Linear extrude uses a 2d shape and extrudes it along a single axis to create a 3d shape.
Let.s create a basic extrusion.
Create a 2d square , use square(10);
![](https://www.openscad.info/wp-content/uploads/2020/06/Selection_846.png)
Precede the square(10); with linear_extrude(10)
![](https://www.openscad.info/wp-content/uploads/2020/06/Selection_847.png)
You can extrude any 2d shape, and you can modify the extrusion along the way.
Adding a twist, if we center the square and add a 360 twist we get a screw-like shape.
Use this code ( the $fn=90, just makes it prettier ).
linear_extrude(10,twist=360)
square(10,center=true);
$fn=90;
To make this shape:
![](https://www.openscad.info/wp-content/uploads/2020/06/Selection_848.png)
Or let’s make a rope real fast. Try out this code ( don’t worry about the decimals for now the just put all three circles evenly around the center.
color( "Tan")
linear_extrude(300,twist=720) {
translate([5,2.88675,0])
circle(5);
translate([-5,2.88675,0])
circle(5);
translate([0,-5.7735,0])
circle(5);
}
$fn=90;
![](https://www.openscad.info/wp-content/uploads/2020/06/Selection_850.png)
You can also extrude polygons using the polygon command.
linear_extrude(2)
polygon(points = [ [2,2],[15,4],[15,-4],[2,-2] ]);
![](https://www.openscad.info/wp-content/uploads/2020/06/Selection_851.png)
The center command in linear extrude, centers the final result not the process. Half of the extrusion will be above the XY plane and half below the XY plane.
![](https://www.openscad.info/wp-content/uploads/2020/06/Selection_852.png)
If you want the axis of the extrusion around the Z-Axis you should center the base object. The code below center the square before the extrusion, and then performs the extrusion. This is important and is the difference between a screw shape and a rope shape.
linear_extrude(10)
square(10,center=true);
![](https://www.openscad.info/wp-content/uploads/2020/06/Selection_853.png)