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I have a square. I want half of it to be circular, like this
sketch of shape
(image is from a topdown view)

I kept loop cutting but I couldn't find a way to make it truly "round" and do it evenly.

NOTE: to be very specific, it should only be round horizontally, the rest should be untouched. Like this:

3D sketch of shape
(please excuse my horrifying drawing)

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  1. Add a Cylinder ShiftA > Mesh > Cylinder, set the Cap Fill Type to Nothing.
  2. Select half of it and delete with X or Delete  modeling cylinder
  3. Select the four vertices along the edge
  4. Extrude to make the long sides
    extrude
  5. Fill the back, top and bottom to get this.
    final result

Note the top and bottom are Ngons, witch in many cases are undesirable. To get a quads all over the top use grid fill.

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With this technique you can add half-circles anywhere on your existing geometry:

  1. Add a Cube and scale it 1.5 in X axis - S+X+1.5:

    enter image description here

    Be sure not to scale it in objectmode. If you do apply the scale with Ctrl+A.

  2. Select 2 front edges and Bevel them - Ctrl+B. Adjust Segments with mousewheel and type 1 for the Offset - it will create a perfect half-circle:

    enter image description here

  3. Ctrl+V > Remove Doubles. Enjoy:)

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You can moddel this shape from a cube.

First add a cube, If you are unsure how do do that, see this question, How to add a cube.

Cube

It does not need to be blue, however it looks cool blue.

Once you have a cube select one of the faces, and set the 3D cursor to that face. If you are unsure how to move the 3D Cursor, see this question How to move the 3D coursor

enter image description here

Once you have a face selected and the 3D cursor set to that face, select Only one edge on that face. it is very important that you Do not move the 3D cursor in this step.

enter image description here

Once you have an edge selected press NumPad 7 This will move your viewpoint to the top view. it is also Very important now that you DO NOT move the 3D cursor. It must be in the same location as before.

enter image description here

Once you are in the top view with the 3D cursor in place and the correct edge selected, press Alt R Then adjust the settings for the spin tool to 180° and a reasonable number of steps.

enter image description here

There is one thing you can skip here but it is better if you do this step. When you spun the edge around 180°, it created an edge where there already was an edge. First, rotate the view to a better position. Press A to select the entire object.

enter image description here

Now click on Remove doubles.

enter image description here

Now that the required mesh has been generated, and there are no duplicate vertices, your object is nearly done. First Rotate the view so you can see this face. Select it and press delete.

enter image description here

A menu will open with delete options. select Face.

enter image description here

Now that the internal face is gone, we must complete the object.

Select these edges.

enter image description here

Press F. This will fill the empty space with a face.

enter image description here

Once the other side is filled, The object will be complete.

Select the edges on the bottom - Same as the top

enter image description here

Then press F to fill this side.

enter image description here

Now your object is complete. Exit edit mode by pressing tab

enter image description here

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Try adding a circle to the scene and making half of it like a square instead of the other way around.

Circle half Circle Extruding

Hope this helps!

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  • 1
    $\begingroup$ Copying other users answers is not recommended. $\endgroup$ – X-27 the fluffy unicorn Mar 10 '15 at 22:50
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    $\begingroup$ I was second if not first to answer this question and at the time I started there were no previous answers. So sue me. $\endgroup$ – Dr. Who Mar 10 '15 at 23:00
  • $\begingroup$ Yes you were second. But apparently David had started writing his answer before you. $\endgroup$ – X-27 the fluffy unicorn Mar 10 '15 at 23:01
  • $\begingroup$ @Dr.Who This answer was not first, don't worry though, nobody will sue you. X-27 David also posted his answer first $\endgroup$ – VRM Mar 10 '15 at 23:02
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    $\begingroup$ @X-27 Let's try to give users the benefit of the doubt that they simply didn't see the other answers, I know it's happened to me. $\endgroup$ – Ray Mairlot Mar 10 '15 at 23:13
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Here's how I would do it,

  • Create a plane

  • select one edge of the plane, hit Shift+S > cursor to selected

  • Shift+A to add a circle

  • Ctrl+V > Remove doubles

  • Delete the part of the arc that resides within the squarea (That's a word now) of the plane

  • select it all (A) and hit E to extrude to desired height.

  • BOOM! victory.

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