[OpenSCAD] Newbie question

lar3ry lar3ry at sasktel.net
Thu May 23 02:18:07 EDT 2019


G. Wade Johnson wrote
> The problem in both cases is a subtle issue that we all dealt with. The
> shape you want to difference cannot be the exact same length as shape
> you are subtracting from. Otherwise, you end up with a very thin
> membrane at each end.

Are you speaking of the preview? If so, I would not consider that a
membrane. It's congruent surfaces interfering with the preview. The render
gives exactly what I would expect, a hollow cylinder.


> Try
> 
> difference() {
>    translate([0,37,15]) rotate([90,0,0]) cylinder(h=20,r=5);
>    translate([0,37,15]) rotate([90,0,0]) cylinder(h=21,r=4);
> }
> 
> I normally have a constant eps, defined at the top of my file, that I
> add to the lengths like this to make it obvious that I am using a fudge
> factor
> 
> eps=0.01;
> 
> difference() {
>    translate([0,37,15]) rotate([90,0,0]) cylinder(h=20,r=5);
>    translate([0,37,15]) rotate([90,0,0]) cylinder(h=20+eps,r=4);
> }
> 
> The second example works the same way with a small addition (like eps),
> Doesn't work as well, adding a 1. (Wall thickness is not what you
> expect in that case.)

I don't get it. The two examples you gave resulted in exactly the same
object.
Yes I do realize that this can sometimes create an STL that has errors, but
if that happens, THAT's when I take steps to get rid of the problem.

Larry



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