# Mathematically precise bisect cuts?

https://youtu.be/raWx4MYjq-4

^^^^^^ Example ^^^^^^^

Hi. Is it possible to make precise bisect cuts in Blender? I can only make them manually, by hand, and imprecisely.

For example, like in the video, say I wanted to bisect the object exactly on one of the 3 lines that you see there. I did it by hand in the example.

I know there are numbers in the Bisect window, but I'm not sure how to operate them. There's no "angle" or anything (which would be helpful, because in this case I could set the bisect to 72 degrees for example (but unfortunately it's not angle based)).

Is this possible?

Well, that's not an easy interface to interpret, is it? First, in the top part of the window is "Plane Point" this is a point on the cutting plane. In your case, I think you should set it to the origin (0,0,0). This is also the pivot point for angle adjustments.

The second part of the window is "Plane Normal." For the Y-Z plane, set it to (1,0,0). For the X-Z plane, set it to (0, 1, 0) and for the X-Y plane, set it to (0,0,1). In the attached screenshot, it is set to (cos(72°), sin(72°), 0) and the poorly drawn white arcs show 72° for the normal referenced from the x-axis and 72° for the cutter plane referenced from the y-axis.

To further expand, lets cut out Suzanne's face at +/1 36°. I put sin(36°) and cos(36°) into a calculator and got 0.588 and 0.809 so lets see how to put those in:

• Wonderful answer! I'm sorry if it's a bit self explanatory after your answer... but, I can't seem to bisect it further for a perfect, 72 degree cut again. Could you help me with that too by any chance? Commented Aug 4, 2020 at 4:16
• Does this answer your question or are you looking for some angles? Commented Aug 4, 2020 at 4:23
• Oh no, I always select all the vertices for the mesh before I bisect. That way the bisect goes through the entire thing. I'm looking for some angles I think; one way I just did it is by rotating the mesh 120 degrees (why 120 degrees? ) and then performing the exact same cut again. (Edit: 120 * 3 = 360... I get it, but I don't understand how to do this using bisect (without rotating the mesh) Commented Aug 4, 2020 at 4:25
• You were asking for 72°, which makes 5 sections. Cosine and sine of 120° is -0.500 and 0.866, swapping them around and changing +/- signs gets you related angles of 30°, 60° and 150° Commented Aug 4, 2020 at 4:42
• Very sorry, I've gotten my degrees mixed about. I've been working on this all day. I want to cut the mesh into 3 sections (360/3 = 120, so 120 degrees) Basically, if you watch the YT video I linked, that is exactly what I want to do, but using math like you did up there. Plugging in Cos 120 and Sin 120 and then flipping them does not cut the mesh perfectly into 1/3 of the original... I found an easier way; make a horizontal cut, rotate the mesh 120 degrees and perform the same cut again. I basically want to do this, but without rotating the original mesh: youtu.be/XtTn8qIc-d0 Commented Aug 4, 2020 at 5:05