This Instructable is written for those just learning and starting out with AutoCAD with the purpose passing on some good advice. It assumes some facility with the computer.
I will show how to make the star above using four basic and extremely useful commands. First, a little intro. I joined the TechShop a couple years ago. I had done electronics design and programming, but was always interested in building things. In my first tour of the TechShop, I was amazed by all the new tools: laser cutters, plasma cutters, ShopBots, and such. To me then, the most impressive machine was the waterjet cutter, which I knew something about from welding classes I had taken. but I never thought I would use one!!! The bottom line for all these machines is that you need a drawing, and AutoCAD is one program to make drawings that can be used by these machines. So I was determined to learn AutoCAD.
I took the introductory course and set out to learn in by myself, with the help of books and tutorials. Despite my enthusiasm and good intentions, I struggled to get drawings done and learn AutoCAD, Finally, I signed up for a course at the local community college, Laney College, in Oakland. About the third week, the instructor told us that in his mind, there were 4 commands that would be extremely useful to make drawings: (1) copy, (2) offset, (3) extend, and (4) trim. It's been a year and a half now, and I am now fairly accomplished with AutoCAD having done a number of projects, some of which were fairly complex.
Throughout this work, I have thought about that class, and my instructor's comments about the "4 commands", and just how useful they are! I wanted to share this idea for learning AutoCAD and to illustrate how these commands might be used to create a simple drawing of a 5-point star. Actually, making the star with these commands really only requires a couple dimensions and a couple rotations.
Step 1: Getting Started Drawing the Star--let's Start With Creating a Vertical Line.
To illustrate these commands--copy, offset, extend, and trim--I thought it might be useful to make a star.
The commands work best when you have your snaps set correctly. They are set at the bottom of the screen in the status bar. I recommend getting to know this. Clicking on Object Snap or 'OSNAP', or the OSNAP icon with the "cross", allows you to select objects with various features of the object: for this example of making a star. To check and change them, right click on "OSNAP" (or the "OSNAP" icon) and select "Object Snap": "endpoint", "center", and "intersection", should be selected. The first step is to create a vertical line. It doesn't matter where it is, or how long it is; just a vertical line line. We can extend it or trim it later. Holding the shift key down when drawing the line will ensure that it the line is vertical.
Step 2: Copy the Line to Other Points on the Screen
Next, make two copies of the line to other points on the screen. You can grab the copy command from the ribbon, or use the command line and type, COPY--actually, just typing "CO" is sufficient.
Step 3: Rotate the Copied Lines 72 Degrees and -72 Degrees
Next rotate the copied lines 72 degrees. It's pretty straight forward: type "ROTATE", select the line and enter the degrees to rotate it. Rotate one line 72 degrees and the other -72 degrees.
Note: to create the (five point) star, we need to find five points equidistant from the center, that is,one a circle around the center. Since there are 360 degrees in a circle, each point will be at 360 degrees/ 5 or 72 degrees.
Step 4: Continuing to Get Lines at 72 Degrees Around the Circle
Using the "copy" command, copy the newly rotated lines to the base of the original vertical line. Actually, you might have used the "move" command, but with the copy, we still have the two rotated lines on the screen. Rotate these lines again by 72 degrees: the one that was rotated 72 degrees is rotated again by 72 degrees, and the one rotated by -72 degrees is rotate similarly by -72 degrees. (At this point you will have a screen like above.)
Step 5: Finding the Five Points on the Circle
After you have rotated the lines by 72 and -72 degrees for the second time, copy this lines to the base of the original vertical line. After you have 5 lines separated equally at 72 degree angles, create a circle of diameter 10 at the point where they all intersect. (To create the Circle, you can use the ribbon at the top, or just type 'C' in the command line and defined the diameter as '10' when prompted.) Clean up any remaining lines on the screen.
Step 6: Extend and Trim the Lines to the Circumference of the Circle
Next, get the lines to extend and terminate exactly on the edge (circumference) of the circle. If your original line was greater than 10 you will have to trim the line; if the original line was less than 10, you will have to extend the lines to meet the circumference of the circle. The commands from the command line are pretty simple: "TRIM" and "EXTEND" respectively.
Step 7: Create a Second Circle Inside the First Circle
After the lines are ten lines (which are also diameters) are drawn through the circle, create a second circle inside the first circle. To do this, type "CIRCLE", or "C", to start the command. (It can also be selected from the "Home" menu in the ribbon.) Place the center of the circle on the intersection of the ten lines, using the "intersection" snap. Make the diameter equal "5", and select, hit okay, or enter, to create the circle.
Step 8: Draw the Outline of a Star
Now that you have the diameter lines (at 72 degree intervals) in the circle and outside and inside points for the circles, we can start to draw the lines for the outline of the star.
Select the line tool from the ribbon (Home menu) or type "LINE", or just "L", and start the line draw tool. Place the mouse pointer over the point at the top of the outside circle. Drag the mouse to the intersection of the inside circle and the next line at 36 degrees in the clockwise direction. Then advance to the intersection of the outside circle and the line at 36 degrees clockwise. Continue this line drawing process until you return to the point at the top of the outside circle. If you make a mistake along the way, don't panic. Just start the procedure up and redraw the line. Next, we will be cutting and trimming lines.
Step 9: Trim the Drawing Lines
Next we use the "TRIM" command to remove the parts of the circle and interior lines that we don't want.
For this, start the command by typing "TRIM" (or select from the Home menu ribbon). To select the area, it is easiest to start at the unused area in the lower right, and drag the mouse across the screen to the upper left. The selected objects (i.e. lines) will appears a lightened and broken. Select "okay" or press enter; you can now remove sections that we don't want.
Step 10: Erase the Lines That You Can Not Trim: Reveal the Star
You probably won't be able to remove all the sections of lines to reveal the finished star. The reason that the lines cannot be trimmed is probably that they are complete lines, and don't really have an intersection. That is, sometimes it appears that a line is intersected by another line, where this is not the case: it might be two collinear lines on either side of a line, so trimming one side or the other of the two lines won't work. In this case, you have to erase one of the lines.
Remember with erasing, even mistakes can be corrected with the "UNDO" command. AutoCAD remembers each command sequence and can dial back to an earlier points. Thus, if you do make a mistake, just "undo" the work until you are at the point where you were prior to making the mistake. When all the lines are trimmed and erased, you should have the 5-point star as above. But we're not finished. Maybe you would have preferred the star more like is used on the American flag, with collinear lines on opposite sides of each other; that is, like would be produced by drawing lines to every second point on the outside circle. To do that, draw the lines as in the next step.
Step 11: Draw the Lines to Make the "American Flag" Star
As in the drawing above, draw the five lines to every other point on the star (those points that were on the outside circle). Again, activate the Line tool by typing "L" (or on the ribbon). Proceeding around the outside circle, and connecting every other point on the star, will produce the screen as above.
Step 12: Finishing the Star
Next, erase all the lines from the star that was draw previously. You will be left with the five lines that create the new star, however, they cross the interior of the star. We will trim these lines, which is simply done with the Trim command. In this case, the lines actually cross the star and should be easily trimmed by selecting all the lines in the drawing, and then clicking on all the points inside of the outside of the shape of the star.
Step 13: And That's the Star--drawn Using Lines and Four Basic Commands
After trimming all the inside lines, you will be left with the star in the drawing above.
This star can be taken to the laser cutter, or the plasma cutter, or the ShopBot, or any other CNC machine, and used to cut the star pattern. And it was created with a minimum of inputs for adding dimensions to the lines: the four commands of copy, offset, extend, and trim, the major part of the work.