When it comes to three js geometry there are a number of built in constructors that can be used to make most basic shapes. One of these is the cone geometry constructor.
The cone Geometry constructor can accept a few arguments. However just the first two are the most important when it comes to a basic example at least. The first one is the radius of the base of the cone and then second is the height or length of the cone from the base to the tip.
To make a half cone I just need to use the last to arguments that are given to the cone geometry constructor.