3D Modeling 16: Understanding Ucs

Although dynamic UCS is brought and improved with the version 2007, you have to be aware of the model space and UCS icon and use them efficiently during a 3D drawing. The subject of this article is the UCS icon.
Fig.1
There is a typical UCS icon in a 3D mode in Figure 1. Its view changes according to your Visual Style setting yet, it always gives the direction of X, Y and Z axes. It is important to create a view angle in a way that the axes are seen through the positive sides as I have mentioned in the first article of the series and 3D Modeling Part 1.

Let us examine the Figure.1 together. At the end of the X and Y axes, there are arrows which indicate the positive directions. Z axis is positive if it raises perpendicular through the X-Y plane and it is negative if it goes down through the same plane (Figure 2).
Fig.2
Let us do some practice.

Command: _CIRCLE Specify center point for circle or [3P/2P/Ttr (tan tan radius)]: 0,0 Specify radius of circle or [Diameter] <5.9971>:
Fig.3 UCS is normal, i.e. the circle is drawn in the plan view.

Command: _ucs Current ucs name: *WORLD* Specify origin of UCS or [Face/NAmed/OBject/Previous/View/World/X/Y/Z/ZAxis] : x Specify rotation angle about X axis <90>: Command: _CIRCLE Specify center point for circle or [3P/2P/Ttr (tan tan radius)]: 0,0 Specify radius of circle or [Diameter] <5.9971>:
Fig.4 The circle is drawn after UCS is rotated 90 degrees around the X axis.

Command: _ucs Current ucs name: *NO NAME* Specify origin of UCS or [Face/NAmed/OBject/Previous/View/World/X/Y/Z/ZAxis] : y Specify rotation angle about Y axis <90>: Command: _CIRCLE Specify center point for circle or [3P/2P/Ttr (tan tan radius)]: 0,0 Specify radius of circle or [Diameter] <6.3737>:
Fig.5 The circle is drawn after UCS is rotated 90 degrees around both X and Y axis.

Notice that Z axis is shown as dashed line. So, upon changing the UCS, we can draw in different planes and the UCS icon follows.

Now, let us use ROTATE command in 3D and make use of Z axis of the UCS icon. Actually when you are drawing in 2D mode, you rotate the drawing around Z axis. The only difference is that you see the Z axis in 3D mode. Simply, the command rotates the X-Y plane around the Z axis.
Fig.6
You can see how a circle perpendicular to the horizontal rotates using ROTATE command in Figure 6 when the UCS icon is in the horizontal mode.

You can pick the origin point of the UCS, move it to the lower left corner on the screen and change its settings when drawing in 3D. To do these you have to run the UCSICON command.

Command: _UCSICON ON/OFF: Turns on and off the icon. Noorigin: Move the icon to the lower left corner. ORigin: Places on the origin point. Properties: Opens the UCS icon settings window.
Fig.7 UCS icon settings window.
I tried to explain the UCS icon in 3D modeling. I will explain the UCS icon in detail and try to help you with upgrading your drawing habits in my next article.