Render fails when threads cut through bottlecap

I'm trying to fit a bottlecap to the decorative bottle shown in the photo below. The owner lost the cap but doesn't want to discard such a lovely bottle.  I have a 3D printer, so I said I would design and print a new bottlecap.  As part of the fitting process, I tried to create a bottlecap with threads that 'cut through' the cap material so that I could visually see how the bottlecap 'innie' threads match up with the bottlecap 'outie' threads.  Imagine my surprise when, after generating the model I wanted, the rendering process completely obliterated the threads, the cuts, and everything else, resulting in a solid cylinder of material.  



Here are some some screenshots showing the problem:


F5 preview of 'normal' thread geometry


Render operation for 'normal' geometry


F5 preview of 'cut through' thread geometry


Render operation for 'cut through' geometry

And here is the code that produced the above geometries:

210430_BottleCap_Normal.scad
210430_BottleCap_CutThrough.scad

The only difference between the two .SCAD files above is that the 'normal' file has 'thread_depth=5.75' in the 'helix_thread' command, and the 'cut through' file has 'thread_depth=5.75'

Any ideas?  I'd really like to be able to print this (maybe modified slightly to add some support for the 'flying thread' portion at the end) so I can visualize the fit to the bottle.

TIA,

Frank

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I'd guess that the BOSL2 library has a bit of a problem with deep thread_depth values.

If you create the thread outside of the difference using

thread_helix(d=37, pitch=7, thread_depth=15.75, thread_angle=25, twist=520, $fn=72);

... you'll see that the thread no longer has a clean, triangular cut.  The result is probably not manifold.  In this case, a thread_depth greater than about 6.5 shows that problem.

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Personally, I would not do a deep thread cut like you did. If you want to see how the threads fit, I would suggest something like this:

include <BOSL2/std.scad>
include <BOSL2/threading.scad>
include <BOSL2/bottlecaps.scad>

difference() {
        translate([0,0,-2.5]) cylinder(18,21.5,21.5);
        cylinder(24,19,19);
        translate([-30,-5,0]) cube([60,10,25]);
        translate([0,0,11])
        thread_helix(d=37, pitch=7, thread_depth=5.75, thread_angle=25, twist=520, $fn=72);
}

The big advantage of this method is that you don't need supports.
And for speed of printing, you might want to cut out some more slots.

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Thanks to both responders; Once I knew what to look for, it was obvious the deep thread setting created non-manifold artifacts.

Also, I tried the 'rectangular cut' idea and this worked nicely as well - thanks!

Frank

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