Taking the original and replacing the 0.0001 with 0.01 (and cleaning up
the indentation):

module tube(d){
linear_extrude(100)
difference(){
circle(d=d);
circle (d=d-0.01);
};
}

module fillet(a,b,c,d){
hull(){
intersection(){
tube(30+a);  //upright tube
translate([-0,0,50])
rotate([0,90,0])
tube(20+b);
}
intersection(){
tube(30+c);  // side tube
translate([-0,0,50])
rotate([0,90,0])
tube(20+d);
}
}
}

$fn=100;
cylinder(d=30,h=100);
translate([-0,0,50])
rotate([0,90,0])
cylinder(d=20,h=50);

for (j= [0:2:90]){
bevel=12.01;
a=bevelsin(j);
b=bevelcos(j);
fillet(0,a,b,0);
}

yields (Windows 11, OpenSCAD 2024.12.26, preview or Manifold:

And the same for preview in 2021.01, though it takes a very long time
there.

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I wasn't really sure how this worked, so I deconstructed it and made sort
of an exploded diagram showing the components of the result.

One thing that makes it a bit hard to follow is the fact that it works
with tubes rather than cylinders.  Some of the shapes being manipulated are
very very small and rely on hull to turn them back into full shapes.  I
switched it over to cylinders and that made it easier to see.

I also changed it over to being a cross-shaped intersection, because
making everything be centered got rid of some translates that were
confusing me, and made some aspects of the diagram below easier.

Here's the diagram.  Each column (0, 30, 60, 90) is one component of the
final curve; each row (A, B, C, D) is a step in the process.  This is a
very low-resolution version so that you have more of a chance of seeing how
the results fit into the final product; the original model had many more
columns so as to produce a smoother curve.

Row A1 is the combination of a horizontal cylinder with the diameter of
the object's horizontal cylinder, and a vertical cylinder that varies in
diameter from the total length of the bevel down to the diameter of the
object's original vertical cylinder.
Row A2 is sort of the transposition of A1; the vertical cylinder is the
diameter of the object's vertical cylinder, and the horizontal cylinder
grows in diameter from the diameter of the horizontal cylinder to the total
vertical length of the bevel.

Row B1 is the intersection of the two shapes in row A1, so it is a segment
of horizontal cylinder that varies in length from the total horizontal
length of the bevel down to the diameter of the vertical cylinder.
Row B2 is similarly the intersection of the two shapes in row A2.

Row C is the union of the shapes in B1 and B2.

Row D is the hull of the shape in C.

Finally, the shapes in row D are unioned to produce the final shape to the
left.

It's a clever technique.

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Good explanation
One thing I would like to add here is that increase of diameters is
not in straight line but in kind of circular traces like cos theta and
sin theta for the 2 solids i.e. vertical and horizontal cylinders.

For example if vertical cylinder is increased in dia by sin 30,
horizontal cylinder is increased by cos 30.

At 0 deg 1 cylinder is increased by 1 unit while the other is at its
original shape.

This will give the desired shape of the fillet

On Thu, 16 Jan, 2025, 9:44 am Jordan Brown via Discuss,
discuss@lists.openscad.org wrote:

 I wasn't really sure how this worked, so I deconstructed it and
 made sort of an exploded diagram showing the components of the result.

 One thing that makes it a bit hard to follow is the fact that it
 works with tubes rather than cylinders.  Some of the shapes being
 manipulated are very very small and rely on hull to turn them back
 into full shapes.  I switched it over to cylinders and that made
 it easier to see.

 I also changed it over to being a cross-shaped intersection,
 because making everything be centered got rid of some translates
 that were confusing me, and made some aspects of the diagram below
 easier.

 Here's the diagram.  Each column (0, 30, 60, 90) is one component
 of the final curve; each row (A, B, C, D) is a step in the
 process.  This is a very low-resolution version so that you have
 more of a chance of seeing how the results fit into the final
 product; the original model had many more columns so as to produce
 a smoother curve.


 Row A1 is the combination of a horizontal cylinder with the
 diameter of the object's horizontal cylinder, and a vertical
 cylinder that varies in diameter from the total length of the
 bevel down to the diameter of the object's original vertical cylinder.
 Row A2 is sort of the transposition of A1; the vertical cylinder
 is the diameter of the object's vertical cylinder, and the
 horizontal cylinder grows in diameter from the diameter of the
 horizontal cylinder to the total vertical length of the bevel.

 Row B1 is the intersection of the two shapes in row A1, so it is a
 segment of horizontal cylinder that varies in length from the
 total horizontal length of the bevel down to the diameter of the
 vertical cylinder.
 Row B2 is similarly the intersection of the two shapes in row A2.

 Row C is the union of the shapes in B1 and B2.

 Row D is the hull of the shape in C.

 Finally, the shapes in row D are unioned to produce the final
 shape to the left.


 It's a clever technique.

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