I've been working on a custom polyhedron, and want to make some circular
holes through it for motor spindles.

I've been using difference and cylinders, but on export it complains "Object
may not be a valid 2-manifold and may need repair!".

I have checked it in thrown together view, but all seems fine and correctly
orientated. I have also tried removing the cylinders and it then doesn't
report any warnings on export.

I've also tested with a simple polyhedron cube and a cube differenced
against that which works fine, which means I can do it in principle, but I
just can't figure out why it's misbehaving for me.

My research across the internet has yielded things to check but doesn't help
at all pinpointing my issue, but I could be missing something obvious.

Any help would be greatly appreciated. My code is below.

Many thanks
Chris

width=80;
length=90;
height=60;

wall_thickness=2;

motor_hole_radius=5;
motor_offset_from_rear=30;

wedge_length=6;
wedge_rise=2;

left_right_top_width=10;

rear_top_length=12;
rear_top_inner_rise=4.7;

module mainBody()
{
polyhedron(
points =
[
//outer bottom
[0,0,(height/2)],//FL (0)
[width,0,(height/2)],//FR (1)
[width, length, 0],//BR (2)
[0, length, 0],//BL (3)

    //outer top points
    [width, length, height],//BR (4)
    [0, length, height],//BL (5)

    //inner left
    [wall_thickness, (length-wall_thickness), wall_thickness],//BL

(Bottom) (6)
[wall_thickness, (length-wall_thickness),
(height-wall_thickness)],//BL (Top) (7)

    //inner right
    [(width-wall_thickness),(length-wall_thickness),wall_thickness],//BR

(Bottom) (8)

[(width-wall_thickness),(length-wall_thickness),(height-wall_thickness)],//BL
(Top) (9)

    //front wedge (back) (as looking from front)
    [wall_thickness,wedge_length,(height/2)],//L (Bottom) (10)
    [(width-wall_thickness),wedge_length,(height/2)],//R (Bottom) (11)
    
    //inner left top
    [(wall_thickness+left_right_top_width), wedge_length, (height/2)],

//front (12)

    //outer left top
    [(wall_thickness+left_right_top_width), wedge_length,

((height/2)+wedge_rise)], //front (13)

    //inner right top
    [(width-(wall_thickness+left_right_top_width)), wedge_length,

(height/2)], //front (14)

    //outer right top
    [(width-(wall_thickness+left_right_top_width)), wedge_length,

((height/2)+wedge_rise)], //front (15)

    //inner rear top
   

[(left_right_top_width+wall_thickness),(length-(wall_thickness+rear_top_length)),
(height-(rear_top_inner_rise+wall_thickness))],//L (16)

[(width-(left_right_top_width+wall_thickness)),(length-(wall_thickness+rear_top_length)),
(height-(rear_top_inner_rise+wall_thickness))],//R (17)

    //outer rear top
   

[(left_right_top_width+wall_thickness),(length-(wall_thickness+rear_top_length)),
(height-rear_top_inner_rise)],//L (18)

[(width-(left_right_top_width+wall_thickness)),(length-(wall_thickness+rear_top_length)),
(height-rear_top_inner_rise)],//R (19)

],
faces = 
[
    [0,1,2,3],//outer bottom
    [3,5,0],//outer left
    [1,4,2],// outer right
    [2,4,5,3],//outer back
    
    [10,6,8,11],//inner bottom
    
    [10,7,6], //inner left
    
    [8,9,11], //inner right
    
    [6,7,9,8], //inner back
    
    [0,13,15,1],//Front wedge (top)
    
    [14,15,13,12],//Front wedge (back)
    
    [12,16,7,10],//inner left top
    
    [0,5,18,13],//outer left top
    
    [11,9,17,14],//inner right top
    
    [15, 19, 4, 1],//outer right top
    
    [17,9,7,16],//inner rear top
    
    [18,5,4,19],//outer rear top
    
    [12,13,18,16],//close left top
    
    [17,19,15,14],//close right top
    
    [16,18,19,17],//close back top
]

);
}
difference()
{
mainBody();
//left motor hole
translate([-1,(length-motor_offset_from_rear),(height/2)]) rotate([0,90,0])
cylinder(h = wall_thickness+2, r1 = motor_hole_radius, r2 =
motor_hole_radius);

//right motor hole
translate([(width-(wall_thickness+1)),(length-motor_offset_from_rear),(height/2)])
rotate([0,90,0]) cylinder(h = wall_thickness+2, r1 = motor_hole_radius, r2 =
motor_hole_radius);
}

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Based on your suggestion, and knowing too little of OpenSCAD, would the solution be to use linear_extrude on the faces that are "sheets" to make them solid?
fred

On Saturday, January 23, 2016 3:58 PM, jon <jon@jonbondy.com> wrote:

Just by visual inspection (after cutting the top off with a cube) the
walls you are drilling the holes through appear to be made of two thin
sheets, rather than being thick and solid...

Jon

Its apparent you have used the method of trying to construct a complex
polygon. This is fairly unusual approach in OpenSCAD, which is designed to
construct objects by boolean ops on simple primitives. It is a common
approach in polygonal modellers.

I'd suggest a different approach where you considered how the object could
be made if you had a pile of blocks, cylinders and spheres and could union
them together or intersect, subtract(difference) them from each other to
make your desired shape.

E.g.
module _wedge_cut(X,Y,Z) {
angle = atan(Z/Y0.5);
translate([-Delta/2,0,Z/2])
rotate([angle,0,0])
cube(size=[X+Delta, Y1.4, Z]);
}

Delta = 0.1;
module wedge(X,Y,Z) {
difference() {
// cube
cube(size = [X, Y, Z]);
// remove top
_wedge_cut(X,Y,Z);
// remove bottom
translate([0,0,Z])
mirror([0,0,1])
_wedge_cut(X,Y,Z);
}
}

difference() {
// primary wedge
wedge(width, length, height);
// subtract interior
translate([wall_thickness,0,wall_thickness2+Delta])
wedge(width-wall_thickness2, length-wall_thickness2,
height-wall_thickness2);
// subtract tube
#translate([-Delta/2,(length-motor_offset_from_rear),(height/2)])
rotate([0,90,0])
cylinder(h = width+Delta, r = motor_hole_radius);
}

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Thanks guys. I did consider using cubes etc to build it up, but thought it
would be easier with a polyhedron - that may have been a mistake.

I thought there was something wrong at the front wedge, but for the life of
me couldn't see it. I'm currently reinstalling my computer OS, so will give
your suggestion a go once it's back.

Cheers
Chris

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