Designing a phone cover, part I

For my next project, I want to play around with my flexible filament. Because I've overcome so many problems, it's time to add some more. But also because I want to make a case for my Nexus 5X.

While designing this case, I got to try out some OpenSCAD operations I hadn't done so much with before, in particular the minkowski() operator. While it works nicely for giving the case an outer rounding, I ran into a nice little problem when doing the interior. You see, the Nexus 5X isn't just a rounded box with rounded edges. The backside is tapering off towards the edges:

So my first model of the phone itself (to be used negatively) using just a hull of four cylinders wouldn't work. Instead, I tried to use an intersection of the hull with a minkowski of a similar hull:

module doubleround(size, corner, chamfer)
{
  $fn=30;

  minkowski(){
    hull() {
      for (x = [0, size[0]]) {
        for (y = [0, size[1]]) {
          translate([x, y, 0])
            cylinder(r=corner, h=size[2]);
        }
      }
    }
    //cylinder(r=radius);
    // Using a sphere is possible, but will kill performance
    sphere(r=chamfer);
  }
}

module nexus_5x_case() {
  intersection() {
    hull() {
      for (x = [rounding, width-rounding]) {
        for (y = [rounding, height-rounding]) {
          translate([x, y, 0])
            cylinder(r=rounding, h=thickness);
        }
      }
    }
    translate([rounding+under_round/2,
               rounding+under_round/2,
               under_round])
      doubleround([width-2*rounding-under_round, 
                   height-2*rounding-under_round,
                   thickness], rounding, under_round);
  }
}

Thanks to nophead for providing me the excellent word "chamfer", though he does it by subtracting an inverted corner.

This unfortunately led to some bumps in the corners:

The better approach turns out to be doing a hull() of an intersection of a cylinder and a large sphere:

module nexus5x_base() {
    under_round = 20;
  intersection() {
    hull() {
      for (x = [rounding, width-rounding]) {
        for (y = [rounding, height-rounding]) {
          translate([x, y, 0])
            intersection() {
              cylinder(r=rounding, h=thickness);
          translate([0,0,under_round])
              sphere(r=under_round, $fn=50);
            }
        }
      }

    }
}

Much better. Alas, my first attempt at printing it fell prey to stripping again, though this time in a rather artistic manner:

So currently my two main problems are stripping and tangling filament. Stripping comes partly from poor Z adjustment and the tangling, but possibly also from poor adjustment of the extruder idler. 

I'm not sure what to do about the tangling filament. I haven't seen any conclusive tips on how to best prevent it, and it seems to be getting worse - I have to babysit the printing process now.