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I wanna cut a mesh by specifying the angle, but knife mode only cut 45° with C key.

I want to cut by the angle around specifying the vertex. cutting by knife

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It isn't currently supported yet, but still possible. You can say that the angle constrain is based on View coordiante, so let's try a tricky way:

  1. Install Precise Align addon (not a must-have, but very effective here);
  2. Select three vertices, including the specified vertex, click Create Empty button in Precise Align panel in the Tool Shelf (T). Click on Swap Empty Origin button to set it at the specified vertex if it hasn't been there. Also make sure Parent to Object is toggled;
  3. Select the empty, ShiftNumpad 7 to Align its local Top view (Local Z axis towards viewer), thus you can make one adjacent edge visually horizontal (or vertical);
  4. Keep the current point of view, R to rotate the empty along its own pivot point by a specified angle (or the complement/supplementary angle, depending on how you rotate it);
  5. Select the mesh object, enter Edit mode, enable Knife tool, press C to constrain horizontally or vertically based on the view (screen) coordinate. Repeat it if needed.
  6. Delete the empty. The mesh rotation will be regressed automatically.

A demo video for better understanding. It's not perfect, but works.

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  • $\begingroup$ Thanks to your kindness!! Especially the demo is very helpful for me to understand how to cut!!!! I felt as if the scales had fallen from my eyes. $\endgroup$ – Momoko Feb 10 '14 at 2:25
  • $\begingroup$ You are welcome. Sometimes it would be helpful to think reversely - If we cannot control the cutting angle, fine, let's rotate the object. :P But indeed, it would be great to have such feature built in. $\endgroup$ – Leon Cheung Feb 10 '14 at 2:53
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Another option is using Knife Project (added in 2.67)

Start by adding an object to use as the cutter, for a straight line at an angle a plane is a good start. Rotate it the specified angle you want. Align it with the object you wish to cut. Scale it so it covers the area to cut.

With the cutter selected shift select the object to cut and enter edit mode. Choose Knife Project, this is available under Mesh Tools in the toolbar or by searching for Knife Project.

The most accurate results would be achieved in a preset orthogonal view to get best alignment. You can also use snapping and entering a calculated location to get a precise placement for the cut.

Up to 2.69 it only cuts the top face. In current trunk builds (to be available in 2.70) you also have the option to cut through, so that it cuts through all sides of a non-flat mesh. This is available in the operator options at the bottom of the toolbar or by pressing F6

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  • $\begingroup$ Good point, I also thought about that. It could be an option for sure, expect the known issue of Knife Project tool, it may fail with very simple mesh, like the OP's case. $\endgroup$ – Leon Cheung Feb 10 '14 at 10:02
  • $\begingroup$ Cutting holes into single faces may fail I've only seen problems using a text object to cut. I haven't seen any issues cutting right across an object from one side to another. $\endgroup$ – sambler Feb 10 '14 at 10:27

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