Rhino Solid Modeling
Solids in Rhino are multiple NURBS surfaces joined into one primitive. Primitives are basic geometric shapes like a cube, cone, sphere, cylinder and pyramid.
All operations concerning creating and editing solids can be found in the menu Solid
Solid modelling: a simple concept of using primitives as basis for modelling
The use of different types of geometry in Rhino was already mentioned in the Introduction to Rhino. For the first exercise we will use Solids to generate a design.
Many industries where accurate 3D models have to be generated use Solids. Solids are widely used in the car, aircraft, mechanical and building industry. A solid is an accurate mathematical description of a volume. This accuracy is crucial for some industries where the tolerances in size are very important. That is one of the reasons why it is used in mechanical engineering.
Rhino has two options of defining a Solid. One is to combine a set of separate surfaces describing a volume into a solid. These surfaces have no thickness and only when combined they define a single, closed volume. These solids are called Poly-surfaces (multiple surfaces). The mathematical description of this geometry is completely different than solid primitives.
The other option is based on the use of a very accurate mathematical defined primitives. Primitives are basic geometric shapes like a cube, cone, sphere, cylinder and pyramid. Several different primitives are available which can be combined into new shapes. The starting point however is always a primitive. This makes it one the one hand easy to use, because the basic building blocks are given, but on the other hand is limits the freedom of designing.
Primitives describe a volume and are called Constructive Solid Geometry or CSG. We will focus on these solids in this part of the explanation because they are easy to make and gives you an idea on the influence of the mathematical description of the geometry, defining the geometry type, on the design process. An influence which you need to understand to make the correct choice in geometry type when you design in the computer.
A few elements which are influenced by the mathematical description are:
- Accuracy of the geometry ( important for manufacturing)
- The shapes it can generate
- The ability to adjust the shape
- The workflow of generating the shapes.
The name of CSG becomes self-explanatory if you look on how the solids can be used to generate geometry. The whole process of generating geometry with CSG is quite straight forward and consists of 3 steps.
- Generating the primitives
- Let the primitives interact with each other (Boolean operations) by:
- subtracting one volume from another
- adding one volume to another
- or define the intersection of two volumes into new geometry
- Editing the resulting volume That means moving corner points, edges or surfaces of the volume.
Generating Solid Primitives
The first step in designing with CSG is the generation of primitives. Rhino has the option of defining several different primitives. Although the typology of the primitive is predefined, additional settings are available which can be changed. If the object is created it will be placed on the grid unless you used the object snap options to align the primitive to another object. The solids can be created by selecting in the pull down menu Solid. The rollout menu gives access to a range of different primitives. Check the command line for additional options once you activated the command.
You can use the primitives as a kind of building blocks similar to a wooden block set used by kids everywhere. In that case you start out with a ground plane and build for example walls and ceilings from cubes and columns from cylinders. Because they can intersect without colliding and there is no gravity in this virtual space they can be moved around freely. This way of generating geometry is quite similar to the form study assignments in previous courses.
However there are additional tools to support the generation of more complex geometry. These options are called Boolean operations.
- Shapes that intersect can be combined into a single new object. The command for this is . Check the command line for additional options.
- If you want to make a hole in an object, you can subtract an object the size of the hole from another object (that remains). The command for this is . Check the command line for additional options.
- The last option is de definition of the common geometry of two objects. The command for this is . Check the command line for additional options.
The command lets you scroll through the different Boolean operation options which can be useful for exploring the different effects of the different Boolean operations. Although the options of volume interaction ( Boolean operations) and the amount of different primitives are limited the final complexity of the geometry can be quite substantial.
Editing Solids The editing of solids is quite straight forward because there are not that many options for editing a solid. The main options are to move one of the surfaces or edges of the solid or to move the corner points. So three components of the solid can be moved.
The corner points
The Edge of the solid
The face of the solid
So all-in all the options for generating and editing CSG can be quite straight forward.
One option of editing a solid is not discussed yet. This option is called Fillet Edge. This option is widely used in mechanical engineering. It rounds off edges. This is primarily not done for aesthetics but for avoiding extreme stresses at the edges of the object and metal fatigue. However you can use it also as an aesthetic option for smoother edges.
- Fillet Edge
- creates round edges
- Blend edge
- almost similar to Fillet Edge, but has more control points
- Chamfer Edge
- creates cornered edges
Cap planar holes
Sometimes when you fillet edges, holes can appear on faces. To close these holes use: