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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.
Another convenience tool AutoCAD gives us is the Polygon command. Polygon allow us to create equilateral shapes having as many sides as we desire. In this lesson, we'll explore the workflow behind the Polygon command. On my screen, I've created some polygon examples. It's important to note that every polygon that we create is based on an imaginary circle. If I pan this up, you can see how a circle could be associated with each of these shapes. In fact, the way we draw polygons is very similar to creating a circle. First, we tell AutoCAD how many sides the polygon has, then we specify the center point, followed by the radius. Now, there is one other thing that AutoCAD needs. It will need to know if the polygon is inscribed or circumscribed. In other words, does the polygon fall on the inside or the outside of the imaginary circle? The way to know which method to use depends on how your polygon was dimensioned. If it was dimensioned from point to point, it's an inscribed polygon, because it falls on the inside of the circle, and this dimension represents the circle's diameter. If the polygon is dimensioned from face to face, it's circumscribed because the polygon falls on the outside of the imaginary circle, and this dimension also represents this circle's diameter. Knowing this, I'd like to create some polygons. I'm going to pan the drawing up, and let's see if we can re-create the general geometry of the stop sign. To launch the Polygon command, we can find it up here in the Draw panel. It actually shares the same menu as the rectangle command. Now, for the number of sides, I'm going to choose 8. I'll be creating the large octagon first. I'll press Enter. I will then click onscreen to specify the center of the polygon. Now, is this polygon inscribed or circumscribed? Well, since it's dimensioned from face to face, this is circumscribed. It falls on the outside of the imaginary circle. I'll choose circumscribed. And then what is the radius of the circle? Well, I can see the diameter is 30, so the radius must be 15. Next, I'd like to create the smaller octagon. To do that, I will relaunch the Polygon command. It becomes the default up here in the Draw panel. I will accept 8 for the number of sides. Now, I need to specify the center. Here's an interesting fact: even though this polygon was created from an imaginary circle, the polygon itself has no center point. So, I'm going to press Escape and cancel this command momentarily. A really quick way to find the center of this polygon would be to launch the Line command and then use my running object snap to snap to the opposite corners.
I'll press Escape when I'm finished. My new polygon will be created from the midpoint of this line. I'll launch polygon again. I'll accept 8. I will then use my running object snap to snap to the middle of this line. This polygon is also going to be circumscribed. And what is the radius of the circle? Well, the larger one had a radius of 15 and I can see the smaller one has a radius that's 1 unit less than that, so I'll type 14 and hit Enter. Now that I'm finished with this line segment, I'll select it and press Delete.
Finally, let's create the carriage bolt geometry that holds the sign to the pole. To view these dimensions, I'm going to click to the lower left and then I'll pull up and click again to create a window selection. Once I've selected that geometry, I'll click the top hot spot on the view cube. This will focus my attention on that area. I will then press Escape to deselect the objects. It looks like the carriage bolt is a hexagon. It also looks like it's dimensioned from point to point so this one is inscribed. We can also see that the center of this polygon falls 3 units below the middle of the top of the sign. Now that I know the dimensions, I'd like to restore my previous view. I could do that by rolling my mouse wheel backwards. Another way would be to come over to this navigation bar. Notice there is a Zoom tool here. If I click the flyout right beneath the tool, I can select Zoom Previous to go back to my previous view. To create the first carriage bolt, I'll launch the polygon command. It has 6 sides. To find the center of the polygon, I'm going to use temporary tracking. I'll type TK and hit Enter. My first tracking point will be the middle of the top of the sign. I will then pull straight down 3 units and hit Enter. Now that I'm where I want to be, I'll hit Enter again to resume the Polygon command. This polygon is inscribed.
And what is the radius of the circle? Well, the diameter is obviously 1 so the radius must be .5. To create the final carriage bolt, I will press the spacebar to relaunch the polygon command. I will hit Enter to accept the number of sides. I'm going to use TK to find the center point. I will snap to the middle of the bottom of the sign and pull straight up 3 units. I will then hit Enter to return the Polygon command. This polygon is also inscribed and has a radius of .5. As you can see, once you understand the difference between an inscribed and a circumscribed polygon, creating these shapes is as easy as drawing a circle.
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