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 move 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.

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, dragging on one of the pluses on the arc that appeared. Then I put suitable figures in the dialog. 

For the axes at the bottom, that is 0, 1 and 0 so it is spinning round the y-axis. 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.


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.

The Verticals

We will create one vertical with the fancy bit at the top, mirror it and then array it.

The first step is to add a cube mesh. It needs to be at the far end of the arch, and sized correctly. Apply the scale. Now add a mirror modifier, mirroring down the x-axis (you will not see anything yet), and then an array modifier. You want about 15 copies. Untick Relative offset, and tick Constant offset, and put a number in the x box, say a 1. You can adjust until you get the right spacing.


To get the arch at the top, go into Edit mode, and select the top face. Click the Spin tool, and as before, drag from the plus. In the dialog box, for the axes at the bottom, that is 0, 1 and 0 so it is spinning round the y-axis. For the angle, try 60°, and move the axis of rotation to get a nice arch. Because we did the mirror and array already, you should see the effect immediately, which helps get it looking right.


Once you have it perfect, come out of Edit mode, and apply the array and mirror modifiers. You can now adjust the length of each vertical so it terminates in the girder - you have to do each one individually, so fiddly, but not technically challenging.

All Together Now

There is further detailing you can add, such as strips along the top and side, but you can sort that out yourself. Save at this point, then save it again, but with a different name, as the next step is a point-of-no-return.

Select all the components, then click the girder to make that the master, and [CTRL]-J to join them together. Apply a mirror modifier along the x-axis, and you are done!

This version has some extra detailing, but the basic principle was the same.




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