In my previous post I created a cone that has the same base radius as the cylinder and a top radius of zero. Since the purpose of this cone is to subtract material from the large cylinder, I need to modify the base radius so there will be a rim around the top of the cylinder after the cone is subtracted. Modifying a solid cone is just as easy as modifying a solid cylinder. I’d like that rim to be about 4 units so I’ll use the cone’s quadrant grips to reduce the radius by 4.

Next, I’ll widen the top radius from zero (a sharp point) to a value of 20.

Notice the top of the cone is below the base because of the way I originally created it. AutoCAD doesn’t care where the top is in relation to the base. If you look at the properties for the cone you’ll see a Base Radius, a Top Radius, and a Height.

Even though AutoCAD doesn’t care if the Top Radius is really on top, there is a difference. If you change the height of the cone, the base maintains its position while the location of the “top” adjusts accordingly. For example, I want to ensure that I have 4 units of material left at the bottom of the cylinder after I subtract the cone. So, I’m going to change the height of the cylinder from 14 to 10 units.

Key concept:

- The base of a primitive object maintains its position when the height changes