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Help with difference of an object from an object generated by an module

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Help with difference of an object from an object generated by an module

OpenSCAD mailing list-2
Hi,
I want to model a curved surface and create a hole in it (around the middle of it)
I have the code that should create this, but instead of difference, it seems to combine the objects.

Here is the code:
the module curvedBase does create a curved surface and rotate it in the horizontal plane
the module main should cut a cylinder from the surface generated by the curved module
The result I get are the 2 combined objects, not the difference

Any help is welcomed. 

========================================
$fn= 360;

w  = 40;      // width of rectangle
h  =   2;      // height of rectangle
l  =  90;      // length of chord of the curve
dh =   4;      // delta height of the curve

main();

module main(){
  difference(){
    curvedBase();
      
    translate([0,20,0])
      cylinder(  20,r1=10, d2=20, center=true);
  }
}

module curve(width, height, length, dh) {
    r = (pow(length/2, 2) + pow(dh, 2))/(2*dh);
    a = 2*asin((length/2)/r);
    translate([-(r -dh), 0, -width/2]) rotate([0, 0, -a/2])         rotate_extrude(angle = a) translate([r, 0, 0]) square(size = [height, width], center = true);
}

module curvedBase(){
  rotate([0,-90,90])
    curve(w, h, l, dh);
}



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lar3ry
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Re: Help with difference of an object from an object generated by an module

lar3ry
I just copied your code and pasted it into OpenSCAD.
Here's a screenshot after a preview.



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JordanBrown
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Re: Help with difference of an object from an object generated by an module

JordanBrown
In reply to this post by OpenSCAD mailing list-2
I too get the expected object with a hole in it.

Perhaps you have View/Thrown Together set?  That makes it look like the objects are combined instead of differenced.

A few comments on the program...

On 4/15/2021 8:52 PM, sorin vatasoiu via Discuss wrote:
$fn= 360;

360 is a lot for $fn.  In more complex models, that's likely to cost you a lot of time.

With $fn=360, your model has 412 facets and looks like this:


With $fa=1 and $fs=1, your model has 115 facets and looks like this:


You can see the difference on the screen.  My bet is that you can't see the difference in plastic.

Roughly, $fa controls the behavior of large circles and $fs controls the behavior of small circles.  The main body of the object ends up the same in either variation, with 1° segments.  The difference is in the hole, which has 360 segments in one variation and about 60 in the other.  Those 60 segments are each 1mm long.

      cylinder(  20,r1=10, d2=20, center=true);

You've asked for a cone with a bottom radius of 10 and a top diameter of 20.  It's unusual to specify a radius for one and a diameter for the other, but more importantly those are the same size so you could have just used r=10 or d=20.


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jon_bondy
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Re: Help with difference of an object from an object generated by an module

jon_bondy
I couldn't find references to $fn, $fa, and $fs on the OpenSCAD cheat
sheet: would it make sense to include them there?

On 4/16/2021 12:48 AM, Jordan Brown wrote:
> With $fa=1 and $fs=1, your model has 115 facets and looks like this:
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lar3ry
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Re: Help with difference of an object from an object generated by an module

lar3ry
In reply to this post by JordanBrown
JordanBrown wrote
>       cylinder(  20,r1=10, d2=20, center=true);

You've asked for a cone with a bottom radius of 10 and a top diameter of
20.  It's unusual to specify a radius for one and a diameter for the
other, but more importantly those are the same size so you could have
just used r=10 or d=20.
I tend to use radius or diameter interchangeably, depending on how I think of the way the part fits other parts, or the use of the part of the part itself. Yes, I might even use radius and diameter for opposite ends of a cylinder for the same reason. But yes, if they are the same size, it's better for readability to make them the same units.

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Re: Help with difference of an object from an object generated by an module

jon_bondy
In reply to this post by jon_bondy
My bad: they are already present

On 4/16/2021 3:06 PM, jon wrote:
> I couldn't find references to $fn, $fa, and $fs on the OpenSCAD cheat
> sheet: would it make sense to include them there?
>
> On 4/16/2021 12:48 AM, Jordan Brown wrote:
>> With $fa=1 and $fs=1, your model has 115 facets and looks like this:
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