Let me make you an (AutoCAD) Star!

Mark Sadler is an, Architect, Designer and much respected CAD consultant and Blogger.

If you like this post, you can get more great tips from Mark at his blog – BestCADTips.com

If you’d like to contribute a post to Cadsetterout.com click here!

Over to you Mark…

Let me make you a Star

In this post, I will demonstrate how to make a five-pointed star using AutoCAD.

In the process of making a star, we will touch on several AutoCAD tools, including the polygon tool, the block tool, the hatch tool, and the trim command.

I’ve always liked stars. I live in Texas, USA, and we’re very big on stars here.  We write songs about them and put them on our flags.  As a kid growing up, I enjoyed using a stick to make a star shape on the ground near my house by scratching five lines in the sand.  It looked something like this:

Now that I’ve grown up (some would argue the point), I use multi-thousand-dollar software and a laptop computer to make an electronic drawing of a star that is accurate to the ten-thousandth of an inch—or centimeter, if you prefer.

A student once asked me in one of my AutoCAD classes,

Doesn’t AutoCAD have a tool to make a star?

I answered, No, not really, but it does have a polygon tool that we can use to make a great star.

Let’s go to AutoCAD.

The first step in making our star is to make a circle.  Why, you ask?  Because the polygon tool uses an imaginary circle in the process of making a polygon, and I’d like to clarify what’s going on with that.

Everyone knows the circle tool. Give AutoCAD a center point (anywhere) and type in a radius length, and you get a circle.  Let’s use a radius of 36 units.

By the way, I don’t use the standard AutoCAD hotkey “C” to make a circle.  The standard circle tool hotkey is “C,” but I prefer to have “C” set to initiate the COPY command, which I use much more frequently than the CIRCLE tool.  I use the Command Alias Editor to set up C to initiate COPY, and set CI to initiate CIRCLE. It saves a lot of time.

The Polygon Tool

Quick question: What is the smallest number of sides that a polygon can have?

(Insert thinking music here . . .)

If you guessed three, you’re correct. Six sides makes a hexagon, five sides makes a pentagon, four sides makes a square, three sides makes an equilateral triangle, and if you try to go less than three sides it’s no longer a polygon, it collapses to become a line.

Going the other direction, what is the largest number of sides a polygon can have?  An infinitely large number.  And the larger the number of sides gets, the more the polygon resembles—you guessed it– a circle.

Next we’ll create the polygon.

The polygon tool is in the Home tab, Draw panel, under the Rectangle pull-down arrow.

Starting the Polygon tool, we notice that nothing happens until you tell AutoCAD how many sides you want your polygon to have.  The command line says:

The default number is 4, but since it is shown between pointy brackets, you must either keep it or change it and then hit [ENTER] to continue.  Type 5 and [ENTER].

Next AutoCAD directs you to specify the center.  With your OSNAP tool turned on, use the Center snap to click in the exact center of the circle.

Now comes an interesting question.  AutoCAD wants to know if you want the polygon Inscribed in circle, or Circumscribed about circle.

Note that if we had not drawn a circle, there would be no circle in our drawing.  Nevertheless, AutoCAD is referring to an imaginary circle in its interior process.  It uses the imaginary circle to set the size of the polygon we’re about to create.But it wants you, the designer, to tell it which of two methods to use.

Let’s take a quick time-out from our star project for a study of AutoCAD’s methods.  Using a triangle as an example of a polygon with only three sides, here’s a drawing of a triangle Inscribed inside a circle:

Now, we will create another triangle which is Circumscribed about the same circle, using exactly the same two snap points:

Wow!  Obviously the second triangle, using the Circumscribed option, is much larger.  It is also “upside down,” compared to the first triangle.

Generally, when using the polygon tool,I just hit [ENTER] to accept the “Inscribed in circle” option, which is the default.  I think it’s always good to understand what AutoCAD is doing behind the scenes, and why the polygon tool requires so many steps.

Creating the Star

So, back to our star project.  After choosing the Inscribed option, click the center point of our circle for the pentagon’s center.  AutoCAD now asks for the Radius of the imaginary circle.

Just click on the top quadrant snap (at the 12:00 position on our circle) to provide a radius point for the pentagon.

I’ve made the pentagon yellow for improved visibility.

Next I’ll connect the five corner points, going around the pentagon, skipping a vertex each time, just like when I was scratching a star in the dirt:

Now we can delete the pentagon, which has served its purpose.  Keep the circle, though, because we will use it in a minute to set the exact center point of our star block.

Next we’ll use the TRIM command to remove the interior lines.  TRIM gives you the option of either choosing cutting edges by selecting them, or just hitting [ENTER] to use all of the lines in your drawing as cutting edges.  I’ll use the latter method here, letting all lines be cutting edges.

Click on the five interior lines as shown above. You now have a beautiful star in a circle:

Let’s hatch in the circle with a solid white hatch.

Creating a Block from the Star

Now let’s make a block of the star.  We will use the center point of the circle as the base point of the block, and we will not include the circle in the block’s objects.

First, I’ll select all of the objects in the drawing, using Ctrl-A.

Now I can remove the circle from the selection set by holding down the SHIFT key and clicking on the circle.  Only the star linework and hatch objects now remain selected.  Type BLOCK.  The Block Definition dialog box appears.  For the block name, let’s use STAR.

Use the Pick point button to place the base point.  Use snap override (Shift+Right-click) to open the snap override options, and choose Center snap.  Click on the circle’s linework to choose its center point as the block’s base point, then hit OK.

Now you have a block which includes the star’slinework and the white hatch, and ithas a base point EXACTLY in the center of the star.  You can now erase the circle.

In case you’re wondering, the white hatch of our star does have a grip point that is near the center of the hatch, but it is not located at the exact center of the star.  Finding the exact geometric center of a five-pointed star like this, without having a circle to go by, is not an easy trick.  Try it sometime.

By having a star block with an exact center base point, you can now accurately place this star into a grid system or other geometric pattern.

Hope you enjoyed this little project.  Thanks again to Paul for letting me provide this guest post.  Cheers, and “Keep on CADDing!”   J

Mark