You need more cuboid with negative rounding. Sometimes you have to union
two nearly-identical cuboid with differing rounding and edges lists.

Also, try building the full inner negative mold first. Use attach and
align to build it. Try to avoid rotate/translate, especially multiple
times.

I find it more productive to do something roughly shaped like this:

module inner_negative_mold() {
  cube() {
    attach(BACK,BACK) cuboid(rounding=-2);
    attach(LEFT,BACK) cuboid(rounding=-2);
   }
}

module driver_phone_box() {
  diff() {
    cube();
    tag("remove") attach(CENTER, CENTER) inner_negative_mold();
  }
}

Also lets me view and work on each module separately.

Cory

On 9/27/26 7:09 PM, Ken via Discuss wrote:

Note that attach(CENTER,CENTER) doesn't do anything.  For this
construction align() may be easier to use than attach().  But definitely
all those random translates make the model incomprehensible and hard to
work with.

The other tool you can use is specifically subtracting masks to parts of
the model.  There are rounding masks in masks.scad you can check out.  That
can let you create more complex roundings on a cube.  And also
rounded_prism can be useful as it's significantly more flexible---but the
roundings are circular so it won't mate with cuboid roundings.

On Sun, Sep 27, 2026 at 10:50 PM Cory Cross via Discuss <
discuss@lists.openscad.org> wrote:

as far as Minkowski is concerned, and this is only what I think it does.

it starts at the origin of the object (this will be the second
argument), and in each direction (just consider the x,y,z), it adds,
point by point that object's radius to the basic shape.

So the main object is increased by the radius of the minkowski object
(say a sphere).

If you were to scale the main object by the ratio of the minkowski
object (same sphere), down, using scale and use (original
object/(minkowsk scale + original object)) as an argument, you should
get close.  I've done this on boxes and enclosures to get rounded corners.

Harvey

On 9/27/26 10:09 PM, Ken via Discuss wrote:

On 9/28/2026 10:28 AM, Harvey white via Discuss wrote:

Not the object's radius.  The object's shape.

Wikipedia has a formal explanation, but it's a bit hard to follow:
https://en.wikipedia.org/wiki/Minkowski_addition

Here's one way to look at it, in 2D:  you have shape A.  You have a
paintbrush in the shape of B.  Drag the paintbrush around the entire
perimeter of A, keeping the origin of the paintbrush on A's outline.

3D is similar, but with a harder-to-imagine 3D paintbrush.

If B is a circle, then it as you said:  add the radius.  But if B is
other than a circle, or if it's not centered on the origin, the result
is different.

Here's a simple non-circle, not-centered example:

minkowski() {
     #square(10);
     square([1,2]);
}

In the picture below, the orange-ish is the original ten-unit square,
and the gold is the part that gets added.  Note that because the second
square is entirely up and right of the origin, stuff is added only to
the top right, and because the second square is taller than it is wide,
more is added on the top than on the right.

In 3D you can get useful effects from this, e.g. by adding a cone.

$fa = 1;
$fs = 1;

minkowski() {
     cube(50);
     cylinder(h=10, r1=0, r2=10);
}

Scale won't do the right thing in the general case.  You really need an
offset operation, but we don't have 3D offset.

Say you have a [20,10] square.  You're adding a one-unit circle to it,
so you scale it by 18/20 to make it be 18 wide, so that when you add the
one-unit circle on each side you're back at 20. But it's only 10 tall,
so scaling it by 18/20 makes it be 9 tall, so when  you add the one-unit
circle on the top and bottom it's 11 tall.

Right now my brain isn't working well enough to come up with more simple
examples where it does the wrong thing, but they exist. Try, for
instance, a hollow box where you need to maintain both the outside
dimensions and the inside dimensions.

On 9/27/26 7:54 PM, Adrian Mariano via Discuss wrote:

Yeah, that's why I said "shaped", I would definitely have /something/
there, but I couldn't think what would be best so a put a placeholder :)

Cory