Cutting a Cylindrical shell

­Hello

I have made a cylindrical shell using binary difference between two cylinders.

I have made a cubic bezier curve using this : https://openhome.cc/eGossip/OpenSCAD/BezierCurve.html

The bezier_coordinate function spits out the coordinates for a polyline to be drawn.

I can project these points on the surface of the cylindrical shell, effectivly "wrapping the curve" on the outer cylindrical surface , The curve is a closed curve,

I want to find a way to cut the cylindrical shell along this line. As if, an arbitrarily small milling bit has milled through the entire depth of the shell , splitting it in different parts.

How is that possible using openscad. Thank you.
 





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­ How can we extract such faces from a shell given some conditions?
Thank you

Von: nop head <[hidden email]>
Datum: 24.04.2021 00:30:51
An: Sayandeep Khan <[hidden email]>, OpenSCAD general discussion <[hidden email]>
Betreff: Re: [OpenSCAD] Cutting a Cylindrical shell

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@...> wrote:

­Hello

I have made a cylindrical shell using binary difference between two cylinders.

I have made a cubic bezier curve using this : https://openhome.cc/eGossip/OpenSCAD/BezierCurve.html

The bezier_coordinate function spits out the coordinates for a polyline to be drawn.

I can project these points on the surface of the cylindrical shell, effectivly "wrapping the curve" on the outer cylindrical surface , The curve is a closed curve,

I want to find a way to cut the cylindrical shell along this line. As if, an arbitrarily small milling bit has milled through the entire depth of the shell , splitting it in different parts.

How is that possible using openscad. Thank you.
 

 

 

 

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­ How can we extract such faces from a shell given some conditions?

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.

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

­ How can we extract such faces from a shell given some conditions?

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.

[ And on autopilot I hit Ctrl+Enter and sent the message when I wasn't ready to... ]

If you have N points in your Bezier curve, you're probably going to have 3*N, or maybe 2*N+1 faces:

• Your top surface is a bunch of triangles with their outside vertices along the Bezier curve that's wrapped around the cylinder, and the inside vertex at the center of the cylinder at some middle-of-the-road Z value
• The shell is a bunch of quadrilaterals with those same outside vertices as the top vertices, and the bottom vertices being those same X/Y values and Z just below the base of the cylinder to be cut.
• The bottom surface might be a set of triangles similar to the top surface, but all with that same just-below-the-base Z value, or maybe you can do it as one big N-gon.

Doing this is a bit intricate, but isn't all that *hard* once you get the pattern down.  Mostly it's a bunch of list comprehensions, and some light trigonometry to walk around the circumference of the cylinder-like polyhedron that you're constructing.

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On 2021-04-24 00:01, Sayandeep Khan wrote:
> I want to find a way to cut the cylindrical shell along this line. As
> if, an arbitrarily small milling bit has milled through the entire
> depth of the shell , splitting it in different parts.
>
> How is that possible using openscad. Thank you.

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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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: [hidden email]
Datum: 24.04.2021 09:30:03
An: OpenSCAD general discussion <[hidden email]>
Betreff: [OpenSCAD] Re: Cutting a Cylindrical shell

On 2021-04-24 00:01, Sayandeep Khan wrote:
> I want to find a way to cut the cylindrical shell along this line. As

> if, an arbitrarily small milling bit has milled through the entire
> depth of the shell , splitting it in different parts.
>
> How is that possible using openscad. Thank you.

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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To unsubscribe send an email to [hidden email]


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On 2021-04-24 12:10, Sayandeep Khan wrote:
> Indeen my curve is not planer any more after wrapping it around a
> cylinder.
> But I like the way you propose.
>
> Thank you.

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

Carsten Arnholm
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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]) [r*cos(theta), r*sin(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);
}

cacb wrote

On 2021-04-24 00:01, Sayandeep Khan wrote:
> I want to find a way to cut the cylindrical shell along this line. As
> if, an arbitrarily small milling bit has milled through the entire
> depth of the shell , splitting it in different parts.
>
> How is that possible using openscad. Thank you.

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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The cutting shape produced by that code has a sharp corner, which means it looks OK for the very thin shell in the demo, but doesn't work so well if the shell is thicker.  This is a result of trying to construct the non-planar "face" defined by the cutting curve, so the approach may work for some curves but not so well for others.  It's as if the cutting bit has turned at a very extreme angle instead of remaining perpendicular to the cylinder.  The path_sweep example keeps the cutting bit perpendicular and will work for any curve whose slope stays finite (the normal has a nonzero component in the z direction).  

Note that my example requires <include<BOSL2/std.scad> which I forgot to include in my post.  

This is what the cutting shape looks like from the skin example:

Ronaldo wrote

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]) [r*cos(i*360/n), r*sin(i*360/n),
a*cos(3*i*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 <[hidden email]> escreveu:

> 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:
>
>

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That solution does appear OK, though it seems better to just use a circle for the top instead of the curved profile.  However, having gone that far, why not just make the entire shape with skin().   Note that this approach only makes one half of the shape the original poster seemed to be requesting.  This could also be constructed using vnf_vertex_array instead of skin.  

include<BOSL2/std.scad>

$fn=90;
r=8;
thickness=1;
len=21;
n=4;
outer_curve = [for(theta=[360:-4:1])
                 [r*cos(theta), r*sin(theta), 10+sin(n*theta)]];
inner_curve = [for(theta=[360:-4:1])
              [(r-thickness)*cos(theta), (r-thickness)*sin(theta), 10+sin(n*theta)]];
skin([inner_curve, outer_curve, path3d(circle(r),len), path3d(circle(r-thickness), len)], slices=0, closed=true);

Ronaldo wrote

> Theg bit perpendicular and will work for any curve whose slope stays
> finite (the normal has a nonzero component in the z direction).


The inclusion of one more section to skin should solve the thickness issue:

include <BOSL2/std.scad>

$fn=90;
r=8;
thickness=1;
h = 20;

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

n = 50;
a = 3;
cyl_curve  = [for(i=[0:n-1]) [r*cos(i*360/n), r*sin(i*360/n),
a*cos(3*i*360/n)] ];


out_cyl_curve = [for(p=cyl_curve) [1.1*p[0], 1.1*p[1], p[2]]] ;
in_cyl_curve  = [for(p=cyl_curve) [(0.9*(r-thickness)/r)*p[0],
(0.9*(r-thickness)/r)*p[1], p[2]]] ;

out_cyl_curve_up = [for(p=out_cyl_curve) p + [0,0,h] ];

//trace_path(cyl_curve, size=.5, closed=true);
intersection(convexity=10) {
  cyl_shell(r,h);
  skin([in_cyl_curve, out_cyl_curve, out_cyl_curve_up],slices=1);
}



>

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