I took jimc's advise and played around with the Extrusion Override control in the Machine Control Panel. I started the print with the same settings I was using yesterday afternoon (see last post; this is Zone 1 on attached photo) and when the first bottom solid layer was about 1/3 complete I dropped the extrusion override to 56%. The remainder of the first bottom solid layer looked pretty good (see Zone 2 on attached photo). Layer 2, which is also a bottom solid layer, began to print and it was apparent that the 56% reduction in extrusion rate was not enough to close the beads (see Zone 3 of the attached photo). I changed the extrusion override back to 100% and finished off about 1/3 of layer with good results (see Zone 4 of the attached photo).
Zone 1 measures 0.51-0.53 mm thick, just as I was reporting previously. Zone 2 measures 0.24-0.26 mm think, which is what is should measure as the FFF is set for 0.25mm layer height. Zone 3 measures 0.49-0.53 mm in the area over Zone 2 which is what you would expect (sum of Zone 2 and the nominal layer height). Zone 4 measures 0.67-0.70 mm over the Zone 1 region and 0.53-0.55 mm over the Zone 2 region.
So you can see from the photo that in Zone 1 the software is over extruding for the first bottom solid layer. This is verified by the fact that when I manual override the extrusion rate to 56% the ridges disappear and the layer height comes into spec (Zone 2). Remember, this is all happening in one print run so this eliminates the effects of trial print to print variability. Also, the fact that the 56% reduction in extrusion rate is insufficient to fill the second bottom solid layer (Zone 3) but 100% extrusion rate closes it solid with almost perfect height (Zone 4) demonstrates that the second bottom solid layer extrusion rate is good.
So while I have additional information its still not clear to me how to resolve this problem. And it is a problem. Tonight I tried to print three small parts on the bed at the same time and because of the ridge formation while working on the second layer when the nozzle passes over areas its already printed on the first layer it rips up the corners and edges because the ridge peaks are in the 0.45 - 0.50 mm height range.
I thought more about the Z end-stop position and while I have checked and double checked position and level, the actual position seems to be a mute point because you can simply use the First Layer Height (FLH) control to add an "electronic" offset to the Z axis starting position. Example, if you have the FLH set to 100% and the layer height set to 0.25 mm the commanded Z position for the first layer will be 0.25 mm BUT if you set the FLH to 125% with the same layer height setting the commanded Z potion for the first layer will be .312 mm (layer height X first layer height). I have verified this for several prints by monitoring the machine control panel position display and stopping prints in progress to measure with feeler gauges. So regardless of where I set the Z end-stop, if its reasonable, I can compensate by using the FLH. Therefore, it don't think the problem is with the Z end-stop position.
Sure seems like a lot of effort to get a part to print correctly. Has anyone experienced this kind of difficulty? Maybe I am overlooking something.
Thanks again for the help.
Thanks for the feedback everyone. More trials this evening. I will forego the details but in short I tried adjustment of the overlap percentage (seems to only effect how much a particular bead overlaps the perimeter shells, not the adjacent beads) and adding additional shells to the perimeter wall to see if I could coax out ridge formation. Turns out that with only two shells at the perimeter you do not get a ridge to form but if you raise that to 10 shells ridges form just like in the solid bottom layers. Conclusion: over extrusion on the first layer is not limited to the bottom solid layer but is also occurs in the perimeter band and skirt; there is simply room on the outside edge of each bead (inboard and outboard) for the material to flow in X-Y without Z displacement.