Wednesday

Understanding UCS (Part 1)

2 Comments Oct-08-2008 | Posted by Erhan Toker | Printable Version | Level |

User Coordinate System (UCS) is a must-know for 3D user’s of AutoCAD. Whenever you start a new drawing in AutoCAD, you are met by a World Coordinate System (WCS) [of course, unless you have a different setting in your template. This coordinate system  has its origins as 0,0,0 and XY plane as paralell to the earth's surface. Think about your screen as earth's surface. An imaginary perpendcular from your screen towards you will be the Z axis. Such a coordinate system will have an icon as in the figure below.

Fig.1.

This icon is called ‘UCS Icon‘. Especially while working in 3D, you should always beware of the state of this icon so that you don't make mistake thw plane you are working on. AutoCAD always draws 2D objects and the bases of 3D objects (the center section of spheres) on this plane on the active XY plane. Using this logic, you'll see that we can define countless XY planes. On the average, you'll need at most 2 UCS settings in 2D and maybe 10 to 15 UCS settings while modeling in 3D. How to use the UCS? That will be another article's topic.

The simplest UCS setting is to shift the origin point.

Command: UCS
Current ucs name: *NO NAME*
Specify origin of UCS or [Face/NAmed/OBject/Previous/View/World/X/Y/Z/ZAxis]<World>:

When you activate the UCS command, you’ll be immediately be prompted to specify the origin. You can use the corner of the plot, the corner of the piece you are working on for your 2D drawings. You should always remember, once you change the origin, all your coordinates will change too, as the coordinates are relative to the origin. While working on rather large drawings where you can loose track, you should be careful.

The second easiest way is to rotate the  UCS about an axis. In 2D drawngs, this is usually done about the Z axis. The rotation follows the right hand rule. It is used when working on drawings that are not paralell or perpendicular to the drawing plot. Figure 2 shows an example.

Command: UCS
Current ucs name: *NO NAME*
Specify origin of UCS or [Face/NAmed/OBject/Previous/View/World/X/Y/Z/ZAxis]<World>: Z
Specify rotation angle about Z axis <90>:15

You can specify the angle of rotation either as numbers, or using two points.

Fig.2.

In the figure, the UCS is rotated about the Z axis in order to work in a plane that is paralell to the walls, drawn in red. As for specifying the angle, we used the two ends of the rel line. I used a red rectangle to represent the new XY plane, althoufgh it is not limited to that rectangle. After setting up the new UCS, I used the RECTANGLE command to achieve this rotated rectangle. You will see that this is a very practical application. The UCS setttings affect everything, the roads you draw, the text you place on them and all other markings.

Notice how the UCS icon has also changed. The little square that used to be at the intersection of the axes has dissappeared, showing us that the we are no more using the WCS.

Now, I have told you about the way UCS works, and its 2D applications. The UCS command is used mainly in 3D drawings, so I will be talking about 3D applications in other articles.