The Plane Cut Tool allows you to slice a Mesh with a plane. This tool actually does several completely different things to your mesh, depending on how it is configured.
The Property Panel for the Plane Cut tool is quite simple, as show to the left. There are two main settings. The Cut Type determines what the cutting plane will do to your mesh. If the operation would involve inserting new boundary loops, then the Fill Type setting controls how those boundary loops are filled (if at all). Details on these settings are shown below.
To position the cutting plane, you use our standard in-scene 3D-transformation widget. See the Transform Tool page for more information on how to use the widget. You can also quickly specify the cutting plane by left-click-dragging in the 3D scene. You will see a red line appear, which allows you to "draw" the cut line. If the wrong side of the model is cut, you can either use the Flip arrow (the short/thick blue arrow), or re-draw the line in the opposite direction.
There are currently three Cut Types in this tool. The images below show the two basic modes - Cut and Slice. In Cut mode, the plane slices through the edges of your Mesh, and the portion of the model that is rendered as semi-transparent will be deleted. In Slice mode, the Mesh is cut in the exact same way, but both sides are kept. Note that when you Accept the operation in Slice mode, you will still have a single Mesh object - we do not automatically split the result into two separate objects. You must do this yourself, for example using Separate Shells.
The third Cut Type mode is Slice Groups. This mode is different than the other two, in that although it does still cut the Mesh edges, it does not change the shape of the model. Instead of separating along the cut boundary, new Face Groups are assigned on one side of the cut plane. The Image below shows an example of using Slice Groups mode on a cube that has a separate group on each face.
If you are in Cut or Slice mode, then your plane cut will be creating holes in the object. By default we automatically fill those holes with planar meshes, as long as the holes are closed. Hence, when you cut a solid, you get a solid. If you are cutting through an existing boundary loop in a non-solid Mesh, then the result will be a shell, rather than a solid. The four fill types are shown to the right, and three are shown in the image below.
No Fill mode simply leaves the holes empty. Minimal Fill mode (below-left) creates a triangulation with the fewest number of triangles possible (in fact it is a Constrained Delaunay triangulation). If you plan on continuing to edit the mesh, this is probably not the mode to use, as it creates poor-quality triangles for further operations. Remeshed Fill applies remeshing to the Minimal Fill result to create a more regular triangle distribution. Currently you cannot control the triangle density, it is inferred from the size of the triangles on the hole boundary. Finally Fixed Fill mode (below-right) clips a fixed triangle pattern against the hole. This creates regular triangles on the interior and small sliver triangles on the boundary. The main advantage of this mode is that it is much faster than Remeshed Fill.
Precise Cuts with Pivots
You can use Pivots to precisely define 3D planes in the scene, and then use that plane as your cut plane. Pivots can be created in many ways, see the Create Pivot tool for details. However, one particularly useful way to use them in Plane Cut is to allow you to use an actual plane mesh for cutting. The example below shows the workflow. Basically, you first position and orient the plane mesh appropriately. Then use Create Pivot to drop a pivot on this plane. Now start Plane Cut, and left-click on the Pivot to set it as the cutting plane (second image).
By default the cut plane will be perpendicular to the Green axis of the pivot. However by repeatedly clicking on the pivot, you can cycle through the Red and Blue axes as well, as shown in the two rightmost images.
Pivots will also allow you to re-use a cutting plane multiple times, for example to cut separate objects with the exact same plane. See the From Last Tool mode in the Create Pivot tool for details on how to create such a Pivot.
Tip and Tricks
One problem that can arise is if your cutting plane precisely intersects a vertex or edge of the mesh. In this case we can still perform the cut, however our algorithm for creating the fill mesh may fail. In this case you have several options. One is to not fill with the Cut tool, but instead fill the hole afterwards using the Minimal or Refined modes of Erase/Fill. Alternately you can try to remesh around the plane cut boundary, so that the cut will succeed. If the cut plane location can be somewhat imprecise, you can try wiggling it slightly, and you may find that the fill succeeds at a slightly different location.