripe and the next so it isn't really "Wall Width". In Vase mode the width of the plastic stripe is dependent on temperature, the material properties, nozzle wear, first layer vs other layers, flow, etc. and the stripe is not trapped. You would expect it to be exactly 1/2 line width on either side of the nozzle centerline, but that's not likely to happen although as you found, you can make it so.
The area under the nozzle is (usually) defined as a rectangle - "line width wide" x "layer height high". But the Vase mode stripe isn't a rectangle because it's molten plastic so the sides bulge into a radius making the shape into a flattened oval. That may be what you are measuring - the additional .05 radius on either side of the Vase mode stripe. Printing conventionally (at .4 line width), the stripe is trapped on one side by a previous stripe so it's .2+ on one side, and .2- on the other side of the centerline. At some point you have to make a decision on how to treat the area under the nozzle. Is it a rectangle, an oval, or 1/2 of each? It's a compromise which of course means that it's wrong either way. I think Simplify3D uses a fudge factor in regards to the size of the rectangle to try to account for the fact that it isn't really a rectangle. Cura seems to be straight forward in considering that it is a rectangle.
I've come to the conclusion (opinion really) that trying to calibrate for flow using any method beyond calibrating the Esteps is useless. Better to use the Mark I Eyeball because as you noted when you "saw the gaps"... when it looks good it's good and when it looks bad it ain't good. I happen to have a microscope to assist my Mark I Eyeball and I have verified that good is different from bad to my personal satisfaction.
When printing in Vase mode I usually go to .6 line width with my .4 nozzle. I like the way it looks.
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