The Knife Tool is one of the essential tools that Blender provides to subdivide a mesh by drawing lines. Any shape can be created by hand using this tool.
To cut an object, first select it and then enter Edit Mode (Tab). Then click the knife icon in the Toolbar to enable the knife tool.
Select the Mesh
There are a few ways to select the mesh on blender. The box, circle and lasso selection tools can be used to quickly select specific vertices, edges or faces.
The Knife Tool is another way to make a cut on a mesh. To use the knife, you need to be in the modeling workspace and have the Tool Shelf visible.
To begin making a cut with the knife tool, you can click-drag or keep the cursor over an edge or vertex of the mesh. After you make a cut, a purple line will appear that will indicate that the cut has been made.
Enable the Knife Tool
The knife tool is a powerful modeling tool for subdividing any surface of the mesh. It is especially useful for rerouting and cleaning up the topology of the mesh.
The Knife tool can be accessed through the quick tools menu or by entering Edit Mode. Pressing K activates the tool and a (green) selection confirmation box appears, highlighting the nearest vertex or edge alongside the cursor sprite.
Occasionally you may need to make a cut that is perfectly horizontal, vertical or at a 45-degree angle relative to the view axis (like a cylinder or sphere). To constrain the cut line to these axes, hold down C while making a cut.
Make a Cut
The Knife tool is a simple, interactive, tool that allows you to selectively place cuts on an object or mesh. It can be activated using either a shortcut key, K or the Toolbar icon on the left hand side of the viewport when in Edit Mode.
To start a cut mouse over an object, surface or edge and click to initiate the cut (a green node will appear to confirm the start or initial point of the cut). Repeat to trace a ‘path’ which is defined by a ‘cut’ line between the positioned nodes.
Design note: When using the Knife tool in this way it tends to produce structure that’s not conducive to good edge or mesh flow (and may require some degree of optimisation). This is constrained by the kinds of geometry Blender represents.
Another useful tool is Face Cutter, which is a plugin that helps to cut faces by defining snapping points. This is a very useful feature and makes it easy to quickly add perpendicular lines on a face.
Apply the Cut
The bigfoot notwithstanding, it’s no secret that Blender isn’t the biggest kid on your block. It can be a daunting task to try and tackle all your 3D modeling, animation and simulation needs at the same time, much less the required budget. Thankfully, a little bit of tinkering and some elbow grease can go a long way in helping you outshine your colleagues. The best way to do this is to learn about what the software has to offer and use it for good, rather than just for bad.