What's going on with this first layer...?  

RE: What's going on with this first layer...?

Seems your zOffset is still too high(not negative enough).  The gaps between extrusion lines shouldn't be there.

By "change in filament diameter" did you mean the extrusion width?   I wouldn't mess with extrusion width until you've dialed in your zOffset.  I made that mistake early on by making things 200% of the nozzle diameter because I couldn't get things to stick.  Friendly folks on here helped me get my zOffset dialed in and I now print at the default extrusion width for a .4mm nozzle.   

RE: What's going on with this first layer...?

Posted by: @livius

I'm not sure what's going on with this first layer.  I've been pretty happy with the first layer height calibration.  It just looks like there isn't enough material dispensed.  What's weird is that it's happening in circular area about the center of the bed.  

A way to check: I guess your print bed is not flat. Check this with a ruler. The easiest way to correct this mechanically is to put thin foil under the printing plate. Besides a well adjusted Z first layer, it is of course also important that the print bed is flat and well leveled, which is often noticeable with large print parts.

RE: What's going on with this first layer...?

Thanks everyone for the input.  BTW, what I meant by "change in filament diameter" is the change in filament diameter going into the extruder, the manufactured diameter.  If it was out of spec from Ø1.75mm nominal, I was thinking it would change the volume flowing onto the bed.  I no longer think that the case based on the input and my troubleshooting.

I set my first layer cal according to the suggestion from @fuchsr, thanks.  It worked great.  Output is here.  I went with -1.840 (top left corner).

Then I printed the advanced bed level correction pattern and got this.  I dropped the back side only till I got this.  Here's a comparison of before/after.  I took a micrometer to the patches and they're all about 0.22mm thick except the center, it's at 0.233mm.  It's obvious that the bed is still low in the middle but I think I've exhausted calibration options on the machine.   I'm kinda happy where I ended up for now, unless there are additional thoughts.

I did mount a dial indicator to the E-axis and drove around to all four corners of the bed.  The right side of the bed is 0.38mm taller than the left and front to back is fairly flat, within 0.03mm.  As a result, I have some questions:

1. Why is the physical bed tilt not impacting my first layer on the perimeter of the bed leveling pattern in the tilt direction?  My bed level correction was in the other direction, where the variation was 0.03mm front/back.
2. The Z Calibration where the X-axis slams into the top brackets to synchronize the Z-Steppers and set the X-axis horizontal, is that accurate?  It seems like the X-axis would be parallel to how those brackets were screwed in and that would necessarily be parallel to the bed.
3. Should I pursue physically leveling the bed with respect to #2?  I'm amazed that the machine can compensate for 0.38mm bed height. 

Let me know what you think.

RE: What's going on with this first layer...?

Also, should i try to squeeze out the additional 0.02mm in layer thickness by tweaking the first layer calibration?

RE: What's going on with this first layer...?

Posted by: @livius

Also, should i try to squeeze out the additional 0.02mm in layer thickness by tweaking the first layer calibration?

Feels a bit ambitious to me 😊