Friday

Arc drawing techniques in AutoCAD

18 Comments Nov-07-2008 | Posted by Orhan Toker | Printable Version | Level |

Hello Daily AutoCAD readers,

In this article, I will try to explain about arc, which is probably one of the hardest entities in AutoCAD to draw. The reason why I have chosen this subject is that probably this will be one of the main subjects which advances an AutoCAD user to one upper level. Moreover, subjects of correct coordinate and unit entry, drawing perpendicular and parallel and developing of drawing techniques in AutoCAD will also be explained within this article.

I guess, the first entity that you learned to draw in AutoCAD is ARC. However, as I don’t know how you are going to use the arc entity, so I will start by explain ARC drawing methods. Because, after introducing the ARC subject, you will see that in addition to 9 different ARC drawing methods that is generally known by users, there are 4 other methods, which I believe will be very useful for you.

Fig. 1 Structure of an ARC entity

First of all, I would like to start by explaining about the structure of ARC entity. ARC is a circle part that passes through 3 points. Thus, 3 points are enough define an ARC. When you enter ARC command in AutoCAD, unless noted otherwise, you can draw an arc by entering start point, middle point and end point. This is the most fundamental way of drawing an ARC (Fig. 1).

This is the point where most of the students, new learners or those who has learned AutoCAD by themselves make a very common mistake. Users use this method even for drawing an arc which is ¼ of a circle and this is no more then drawing and arc arbitrarily (Fig. 2).

Fig.2 Difference between a correctly drawn door and another

If you are saying “What difference does it make!?”, here is the answer: Maybe you have a good eye vision and you can draw the arc very precisely. It may even not be noticed by eye. However, we are drawing technical drawings in AutoCAD and if our drawings are correct according to technical point of view, then this will save us from possible problems in the future. Especially if you are drawing drawings for manufacturing! Now, let’s take a look at how to draw a door arc. I assume that you have the doorway and the door already drawn. It is quite simple to draw the arc. You will enter ARC command and then you will select CENTER option.

Fig.3 Use of main points that make up an arc in a door drawing.

Then, select START and END points as shown in figure and your door is ready. Here, the sequence of start and end points are very important, because AutoCAD tries to draw the arc in counter clock wise direction, thus the angular direction. I would like to give another example for drawing a simple arc:

Let’s draw the arc that is shown in red in Fig.4. Angle of this arc is 180 degrees and its starting and end points are known. Thus, we have to draw our arc by entering starting point, end point and angle. Run the command and select the starting point. Then enter ENDPOINT option and select end point. Now, select ANGLE option and enter angle as 180 degrees. Don’t forget that if you enter starting and end point in reverse order, then you have to enter the angle as -180 degrees. I suggest that you try and see. Practice is practice.

Fig.4 Drawing tangent arc

Now, we can start professional drawings. Here, I will give an AutoCAD tip. When drawing in AutoCAD, if you are prompted to pick a point, press ENTER. By this way, you can continue by selecting the end point of the entity that you have drawn last. Draw a new line and exit. Then, enter the line once more and when AutoCAD prompts for the starting point, then press ENTER, then you will see that you are continuing from the end point of the last line. Then, do the same for drawing an arc after the line. That’s the surprising part. If you attempt to draw an arc after line or line after arc; then if you press ENTER instead of entering the starting point of the new one, you will see that the new one starts tangent to the old one. Let’s do the previous example by this way.

Fig.5 Continuing the line with an arc tangent to it.

Draw a line of 50 units as shown in Fig.5. Then, enter ARC and select the starting point by pressing ENTER. You will get an arc that is tangent to the previous line at the end of your cursor and it will be reshaped as you move your cursor. Position your cursor 90 degrees towards downward direction, go 50 units and press ENTER. By this way, you have drawn an arc that makes an angle of 180 degrees and starting point of which is -90 degrees and 50 units away from the endpoint of line. But, why did AutoCAD drew an arc in clock wise direction this time? Because, it had to continue after the previous line. After this, you can draw the line with 50 units after continuation of the arc with the same method.

Another arc drawing method is converting POLYLINE into curves. In Fig. 6, you can see that a closed polyline, which is drawn by using PLINE command, is converted into curves by using PEDIT command with SPLINE and FIT options. Please pay attention to vertex points and red polygon.

Fig.6 Converting a closed POLYLINE entity into curves by using PEDIT command.

SPLINE option tries to take mid-points of segments of polyline as tangent points. And FIT option tries to make turns at vertec points of polyline. Once you convert a polyline entity into curves, then you can stretch the tangent points of the new entity. This way does not interfere the tangent orientation. However, if FIT option is used, then you cannot edit the entity. You will notice that the vertexes are disoriented as you edit. This last method is more commonly used for free drawing techniques. It is quite difficult to draw an arc that has definite dimensions by using this method. There is another entity, named as SPLINE in AutoCAD, which is used for drawing more complex curves. But, its use is not that practical and it can not be edited by using PEDIT command. So, I leave it to you to try the command by your self.

In this article, I tried to open the road to drawing complex curved entities by explaining the most fundamental method, which is drawing ARCs. However, please note that, here I cannot give all of the methods that you may use for drawing curved entities. In addition, you can develop different drawing methods that I could not think of, and come up with the same result by using other ways. The only important point that you should pay attention to is not to deviate from the correct dimensions.

Good bye.