Cutting holes in an object can be a bit time consuming in Blender depending on the model and the shape we want but there are a few tools provides to make this process easier. So, how do we cut out a shape?
In the rest of this article, we'll go over how to cut out a shape using the knife tool as with the boolean modifier. We'll also go into details about both methods and some reasons to use them.
First select the object you want to cut and then set the object interaction mode to edit. Now to access the knife tool we can either press K or on the left we can select the knife tool.
Related content: Blender knife tool tutorial
With the knife tool enabled we can start clicking to create a cutout on our mesh. We can also remove points on our cut by pressing CTRL + Z, while in knife mode all the shortcuts can be seen in the status bar at the bottom of the window.
Once we're happy with the cutout we can press enter to add the vertices and edges of our cut to the model. Note that the cut must be on a connected surface.
Sometimes when we try to use the knife tool on a connected surface it still won't work. This can happen when there's a fold in the model and our cut doesn't trace it properly. Sometimes cuts also don't work properly with N-gons so you may need to triangulate N-gons first.
One quick way to select all the N-gons on our models is to first select the model and then set the object mode to edit. Go to select > select all by trait > faces by sides set the number of vertices to 4 and set the type to greater than.
Now go to face > triangulate, we can also do this after we've cut our geometry since the knife tool often creates N-gons.
We could achieve a similar effect by moving around vertices and subdividing edges to get a better outline and then proceed to use the shape like we would with the knife cutout. The reason we use the knife tool is because its much quicker and won't distort the geometry.
Once the cutout is made, we can select the inner part and extrude it inwards or delete it and continue model around the hole.
Related content: Extrude in Blender: Basics, along curve, circles, manifold and more
The benefit of the knife tool is that it is quick and we can cut out custom shapes. But a couple oof drawbacks include that it is hard to make precis shapes, like circular holes. It is also hard to make a cut all the way through an object. In both of those cases a boolean operation is a better choice.
First Create a cutter object and place it where you want to cut your object. In the properties tab select the modifier properties and add a Boolean modifier. Set the boolean object to the cutter object.
Related content: How to use boolean modifier in Blender
If we want to keep the part we cut out we can create a copy of the object but with the boolean set to intersect.
Sometimes the boolean will not work properly, a few things we can try are changing the solver from exact to fast or by moving one of the objects very slightly. There are certain edge cases where a boolean modifier just won't work and in those cases, it may be better just to use the knife tool.
You can also read this article for more detailed instructions on how to solve boolean problems.
Related content: Boolean modifier problems and how to solve them
The reason we might use this method instead of the knife method is because it allows us to cut out exact shapes and can be much quicker. This method also allows for quick iteration.
One downside with booleans is that the geometry they output isn't perfect and requires a bit of clean-up to get good poly flow and to remove N-gons.
Related content: Blender N-gon guide
We may also want to cut using a boolean without actually creating a hole in the model similar to how the knife tool works. We can achieve this effect with a few extra steps.
First select the object we're going to be cutting and duplicate it. In the first object's properties go to the modifier properties and add a boolean modifier then set the object to the cutter object. Set the mode to difference.
In the copied object go to the modifier properties and add a boolean modifier and set the cutter object then Set the mode to intersect.
Once we have the booleans working on both objects we can apply the boolean on each object.
Now merge the two objects together and then set the object interaction mode to edit. Press A to select everything and then at the top of the window go to Mesh > Cleanup > Merge by distance. Now we should have an edge loop that matches the shape of the cutter object.
This can be useful as it will act just like the knife tool but like mentioned above will allow us to model the exact shape we want to cut rather then eyeballing it or creating it by hand.
Like mentioned above though the resulting geometry from booleans isn't perfect so if we don't want to go through the drop of cleaning up the geometry the knife tool may be a better option.
With the knife tool we can easily cut a model and make sure the geometry is cut exactly how we want it. With Boolean we can quickly adjust the shape of our cut as well as using other objects to create our cutout.
Both have there pros and cons but should be able to cover most cases when we need to cutout a shape in an object.
Thank you for your time.
