Autocad Commands

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Using ARC Command

Using ARC Command.

One of AutoCAD's basic drawing tools is the ARC. This command is used for drawing all sorts of arcs, as well as complex curves. This article aims at enlightening us with all the features of the ARC command.

The ARC command can be found in the "Draw" panel under the "Home" tab, in the ribbon menu. The command can also be accessed by typing ARC or A in the command line.
Let's have a better look at what happens in the command line, to be able to draw.

Command: _ARC
Specify start point of arc or [Center]: (1)specify starting point
Specify second point of arc or [Center/End]: (2) specify second point
Specify end point of arc: (3) specify end point

As you can see, we constructed a random ARC. Like in many other commands, there are some techniques that give you the control while the command is running. Let's learn more about the options in the ARC menu and see how they all work with an an example each.

1. Constructing an Arc Starting with a Point

A closer look at the ARC menu shows us that starting with an initial point, there are 7 methods of drawing an arc, including specifying 3 random points. Let's see:

1.a. Start, Center, End

This method helps us to construct arcs with a known center point by specifying the starting and ending points.

The command line will look like this:

Command: ARC
Specify start point of arc or [Center]: (1) specify a starting point.
Specify second point of arc or [Center/End]: c (this tells the command that you'll be specifying a center point ).
Specify center point of arc: (2) specify the center point.
Specify end point of arc or [Angle/chord Length]: (3) specify the end point.

While using this technique, please keep in mind that the arc is drawn counter-clockwise. Starting with the point marked (3) would have resulted in something like this instead:

1.b. Start, Center, Angle

This is used for drawing an arc with the starting point, the center point and the sustained angle.

Command: _arc
Specify start point of arc or [Center]: (1) Specify the start point
Specify second point of arc or [Center/End]: _c (we'll be specifying the center point)
Specify center point of arc: (2) mark the center point
Specify end point of arc or [Angle/chord Length]: _a (we'll be specifying the sustained angle)
Specify included angle: 45 (specify the angle on-screen or enter numerical value)
Start, Center, Length

Besides the starting point and the center point of the arc, if the chord length is known, an arc can be constructed.

Command: _arc
Specify start point of arc or [Center]: (1)
Specify second point of arc or [Center/End]: _c (we'll be specifying the center point)
Specify center point of arc: (2) mark the center point
Specify end point of arc or [Angle/chord Length]: _l (we'll enter the length of the chord)
Specify length of chord: (either specify on-screen or enter numeric value)
Start, End, Angle

Allows you to construct an arc of known starting and ending points by specifying the angle sustained.

Command: _arc
Specify start point of arc or [Center]: (1)
Specify second point of arc or [Center/End]: _e (we'll be specifying the end point)
Specify end point of arc: (2) mark the end point
Specify center point of arc or [Angle/Direction/Radius]: _a (we'll be specifying the angle)
Specify included angle: 90 (specify the angle on-screen or enter numerical value)

Note: Specifying the points in reverse sequence will give you a reversed arc with this option too.

1.c. Start, End, Direction

This technique is actually the most useful among all, and if it hadn't been included in the menu, there's no way we could access it from the command prompt.

In the example above, we specified the end point (2) so that the arc will sustain a 90-degree angle. This caused the arc to actually intersect the door just a little bit. Let's retry with our new technique. It'll be more accurate this time.

Command: _arc
Specify start point of arc or [Center]: (1)
Specify second point of arc or [Center/End]: _e (we'll be specifying the end point)
Specify end point of arc: (2) mark the end point
Specify center point of arc or [Angle/Direction/Radius]: _d (we'll be specifying direction)
Specify tangent direction for the start point of arc: (90 degrees and upwards)

As you can see, specifying the tangent direction of the desired arc enables us to define an arc ending correctly at the specified point.

1.d. Start, End, Radius

Command: _arc
Specify start point of arc or [Center]: (1) starting point for the arc
Specify second point of arc or [Center/End]: _e (we'll be specifying the end point)
Specify end point of arc: (2) mark the end point
Specify center point of arc or [Angle/Direction/Radius]: _r (we'll be specifying the radius)
Specify radius of arc: (specify on screen or enter numerical value)

2. Constructing an Arc Using the Center Point

 

2.a. Center, Start, End

Let's re-do the same sample using the center point.

Command: _arc
Specify start point of arc or [Center]: _c (we'll be specifying the center point)
Specify center point of arc: (1) mark the center point
Specify start point of arc: (2) mark the starting point
Specify end point of arc or [Angle/chord Length]: (3) mark the end point of arc.

Note: It is important to specify the points in counter-clockwise (AutoCAD's angle direction). Doing it the other way will result in a complementing arc.

2.b. Center, Start, Angle

Command: _arc
Specify start point of arc or [Center]: _c (we'll be specifying the center)
Specify center point of arc: (1) mark the center
Specify start point of arc: (2) mark the start point
Specify end point of arc or [Angle/chord Length]: _a (we'll be specifying the sustained angle)
Specify included angle: (specify the angle on-screen or enter numerical value) (3)

2.c Center, Start, Length

This could have been excluded from the menu :) Although it's not often used, I'd like to mention it.

Command: _arc
Specify start point of arc or [Center]: _c (we'll be specifying the center point)
Specify center point of arc: (1) mark the center
Specify start point of arc: (2) mark the start point
Specify end point of arc or [Angle/chord Length]: _l (we'll be specifying the length)
Specify length of chord: (specify the length of the chord on-screen or enter numeric value) (3)

3. Constructing Complex Curves Using the "Continue" Feature

We've talked about arc-arc and line-arc continuity in our diaries. AutoCAD let's you continue constructing from the end point of the last object you created. If the object happens to be an arc, the second arc will be tangent to the first arc.

Let's construct a spiral using this feature.

1. Let's create a helping line for the spiral.


2. Use the DIVIDE command to split the line into 9 pieces.


3. If you can't see the divided parts, set the PMODE system variable to 3 to make them visible.


4. Make sure "NODE" is selected in the object snap settings.


5. Let's construct an arc of known starting and ending points, sustaining 180 degrees, by marking points 1 and 2.


6. Now run the arc command again. AutoCAD will prompt you with "Specify start point of arc:". Hit ENTER to select the "Continue" option.


7. You should now have a second arc appearing at the tip of your cursor.


8. Select the point mark 3 and continue until your spiral is complete.

Keep in mind that the AutoCAD command prompt has many secrets amongst its options.

 

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AutoCAD Commands and Methods


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