# What is SOLID MODELLING?

Solid modelling is the process of digitally creating a 3D design using CAD software. Solid and Surface are the 2 main types of modelling. Surface models are used for visual purposes. On the other hand, solid models are for real world fabrication and computer simulations. First lets start with the basics, The difference between 2D & 3D CAD.

2D drawings only include the x and y axis. 3D uses an extra dimension to form a solid object using the x,y & z axis.

2D drawings can communicate a design sufficiently to a human. However, if you wanted to produce the CAD design using computer aided machinery, it will need to be 3D. Here you can see a 2D CAD model vs a 3D CAD model, built using solid modelling. Now lets look at the definition of a solid model.

## Solid modelling definition

A solid model is a more complete representation of a surface (or wireframe model). Furthermore, a solid model uses both geometric and topological data, which represents a solid completely. Geometric data addresses shapes, sizes and positioning in a 3D environment or coordinate system.

Topology deals with what people call ‘relationship information’. In this case, its non geometric, non-scalar data.

This square shows both, topological and geometric information. Firstly, the geometric is the 100mm length of the square’s side. Whereas, the topological is the perpendicular relationship, of the vertical and horizontal sides, joining at a 90 degree angle.

### Lets look at how solid models are made up of primitive shapes and primitive shapes made up of 2D sketches….

Solid models are made up of primitive shapes, like cubes, spheres or cylinders. Primitive shapes are made of 2d sketches. For example, a solid cube is made up of 6 flat squares surfaces that join together.

To Summarise, any solid 3D model can be broken down in to sub-shapes, no matter how complex. In addition, the subshapes are formed with 1 or more 2d sketches. Finally, think about the design below.  A number of cylinders, squares and triangles complete the design.

FULL SOLID MODEL

PRIMITIVE SHAPES

2D SKETCHES

#### Two-Dimensional Drawing

2D sketches are the oldest form of 3D modelling. Models are
made up of edges and vertices, with no z axis. You cannot represent complex surfaces and it does not show interior details.

#### Three Dimensionsal Drawing

3D includes all x,y & axis or 3 spatial dimensions. a 3D model contains edges and vertices as with 2D, however these edges are filled in with surfaces and interior detailing.

WHAT CAN I DO WITH A 2D DRAWING?

Usually, architects would express a building with a large variety of 2d drawings. However, in modern day manufacture 3d drawings are always required.  What you can do with a 2d drawing depends. If its a pencil drawing on paper, not much. If it’s a 2d drawing modelled with 2d CAD software you can cut flat sheet material with a cnc cutter. You can map out a floor plan or have a 3D CAD guy build the model in 3D. Other then that, not much.

WHAT CAN I DO WITH A 3D DRAWING?

Firstly, you can send to a 3d printer to make a physical prototype, or simulate forces acting on the design. Secondly, you can use to make artwork for all marketing or for communication purposes with anyone anywhere instantly. In addition, if you want to mass produce you’ll usually make a mold or production tool. Furthermore, you can preform motion studies to explore how mechanical parts or systems function. The list goes on and on. In conclusion, all these actions require 3D CAD files.

##### 2D shapes are used with the primary feature forming operations

As mentioned earlier, nearly all 3d forms are made up of, or start as a 2d shape. With most parametric solid modelling, forms are built from 2d sketches using the 6 primary forming operations as shown below.

EMBOSS/EXTRUDE

Extruding is the primary forming operation. It involves taking a flat 2d sketch and extruding it up so it has a thickness.

CUT EXTRUDE

Cut extrude is the opposite of boss extrude, it involves using a flat sketch to cut material from a thickened shape.

REVOLVE BOSS

Revolve boss allows us to take a flat sketch and rotate it around a center to create a solid shape.

SWEEP BOSS

Sweep boss takes a flat sketch and extrudes it along a path. Can include guide curves to manipulate the uniformity.

LOFT EMBOSS

Loft emboss takes 2 separate flat sketches and joins them together with a often curved and unique surface.

BOUNDARY BOSS

Boundary boss is very similar to loft boss however it can join open open paths as well as closed flat shapes.

Take the image below for example, this form was modelled using all 6 operations.

Base built
using LOFT
operation
to
join the 2d circle
and square base

represented by
the yellow
highlighted
area

microworkshops
Logo

EMBOSSED
using a single
2d sketch

SWEEP
OPERATION

takes circular
2d sketch and
sweeps it along
spiral path to
create solid bar

REVOLVE
OPERATION

takes a sketch split
vertically down the
center. the 2d sketch
is revolved around
the center axis to
create solid shape

BOUNDARY
EMBOSS
OPERATION

a solid body joins
the 3 separate 2d sketches in the stem of the design
can manipulate the
outer surface profile
for a more freeform
geometry

CUT EXTRUDE
OPERATION

This is the inverse operation of boss extrude. Cut extrude is cutting material away to create a circular array of holes the base platform of the model

A powerful operation that isn’t included in the 6 primary operations is BOOLEAN. Boolean is a example of constructive solid geometry, or CSG

CSG constructs solid models by combining shapes. The boolean operation come in 3 forms:

• Union (unite) – this simply joins 2 shapes together into one single solid body
• Difference (cut) – Subtracts the volume of one shape from another shape.
• Intersection  – Keeps the volume when 2 shapes intersect.

So these 6 shape forming operations can build almost any design you could ever imagine…. For more in depth example watch our video below.

Now you have a concept of how models are actually built in 3d CAD software. So then, lets talk about a slightly more advanced solid modelling concept, called PARAMETRIC MODELLING.

#### What is PARAMETRIC MODELLING?

Parametric modelling describes a designers ability to change a measurement in a 3d model. Which in turn, changes the shape or size of a models geometry. In other words, measurements control the model.

Design intent is how a model reacts when a measurement value is changed. Take for example, changing the diameter of screw holes in a table design. That table is made up of forming operations like 6 mentioned above. In this context, the screw holes were made using the cut operations.

This cut operation was made from a 2d sketch of a circle, which had a measurement value assigned to it. These screw sizes were later changed, for some reason. Now, the designer can easily open the model and change the diameter value at any time. Thus, updating the screw holes. And hence comes the value of parametric modelling. If its modelled with the correct design intent, it should be easy to change design details retrospectively. Contact microworkshops on 0203 0341 334 to speak with one of our parametric modellers. Or check out our CAD design services for further information.

###### Advantages of solid 3D modelling

Easy to interpret with multiview perspectives.

Cheaper and faster than building a physical model.

Can be used for visuals when presenting or selling.

Directly used for CAM manufacture like, 3d printing or CNC.

Allows mass and volumetric analysis. For example, weight, center of mass, volume and inertia.

Can be exported into large variety of file formats depending on its purpose.

Solid modelling is usually parametric. Meaning it’s size and detail changes can be made retrospectively.

Extremely useful for flexible design iteration.