Circles and arcs

I wrote about curves previously, but if your curve is a circle - or a section of a circle, an arc, - you have other options.

Cylinder

The most obvious is to just use a cylinder mesh. Remember to give it enough sides that it will resemble a smooth curve when printed. As a rough guide, at least 2 sides for every millimeter in the diameter. Too many, and you are slowing the computer down unnecessarily, too few and the steps will be visible.

If you want a hollow cylinder, create two, one inside the other, with the inner one a bit longer. Then on the outer one add a Boolean modifier, and apply the inner one,

I like to put all the meshes that are just there for Boolean operations into their own collection. You can then hide that collection, and see the hole.

If you want a section of a circle, you can just delete the vertices you do not want, and add faces as required. However, if you have a more complicated shape composed of several concentric cylinder that gets harder to do, and you are better off using the Spin tool.

Spin Tool

The Spin tool is available in Edit mode. Select a face (or several), then click on the tool. An arc will appear with + at each end, just drag from one end. It will do a spin, but just accept is will be wrong, and then you can put in the numbers you want in the dialog.

Note that you can change the axis of the spin by selecting X, Y or Z at the top left before dragging.

So let us see it in action. I am going to make a cast iron arch for a bridge. You can see what I am aiming for here, one of many bridges in Castlefield:

The arch is symmetrical, so I will create the left side, then join the components altogether and use a mirror modifier to generate the right hand side.

The Curved Girder

The basis is a plane mesh, rotated to be vertical and facing the x-axis, and then modified to give the right cross-section. This was done in Edit mode by subdividing the vertical edges to give two extra vertices on both, and then extruding out from the top and bottom on each side.

Still in Edit mode I selected all the faces, and went to the spin tool, and clicked the Y.

Now I dragged on one of the pluses on the arc that appeared to get the basic shape. Then I put suitable figures in the dialog. The angle is 40°, and the number of steps I picked 64.

You then need to set the location of the spin axis. Which ever axis you are spinning round, I suggest first changing the view so you look along that axis. You can the drag the arrows on the spin axis to move it where you want. You will probably want the spin axis to lie along a line passing through the face you originally selected so you get a smooth transition into the curve.

In this case, the axis of spin has to be at zero on the x-axis, and a long way down the z-axis. I put it at -40.

If you are doing this to a certain size, I suggest first creating a cube that is that size. you can then set the parameters for your Spin tool to fit the box.

I then created one vertical with the fancy bit at the top, mirrored it and then arrayed it. The arch is again made using the spin tool.

This version has some extra detailing, but that is the basic principle.

So which do I use?

If you want a complex shape - that is, multiple curves, or a curve that is not a circle or ellipse - you need to use a Bezier or NURBS. For precise positioning and angles (eg tubing), use Bezier; for more flowing and natural things (ropes and cables).

If it is a simple curve - specifically a circle - but a complex outline use the spin tool.

If it is a non-uniform shape along the curve - that is, the outline is different at different points along the curve, as in the last image on the other page - you will have to use an object deformed to follow a curve.

For a simple circle or ellipse, use a cylinder.