**AutoCAD 3D Modelling:**

Before study the 3D modelling, it would be desirable to study the 3D co-ordinate systems.

**3D Co-ordinate systems**

**1. Absolute 3D co-ordinate system: **

XY plane is assumed to be parallel to the computer screen. +Z axis is perpendicular to the computer screen projecting towards the user.

A point with co-ordinate 3, 3, 5. Where 3 units along +x axis, 3 units up along +y axis and finally 5 units towards the user (along +z axis).

**2. Relative cylindrical co-ordinate system:**

A point at 4<30, 5 is marked using relative cylindrical co-ordinate system. 4 units from the last reference point along +x axis, 30 degrees from the x-axis measured

in the anticlockwise directionin the xy plane and 5 units along the +z axis.

**3. Relative spherical co-ordinate system:**

A point at 5<30<40 is marked using relative spherical co-ordinate system. 5 units from the last reference point along +x axis, 30 degrees from the x-axis measured in the anticlockwise direction in xy plane and 40 degrees up from xy plane.

**Types of 3D modelling:**

3D modelling is of three types. They are

1. Wire frame modelling

2. Surface modelling

3. Solid modelling

**Wire frame modelling:**

Wire frame model is a 3D representation of objects by using mostly lines, arcs and circles. It is skeletal and does not have surfaces. Only edge details are shown by a wire frame model.

**Surface modelling:**

In surface modelling, the 3D object is made up of opaque surfaces (3D faces). The inner portion of the object is hollow. The details available behind the surfaces are hidden.

**Solid modelling:**

In solid modelling, a solid 3D object is produced. It is like a solid object made of a material like wood. The solid thus produced can be cut into pieces. Mass properties of solid models can be determined.

Some of the 3D autocad commands are listed below:

1. **Box:** box is used to draw three dimensional solid box. The command is BOX. It can be selected from solids toolbar or draw menu – solids

2. **Cylinder**: cylinder is used to draw three dimensional solid cylinder. The command is CYLINDER. It can be selected from solids toolbar or draw menu – solids

3. **Elliptical**: The command is CYLINDER. It can be selected from solids toolbar or draw menu – solids

4. **Cone**: Cone is used to draw a three dimensional cone. The command is CONE. It can be selected from solids toolbar or draw menu – solids

5. **Sphere**: Sphere is used to draw a three dimensional solid sphere. The command is SPHERE. It can be selected from solids toolbar or draw menu-solids

6. **VPoint**: VPOINT is used to specify the viewing direction for a three dimensional visualization of the drawing. The command is VPOINT. It can be selected from view menu – 3D view point (Tripod / Vector). Vpoint locates the observer in a specified position to look at the drawing back at the origin 0,0,0.

7. **3D FACE**: 3D Face is used to draw a three dimensional face. The command is 3D face or 3F. it can be selected from surfaces toolbar or Draw menu – Surface. 3D face is used to draw a three or four sided surface anywhere in 3D space. Each corner point can be assigned different Z – coordinate values.

8. **Elevation**: ELEV is used to draw elevation and set extrusion thickness properties of new objects.