User Co-ordinate Systems AutoCad 2010 Tutorials

User Co-ordinate Systems


Introduction

This tutorial describes what UCSs are, why we need them and how to use them. The correct use of UCSs with AutoCAD is the key toproducing good 3D models and they can also help with 2D work. If you just want to quickly find a description of the UCS options, click on the appropriate button on the QuickFind toolbar below.What is a UCS and why do I need one?

AutoCAD started life as a two-dimensional drafting program. It was not designed for 3D. Almost all the AutoCAD drawing and editcommands can only work in 2D (the exceptions being commands like 3DPOLY and 3DFACE). When Autodesk, the makers ofAutoCAD incorporated 3D into the program they needed some method for doing so without completely rewriting the software. The methodthey decided upon has become known as UCS, User Co-ordinate Systems.When you first start up AutoCAD you are presented with a plan view of the drawing area. In the bottom left hand corner of the drawing area is an icon, known as the UCS icon. The icon looks like the illustration on the right and shows 3 specific bits of information. First, the icon contains a figure X and an arrow which points from left to right along the bottom of the screen. Second, the icon contains a figure Y and an arrow head which points from bottom to top along the left hand side of the screen. These first two parts of the icon indicate the position and direction of the X and Y axes. As your cursor moves over the screen area you can see the change in X and Y co-ordinates by watching the co-ordinate statusarea at the bottom left of the screen (X,Y,Z). X and Y co-ordinates increase in the direction indicated by the UCS icon. By implication the Z axis points straight out of the screen towards us. If you have just opened a new drawing the Z co-ordinatewill appear as "0.0000" in the status bar and will not change as you move the cursor because you are only moving in the XY Plane. The third piece of information contained in the UCS icon is the letter W. The W stands for "World" and indicates that you are using the World Co-ordinate System.You can think of the World Co-ordinate System as representing the real world. The 2D drawing plane that you see in plan when you first start AutoCAD can be thought of as the ground under your feet. This plane is known as the XY plane.As mentioned above, almost all drawing with AutoCAD happens in 2D. For example to draw an open ended box you would simply draw a rectangle in plan and then use Change Properties to give it a thickness. Notice that to produce this 3D box you have only worked in 2D. To produce a 3D effect all you have to do is to change one of the 2D rectangles parameters i.e. its thickness. No actual drawing was done in 3D.This method works very well for simple 3D objects, but say you wanted to draw a circle on one of the vertical faces of the box you have just drawn. Using only the World Co-ordinate System this would be impossible because circles (like many other AutoCAD entities) can only be drawn in the XY plane.AutoCAD gets round this problem by allowing you to move the XY plane into a different position. For example, to draw a circleon the vertical face of a box you would need to move the XY plane in such a way that it lay coplanar (in the same plane) with the vertical face of the box.By moving the XY plane you are by definition changing the co-ordinate system. In fact AutoCAD takes this analogy and turnsit around. To move the XY plane the user (i.e. you) must create a new co-ordinate system, in other words, you must definea User Co-ordinate System.

Orientation of the UCS icon in an oblique view with the World Co-ordinate System.	Orientation of the UCS icon in an oblique view with a User Co-ordinate System which is coplanar with the front vertical face of the box.

As you can see from the two illustrations above, the UCS icon shifts its position to indicate the orientation of the currentUser Co-ordinate System. Also, when you are working in a UCS the W disappears from the icon to indicate that you are no longer in the World Co-ordinate System.
Top of pageHow do I define a UCS?

AutoCAD provides a number of ways to define a User Co-ordinate System. All of these options are available to you when you use the UCS command.The UCS Command

Toolbar
Pull-down	ToolsUCSvarious options
Keyboard	UCS

When you start the UCS command from the keyboard or from the toolbar you are presented with lots of options on the command line:Origin/ZAxis/3point/OBject/View/X/Y/Z/Prev/Restore/Save/Del/?/<World>:These options will be described later.Note that the pull-down menu allows direct access to all of these options. You can also gain direct access to the options from the fly-out UCS buttons on the standard toolbar or from the UCS toolbar.First let's have a look at a practical example of defining a UCS using the 3point option.

The 3 Point OptionToolbar 3 Point UCSPull-down ToolsUCS3 PointKeyboard UCS Return 3The 3point option prompts you to pick 3 points in space which it uses to define the position of the new XY plane. The three points represent three positions in the new XY plane. The first point will become the origin of the new co-ordinate system. The second point can be any point on the positive portion of the X axis. The third point can be any point on the positive portion of the Y axis. The illustration below shows the three points you could pick to define a UCS with an XY plane which is coplanar with the front face of the box.This is a very common sequence which you may need to use in order to draw windows and doors on a building elevation.location of the 3 pointsMake sure you use the Endpoint Osnap to pick the corners of the box.Top of pageAn ExampleLet's go through the sequence of defining a UCS and then using it to draw on the vertical face of a box.1. Draw the boxStart AutoCAD and draw a square with sides of 50 drawing units using the Rectangle command.Select Rectangle from the Draw toolbar. RectangleAt the Chamfer/Elevation/Fillet/Thickness/Width/: prompt, pick a point near the middle of the drawing area.At the Other corner: prompt, enter @50,50 at the keyboard (the @ indicates a relative co-ordinate). The square is now drawn at the required size.Using Properties give the square a thickness of 50 drawing units.Select Properties from the Object Properties toolbar. PropertiesSelect the rectangle at the Select objects: prompt and enter 50 in the Thickness edit box.Next, change the view using the DDVPOINT command, setting the angle from the X Axis to 245 degrees and the angle from the XY Plane to 30 degrees. You can find this command on the view pull-down (View3D ViewpointSelect…)Your box should now look similar to the one in the illustration above.2. Try to draw a circleTry to draw a circle on the front vertical face of the box. Start the Circle command (select Circle from the Draw toolbar), pick a centre point near the middle of the front vertical face of the box and enter a radius of 20. Notice that the circle ends up flat in the World XY plane.3. Start the UCS CommandStart the UCS command by typing "UCS" at the keyboard or by selecting UCS from the UCS toolbar.Origin/ZAxis/3point/OBject/View/X/Y/Z/Prev/Restore/Save/Del/?/:Type "3" to select the 3point option at the prompt.Origin point :The default (0,0,0) refers to the origin of the current UCS. Using the Endpoint Osnap select the lower front left hand corner of the box (refer to the illustration above).Point on positive portion of the X-axis :Using Endpoint again, select the lower front right hand corner of the box.Point on positive-Y portion of the UCS XY plane :Pick the upper front left hand corner of the box (don't forget Endpoint!).Note: The default co-ordinate values shown in triangular brackets at your command prompt will probably be different from the ones shown here, this will make no difference to the final result.Notice that two things have happened to the UCS icon. The icon has changed position to reflect the orientation of the current UCS and the W has disappeared to tell you that you are no longer in the World Co-ordinate System.4. Now try drawing that circle againNow that you have successfully defined a UCS you should be able to draw that circle on the front face of the box. Draw a circle of 20 units radius with its centre near the centre point of the front vertical face. Notice that this time the circle is drawn exactly where you wanted it.5. Give it a thicknessOnce you have defined a UCS all AutoCAD commands will work relative to the new co-ordinate system. In other words AutoCAD treats the UCS just as if it were the WCS (World Co-ordinate System). For example if you give an object a thickness, you know that the object will be extruded in a direction which is perpendicular to the XY plane. Since our XY plane is now in a vertical position relative to the WCS any thickness applied to an entity will cause an extrusion in a horizontal direction relative to the WCS.Use Properties Properties to give the new circle a thickness of 20 drawing units.6. Define another UCSTry the UCS 3point option again, start the command by selecting 3 Point UCS from the UCS toolbar to pre-select the 3 point option and this time define a UCS on the left hand face of the box. Draw another circle and give it a thickness of 20 units.Your finished drawing should look something like the one on the right.7. Experiment with the Plan commandThe Draw and Modify family of commands are not the only ones which work with respect to the UCS. View commands like Plan also change to take the new co-ordinate system into account. You can create a plan view of the current UCS by typing PLAN and Return at the command prompt (current UCS is the default option) or you can select it from the pull-down menu at View3D ViewpointPlan ViewCurrent UCS.Try using the Plan command with different UCS orientations. With a UCS defined as coplanar with the vertical face of a box the UCS plan view will correspond to the WCS elevation of the same face.Remember, you can return to the World UCS at any time by selecting World UCS from the UCS toolbar or by typing UCS Return 3 at the keyboard.The two illustrations below show this situation. The one on the left shows the result of a plan view with a UCS defined coplanar with the left hand vertical face of the box which you drew in the above exercise. As you can see the result is an elevational view of that face relative to the WCS. The one on the right shows the result of a plan view of the same box with the UCS set to World. In other words this is the true plan view. Before we go on to have a look at the other UCS command options let's have a look at another command which can help to make life easier when working with UCSs.