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AutoCAD Essentials is a multi-part series that takes a more modular approach to this massive program, used for everything from 2D and 3D CAD design, drafting, and modeling to architectural drawing and engineering projects. In this installment, author Jeff Bartels concentrates on the particulars of creating basic geometry in AutoCAD, including assigning imperial or metric units of measurement, using object snaps to control accuracy, and drawing and transforming basic lines and shapes. The last chapter in the course tests your newfound skills in a short project.
Making a change to one object can often require the modification of other geometry in a drawing. Fortunately, AutoCAD has some tools to help us clean up line work after making changes. In this lesson, we'll look at the Trim and Extend commands. On my screen, I have a simple drawing. Let's make a change. I'm going to select this circle, and I'll come over to the Properties palette and I'll change its radius from 1.5 to 2.5. When I'm finished, I'll press Escape. Now, I've got a problem. These lines that used to meet the edge of the circle now cross over. I'd like to trim these back to meet the new circle edge.
To do this, I'll use the Trim command. Trim can be found in the Modify panel--the icon's right here. After launching the command, I will select my cutting object, or objects, and press Enter, I can then select the objects I'd like to trim. And notice how they get pulled back to meet that cutting object. When I'm finished, I'll press Enter. So when using Trim, we use one object to cut another. Let's work in the other direction. I'm going to select this circle again. I'll come over to Properties, and I'm going to change its radius back to 1.5. Now I've got the exact opposite problem. I need to project these lines out to meet the new circle edge. We'll do this using the Extend command. Extend can also be found in the Modified panel. In fact, it shares the same menu as Trim. I'll launch Extend and I will then select my Boundary Edge and press Enter. Then I will select each object I would like to extend to meet that boundary.
When I'm finished, I'll press Enter. As you can see, the workflow behind Trim and Extend is identical. In fact, these commands are so similar that if you launch one of them, you can access the other by simply holding down the Shift key. Let's take a look. I'm going to zoom out, and I'll pan the drawing over. I'd like to convert this geometry into the shape of a ladder. To do that, I will obviously have to trim off some of the line work and extend some of the others. I'm going to start using the Trim command. I'll create a crossing window to select these vertical lines--these will represent my cutting edges-- and I'll press Enter.
I can then select each of the objects I'd like to trim. Now, I could do that one at a time, or I could use a crossing window to select those objects. Once I'm finished trimming, this is where we would typically launch the Extend command. Take a look at the cursor though. It says, "Select object to trim or shift-select to extend." If I'm in the Trim command and hold down my Shift key, Trim converts to Extend and my cutting edges become boundary edges. When I'm finished, I'll press Enter. Now that we understand the basic workflow behind Trim and Extend, let's try and use these commands in a practical example.
I'm going to zoom out, I'll pan the drawing over again. On my screen, I have a double window with an arched top. Let's say that I'd like to convert this geometry into a single window with more of a Gothic top. I'll start by deleting the geometry that I don't need. I'm going to click in this right- side window, and I'll create a crossing selection, to select these dividers. I'll press Delete to remove them. I will then select the top inside edge and the bottom inside edge.
I will also select this geometry on the right side and press Delete, I'll zoom in a little. Now, I'd like to extend this outside edge up and down to meet the top and bottom edge of the window. I'll launch the Extend command to do that. I'll select the top and bottom edge of the window as my boundary edges and press Enter. I will then click either end of this line to project the geometry out to meet those edges. When I'm finished, I'll press Enter. Now I can use my revised line work to trim off the top and bottom edge.
Let's launch the Trim command. I'll select the right side of the window and press Enter. I will use this right side to trim off the top and the bottom. Let's pan the drawing down. And it doesn't look like I'm going to need these radial dividers any more, so I'll use a crossing window to select these, and I'll press Delete. It looks like the top left side of my Gothic window is in good shape. I would like to copy this geometry on the right over to create the top right side of the window. So, I'll launch the Copy command. I'll use a crossing window to select the geometry and press Enter.
I'll pick it up from the endpoint of the large arc, and I'll place it to the upper-right corner of the window. When I'm finished, I'll press Enter. Finally, I have to do a lot of trimming. I'll move up and launch the Trim command. And rather than going in and trying to pick each individual cutting edge that I need, take a look at the cursor. AutoCAD's saying, "Select cutting objects or a cutting object. So I can go through and quickly remove the objects I don't need, either by clicking them one at a time or by using a crossing window. Zoom in a little closer.
I'll take off this piece, and this one, and this one. Fortunately, every object is able to be used as a cutting object. And you don't have to worry about making a mistake, like I just did. If you look at the command line, you'll find an Undo option down here. If you click that, it will restore the last object that you trimmed. Now that I'm finished, I'll press Enter. As you can see, using the Trim and Extend commands, we don't always have to create new line work. We can use existing geometry to help develop our final design.
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