The precise cause is that rotation by 90 degrees is not exact in OpenScad
because it converts degrees to radians before doing the trig functions.

If you redefine rotate as follows: -

module rotate(a)
{
cx = cos(a[0]);
cy = cos(a[1]);
cz = cos(a[2]);
sx = sin(a[0]);
sy = sin(a[1]);
sz = sin(a[2]);
multmatrix([
[ cy * cz, cz * sx * sy - cx * sz, cx * cz * sy + sx * sz, 0],
[ cy * sz, cx * cz + sx * sy * sz,-cz * sx + cx * sy * sz, 0],
[-sy,      cy * sx,                cx * cy,                0],
[ 0,      0,                      0,                      1]
]) children();
}

Then F6 works because GCAL can work with coincident faces if they are
exactly coincident. F5 still gives shimmering holes due to Z buffer
fighting so you still need to make the cylinders longer to fix that.

On 10 March 2016 at 22:04, Eric Buijs ebuijs@mac.com wrote:

nop head,

Interesting observation; however, it seems to me that your rotate would
require the same convertions.

2016-03-10 20:40 GMT-03:00 Eric Buijs ebuijs@mac.com:

Why not using a simpler way like

H  = 90;
W  = 5;
D  = 55;
eps = 0.005;
H1  = H - 2 * W;
H2  = H + eps;
$fn = 360;

difference() {
cube([ H, H, H], center =true);
cube([H1,H1,H1], center =true);
cylinder(d = D, h = H2, center = true);
rotate([90,0,0]) cylinder(d = D, h = H2, center = true);
rotate([0,90,0]) cylinder(d = D, h = H2, center = true); }

Just my $0.02,

Jean-Paul
N1JPL

Yes but the OpenScad sin and cos functions exposed to the user has special
cases for 0, 30, 45, 90, etc. Unfortunately it doesn't use those internally
for rotate and generating circles.

On 11 March 2016 at 00:52, Ronaldo Persiano rcmpersiano@gmail.com wrote: