I think you need to convert the point list into a polyhedron by adding a
second set of points on a circle bigger than the tube and further away then
its end. Then list all the faces to connect those points to your curve.
That would give a 3D shape that you can subtract from the tube to cut it.

On Fri, 23 Apr 2021 at 23:02, Sayandeep Khan seany@eclipso.de wrote:

You could simply duplicate the points and give them two different z values,
one where you want the cut and the other far away. Concatenate the lists
and pass that as the point list for polyhedron. Then for the faces list you
need to make the walls and the endcaps by listing triangles that index the
point list.

On Sat, 24 Apr 2021 at 00:04, Sayandeep Khan seany@eclipso.de wrote:

On 4/23/2021 4:03 PM, Sayandeep Khan wrote:

I'd be happy to cook up a demonstration, but I don't have time right now.

Here's the basic theory...

Have you looked at how you build a polyhedron?  You define a bunch of
points, and then you connect groups of those points into faces that all
add up to being the polyhedron.

The idea here is that you're going to build a polyhedron that mostly
looks like a cylinder a bit bigger than the cylinder that you're going
to cut, except that the top of this polyhedron is wavy, using your
Bezier curve.

On 2021-04-24 00:01, Sayandeep Khan wrote:

If your curve is planar, there are simple ways to do it. If the curve is
general and non-planar, the suggestions given by others here apply, but
constructing a polyhedron from the ground up can be cumbersome.

Perhaps another approach is workable by first modelling the small drill
bit as a small polyhedron, then place copies of it, properly oriented,
in every point along the split curve. Union pairs of neighbouring drill
bits and use them to subtract from the cylindrical shell. When done for
all pairs, the result is a split cylinder.

Carsten Arnholm

Indeen my curve is not planer any more after wrapping it around a cylinder.
But I like the way you propose.

Thank you.

--- Ursprüngliche Nachricht ---
Von: arnholm@arnholm.org
Datum: 24.04.2021 09:30:03
An: OpenSCAD general discussion discuss@lists.openscad.org
Betreff: [OpenSCAD] Re: Cutting a Cylindrical shell

On 2021-04-24 00:01, Sayandeep Khan wrote:

If your curve is planar, there are simple ways to do it. If the curve is

general and non-planar, the suggestions given by others here apply, but

constructing a polyhedron from the ground up can be cumbersome.

Perhaps another approach is workable by first modelling the small drill

bit as a small polyhedron, then place copies of it, properly oriented,
in every point along the split curve. Union pairs of neighbouring drill

bits and use them to subtract from the cylindrical shell. When done for

all pairs, the result is a split cylinder.

Carsten Arnholm

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On 2021-04-24 12:10, Sayandeep Khan wrote:

Actually, I meant to say use hull() rather than union() for the drill
bit pairs

Carsten Arnholm

This approach will work, but it will be very slow.  I think the right
approach is to use a sweep to create the cutting polyhedron.  As you say,
it's hard to construct a polyhedron from the ground up, which is why you
should instead use helper functions to do it.  Many libraries provide a
sweep function such as dotSCAD, Parkinbot's naca sweep, and BOSL2.
Using BOSL2:

https://github.com/revarbat/BOSL2/wiki/skin.scad#functionmodule-path_sweep

$fn=90;
r=8;
thickness=1;
len=21;

curve = [for(theta=[0:4:359]) [rcos(theta), rsin(theta),
10+sin(4*theta)]];

difference(){
cylinder(r=r, l=len);
down(.5)cylinder(r=r-thickness, l=len+1);
path_sweep(left(.05,square([1.1,1])), curve, closed=true, method="manual",
normal=UP);
}

http://forum.openscad.org/file/t2477/cutcyl.png

cacb wrote

--
Sent from: http://forum.openscad.org/

An alternative approach is to use BOSL2 skin() to make a polyhedron to cut
away the cylinder unneeded part. Oskar Lange's skin() possibly would work
too.

include <BOSL2/std.scad>

r = 10;
h = 20;
module cyl_shell(r,h) {
difference() {
cylinder(r=r,h=h,center=true);
cylinder(r=r-0.1,h=h+.1,center=true);
}
}

n = 50;
a = 3;
cyl_curve = [for(i=[0:n-1]) [rcos(i360/n), rsin(i360/n),
acos(3i*360/n)] ];

cyl_curve_up = [for(p=cyl_curve) p + [0,0,100] ];

//trace_path(cyl_curve, size=.5, closed=true);
intersection() {
cyl_shell(r,h);
skin(1.01*[cyl_curve, cyl_curve_up],slices=1);
}

Em sáb., 24 de abr. de 2021 às 13:23, adrianv avm4@cornell.edu escreveu: