Some general rules about 3D coordinates and work planes.

Before we can set the properties of a work plane, we have to explain the AutoCAD co-ordinate system (UCS, WCS, etc... ).

Absolute coordinates: e.g.: 20,40,50

Values:

X = 20

Y = 40

Z = 50

The coordinates 20, 40, 50 are the absolute coordinates from the origin of the current WCS(UCS).

 

Relative coordinates. e.g.: @37,-25,50

@-sign indicates relative values from the last point.

Values:

X = 37

Y = -25

Z = 50

The relative coordinates @37,-25,50 define a coordinate at a distance of X = 37, Y = -25, en Z = 50 from the last point entered (20,40,50).

 

 

     

To determine the positive direction of the Z-axis you can use the right-hand rule:

- Thumb = direction of the positive X-axis

- Index = points to the positive Y-axis

- The remaining fingers bent inward give you the direction of the positive Z-axis.

         

When you start a new drawing in a WCS coordinate system, the positive Z-axis points towards you.

There are three different coordinate systems:

 

1°-WCS World coordinate system.

 Standard coordinate system (origin at the bottom left).

 

WCS TOP view

 

WCS BOTTOM view

 

         

2°-UCS User coordinate system.

User defined coordinate system (For each UCS you set a different origin, x-axis, y-axis and z-axis).

 

UCS TOP view
not linked to   origin

 

UCS BOTTOM view

not linked to origin

 

UCS BOTTOM view

linked to origin(+)

 

3°-OCS Object coordinate system.

This coordinate system cannot be viewed or modified. It is embedded in each object and is stored in the AutoCAD database. This OCS has direction and origin from the UCS in use at the moment that the object is created.

You can make the UCS coincide with the OCS by placing the work plane according to an object.

 

Correctly setting the UCS-planes is a determining factor for good results in 3D-drawing. Once you have set the UCS wrong, the drawing is lost because you will repeatedly encounter objects with wrong co-ordinates.   

 

With the UCS you can define a co-ordinate system in space. This allows us to use all 2D commands in 3D.

In a 3D space we can only draw lines, 3D polylines and 3D objects. All other commands are executed in the current UCS.

 

You have to avoid reusing too many times the current UCS to create a new UCS: otherwise this will multiply previous errors. In other words: always return to the WCS before creating a new UCS!

 

Importance of Osnap when setting the UCS.

 

When you use Osnap options (see Basic Training), AutoCAD will ignore all other selection methods.  This means that whatever the settings are for UCS, elevation, etc. you will select the correct X, Y, Z-coordinate when you use the Intersection-command.

Without Osnap options, any point selected randomly on the screen will always be on the current XY-plane or UCS unless Elevation is set. Beware of the Osnap Nearest Option! Also keep in mind the size of the selection square.

What AutoCAD defines as the nearest point is not always the same as the nearest point we perceive ourselves.